TITLE ANTAGONIST ANTIBODIES AGAINST GDF-8 AND USES IN TREATMENT OF ALS AND OTHER GDF-8-ASSOCIATED DISORDERS This application claims the benefit of priority from U.S. Provisional Patent Application No. 60/709,704, filed August 19, 2005, the contents of which is hereby incorporated by reference herein in its entirety. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH This invention was made with U.S. Government support under grants from the National Institutes of Health (R01 GM-48661 and R01 GM-56707). The Government has certain rights in the invention. BACKGROUND OF THE INVENTION Field of the Invention The technical field of the invention relates to growth and differentiation factor-8 (GDF-8) antagonists, in particular, antibodies against GDF-8, e.g., mouse, human and humanized antibodies and their fragments, particularly those that inhibit GDF-8 activity in vitro and/or in vivo. The field further relates to treating, ameliorating, preventing, prognosing, or monitoring GDF-8-associated disorders,e.g., muscle disorders, neuromuscular disorders, bone degenerative disorders, metabolic or induced bone disorders, adipose disorders, glucose metabolism disorders or insulin-related disorders, particularly amyotrophic lateral sclerosis ("ALS"). Related Background Art 10004; Growth and differentiation factor-8 (GDF-8), also known as myostatin, is a secreted protein and member of the transforming growth factor-beta (TGF-ß) superfamily of structurally related growth factors. Members of this superfamily possess growth-regulatory and morphogenetic properties (Kingsley et al. (1994) Genes Dev. 8:133-46; Hoodless et al. (1998) Curr. Topics Microbiol. Immunol. 228:235-72). Human GDF-8 is synthesized as a 375 amino acid precursor protein that forms a homodimer complex. During processing, the amino-terminal propeptide, known as the "latency-associated peptide" (LAP), is cleaved and may remain noncovalently bound to the homodimer, forming an inactive complex designated the "small latent complex" (Miyazono et al. (1988) J. Biol. Chem. 263:6407-15; Wakefield et al. (1988; J. Biol. Chem. 263:7646-54; Brown et al. (1999) Growth Factors 3:35-43; Thies et al. (2001) Growth Factors 18:251-59; Gentry et al. (1990) Biochemistry 29:6851-57; Derynck et al. (1995) Nature 316:701-05; Massague (1990) Ann. Rev. Cell Biol. 12:597-641). Proteins such as follistatin and its relatives also bind mature GDF-8 homodimers and inhibit GDF-8 biological activity (Gamer et al. (1999) Dev. Biol. 208:222-32). An alignment of the deduced GDF-8 amino acid sequence from various species demonstrates that GDF-8 is highly conserved (McPherron et al. (1997) Proc. Natl. Acad. Sci. U.S.A. 94:12457-61). The sequences of human, mouse, rat, porcine, and chicken GDF-8 are 100% identical in the C-terminal region, while baboon, bovine, and ovine GDF-8 differ by a mere 3 amino acids at the C-terminus. The high degree of GDF-8 conservation across species suggests that GDF-8 has an essential physiological function. GDF-8 has been shown to play a major role in the regulation of muscle development and homeostasis by inhibiting both proliferation and differentiation of myoblasts and satellite cells (Lee and McPherron (1999) Curr. Opin. Genet. Dev. 9:604-07; McCroskery et al. (2003) J. Cell. Biol. 162:1135-47). It is expressed early in developing skeletal muscle, and continues to be expressed in adult skeletal muscle, preferentially in fast twitch types. Additionally, GDF-8 overexpressed in adult mice results in significant muscle loss (Zimmers et al. (2002) Science 296:1486-88). Also, natural mutations that render the GDF-8 gene inactive have been shown to cause both hypertrophy and hyperplasia in both animals and humans (Lee and McPherron (1997) supra). For example, GDF-8 knockout transgenic mice are characterized by a marked hypertrophy and hyperplasia of the skeletal muscle and altered cortical bone structure (McPherron et al. (1997) Nature 387:83-90; Hamrick et al. (2000) Bone 27:343-49). Similar increases in skeletal muscle mass are evident in natural GDF-8 mutations in cattle (Ashmore et al. (1974) Growth 38:501-07; Swatland et al. (1994) J. Anim. Sci. 38:752-57; McPherron et al., supra; Kambadur et al. (1997) Genome Res. 7:910-15). In addition, various studies indicate that increased GDF-8 expression is associated with HIV-induced muscle wasting (Gonzalez-Cadavid et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95:14938-43). GDF-8 has also been implicated in the production of muscle-specific enzymes (e.g., creatine kinase) and myoblast proliferation (WO 00/43781). I n addition to its growth-regulatory and morphogenetic properties, GDF-8 is believed to participate in numerous other physiological processes, including glucose homeostasis during type 2 diabetes development, impaired glucose tolerance, metabolic syndromes (i.e., a syndrome such as, e.g., syndrome X, involving the simultaneous occurrence of a group of health conditions, which may include insulin resistance, abdominal obesity, dyslipidemia, hypertension, chronic inflammation, a prothrombotic state, etc., that places a person at high risk for type 2 diabetes and/or heart disease), insulin resistance (e.g., resistance induced by trauma such as burns or nitrogen imbalance), and adipose tissue disorders (e.g., obesity, dyslipidemia, nonalcoholic fatty liver disease, etc.) (Kim et al. (2000) Biochem. Biophys. Res. Comm. 281:902-06). A number of human and animal disorders are associated with functionally impaired muscle tissue, e.g., amyotrophic lateral sclerosis ("ALS"), muscular dystrophy ("MD"; including Duchenne's muscular dystrophy), muscle atrophy, organ atrophy, frailty, congestive obstructive pulmonary disease (COPD), sarcopenia, cachexia, and muscle wasting syndromes caused by other diseases and conditions. Currently, few reliable or effective therapies exist to treat these disorders. The pathology of these diseases indicates a potential role for GDF-8 signaling as a target in the treatment of these diseases. ALS is a late onset and fatal neurodegenerative disease characterized by degeneration of the central nervous system and muscle atrophy. ALS typically initiates with abnormalities in gait and loss of dexterity, and then progresses to paralysis of limbs and diaphragm. While most cases of ALS are sporadic and are of unknown etiology, 5-10% of cases have been shown to result from dominant familial (FALS) inheritance. Approximately 10-20% of FALS cases are attributed to mutations in the Cu/Zn superoxide dismutase (SOD1) gene (reviewed in Bruijn et al. (2004) Ann. Rev. Neurosci. 27:723-49). SOD1 is a heterodimeric metallo-protein that catalyzes the reaction of superoxide into hydrogen peroxide and diatomic oxygen, and as loss of SOD 1 does not result in motor neuron disease (Reaume et al. (1996) Nat. Genet. 13:43-47), it is believed to induce disease by toxic gain of function (reviewed in Bruijn et al., supra). The specific mechanisms of SOD 1-induced neuronal cell death are unclear, and may involve alterations in axonal transport, cellular responses to misfolded protein, mitochondrial dysfunction, and excitotoxicity (Bruijn et al., supra). The degeneration of motor neurons observed in ALS may occur via multiple mechanisms, including uptake or transport disruption of trophic factors by motor neurons (reviewed in Holzbaur (2004) Trends Cell Biol. 14:233-40). Thus, ALS might be treated by therapies that rejuvenate a degenerating neuron by providing an optimal survival environment. A nerve's environment includes nonneuronal cells such as glia and the muscle cells innervated by the motor neuron. This environment provides trophic and growth factors that are endocytosed by the neuron and transported via retrograde axonal transport to the cell body (Chao (2003) Neuron 39:1-2; Holzbaur, supra). FALS has been modeled in both mouse and rat by the overexpression of mutant SOD1 (Howland et al. (2002) Proc. Natl. Acad. Sci. U.S.A. 99:1604-09). Transgenic mice overexpressing the G93A form of mutant SOD1 display muscle weakness and atrophy by 90 to 100 days of age, and typically die near 130 days of age (Gurney et al. (1994) Science 264:1772-75). However, the underlying SODG93A-induced pathology, which includes grip strength weakness and loss of neuromuscular junctions, is significant as early as 50 days of age (Frey et al. (2000) J. Neurosci. 20:2534-42; Fisher et al. (2004) Exp. Neuro. 185:232-40; Ligon et al. (2005) NeuroReport 16:533-36; Wooley et al. (2005) Muscle Nerve 32:43-50). Transgenic rats expressing the SODG93A mutation follow a similar time course of degeneration (Howland et al., supra). Recent work has suggested that the development of pathology is not cell autonomous, consistent with the hypothesis that the degeneration of motor neurons observed in ALS occurs via multiple mechanisms, including the disruption of uptake and transport of trophic factors by the motor neuron (see above). Clement and coworkers have used chimeric mice to show that wild type nonneuronal cells can extend survival of motor neurons expressing mutant SOD1 (Clement et al. (2003) Science 302:113-17). These observations have led to the the investigation of therapies that might slow neuronal degeneration by providing an optimal microenvironment for survival. For example, treatment of the SODG93 A mouse via direct intramuscular injection of virally expressed growth factors (including IGF-1, GDNF and VEGF) prolongs animal survival (Kaspar et al. (2003) Science 301:839-42; Azzouz et al. (2004) Nature 429:413-17; Wang et al. (2002; J. Neurosci. 22:6920-28). In addition, muscle-specific expression of a local IGF-1-specific isoform (mIGF-1) stabilizes neuromuscular junctions, enhances motor neuron survival and delays onset and progression of disease in the SODG93A transgenic mouse model, indicating that direct effects on muscle can impact disease onset and progression in transgenic SOD1 animals (Dobrowolny et al. (2005)J. Cell Biol. 168:193-99). Links between muscle hypermetabolism and motor neuron vulnerability have also been reported in ALS mice, supporting the hypothesis that defects in muscle may contribute to the disease etiology (Dupois et al. (2004) Proc. Natl. Acad. Sci. U.S.A. 101:11159-64). Thus, enhancing muscle growth should provide improved local support for motor neurons, and therefore result in therapeutic benefits. Inhibition of myostatin expression leads to both muscle hypertrophy and hyperplasia (Lee and McPherron, supra: McPherron et al., supra). Myostatin negatively regulates muscle regeneration after injury, and lack of myostatin in GDF-8 null mice results in accelerated muscle regeneration (McCroskery et al., (2005) J. Cell. Sci. 118:3531-41). Myostatin-neutralizing antibodies increase body weight, skeletal muscle mass, and muscle size and strength in the skeletal muscle of wild type mice (Whittemore et al. (2003) Biochem. Biophys. Res. Commun. 300:965-71) and the mdx mouse, a model for muscular dystrophy (Bogdanovich et al. (2002) Nature 420:418-21; Wagner et al. (2002) Ann. Neurol. 52:832-36). Furthermore, myostatin antibody in these mice decreased the damage to the diaphragm, a muscle that is also targeted during ALS pathogenesis. It has been hypothesized that the action of growth factors, such as HGF, on muscle may be due to inhibition of myostatin expression (McCroskery et al. (2005), supra), thereby helping to shift the "push and pull," or balance, between regeneration and degeneration in a positive direction. Thus, GDF-8 inhibition presents as a potential pharmacological target for the treatment of ALS, muscular dystrophy (MD), and other GDF-8-associated disorders, e.g., neuromuscular disorders for which it is desirable to increase muscle mass, strength, size, etc. With the availability of animal models (mouse and rat) of ALS, it is possible to test therapeutics in two different species, thus increasing the confidence of therapeutic effects in humans in vivo. In addition to neuromuscular disorders in humans, there are also growth factor-related conditions associated with a loss of bone, such as osteoporosis and osteoarthritis, which predominantly affect the elderly and/or postmenopausal women. In addition, metabolic bone diseases and disorders include low bone mass due to chronic glucocorticoid therapy, premature gonadal failure, androgen suppression, vitamin D deficiency, secondary hyperparathyroidism, nutritional deficiencies, and anorexia nervosa. Although many current therapies for these conditions function by inhibiting bone resorption, a therapy that promotes bone formation would be a useful alternative treatment. Because GDF-8 plays a role in bone development as well as muscular development, regulation of GDF-8 is also an excellent pharmacological target for the treatment of bone-degenerative disorders. Thus, a need exists to develop compounds and methods of treatment that contribute to an overall increase in muscle mass and/or strength and/or bone density, etc., particularly in humans, and particularly in those suffering from ALS and other muscle-wasting diseases as well as bone-degenerative disorders. Generating neutralizing antibodies and other small molecules with enhanced affinity to GDF-8 is an excellent pharmacological approach to treat these disorders. SUMMARY OF THE INVENTION The GDF-8 antagonists of the invention relate to antibodies (e.g., intact antibodies and antigen-binding fragments thereof), which are referred to herein as "anti-GDF-8 antibodies" or"GDF-8 antibodies." In one embodiment, an anti-GDF-8 antibody reduces, neutralizes, and/or antagonizes at least one GDF-8-associated activity (i.e., "GDF-8 activity"). The present invention thus provides methods to treat various bone, muscle, glucose and adipose disorders associated with GDF-8 activity using these anti-GDF-8 antibodies. The present invention discloses that GDF-8 antagonists, e.g., GDF-8 antibodies, are highly effective therapeutics when used to treat animals suffering from ALS, and that administration of such antibodies reduces the wasting of muscles targeted during ALS pathology, e.g., diaphragm, gastrocnemius, etc. In addition, the present invention discloses that these antagonists are highly effective at increasing muscle mass and grip strength in ALS-afflicted animals. As a result, the invention teaches that anti-GDF-8 antibodies are effective compositions to treat GDF-8-associated disorders, e.g., ALS, muscle wasting disorders or other disorders that result from GDF-8 dysregulation. In one aspect, the invention features a method of treating (e.g., curing, suppressing), ameliorating, or preventing (e.g., delaying or preventing the onset, recurrence or relapse of) a GDF-8-associated disorder in a subject. The method includes: administering to the subject a GDF-8 antagonist, e.g., an anti-GDF-8 antibody, in an amount sufficient to treat or prevent the GDF-8-associated disorder. The GDF-8 antagonist, e.g., the anti-GDF-8 antibody, can be administered to the subject alone or in combination with other therapeutic modalities as described herein. The GDF-8 antibody can be administered therapeutically, prophylactically, or both. In one embodiment, the subject is a mammal, e.g., a human suffering from a GDF-8-associated disorder, including, e.g., bone and muscle disorders. Preferably, the subject is a human. More preferably, the subject is a human suffering from a GDF-8-associated disorder as described herein. In one embodiment, the present invention provides safe and effective therapeutic methods for diagnosing, prognosing, monitoring, screening, treating, ameliorating, and/or preventing GDF-8-associated disorders, e.g., muscle disorders, neuromuscular disorders, bone-degenerative disorders, metabolic or induced bone disorders, adipose disorders, glucose metabolism disorders, or insulin-related disorders which include, but are not limited to, glucose homeostasis, type 2 diabetes, impaired glucose tolerance, metabolic syndrome (i.e., a syndrome involving the simultaneous occurrence of a group of health conditions, which may include insulin resistance, abdominal obesity, dyslipidemia, hypertension, chronic inflammation, a prothrombotic state, etc., that places a person at high risk for type 2 diabetes and/or heart disease), insulin resistance (e.g., resistance induced by trauma such as burns or nitrogen imbalance), adipose tissue disorders (e.g., obesity, dyslipidemia, nonalcoholic fatty liver disease, etc.), HIV-induced muscle wasting, muscular dystrophy (including Duchenne's muscular dystrophy), amyotrophic lateral sclerosis ("ALS"), muscle atrophy, organ atrophy, frailty, congestive obstructive pulmonary disease, sarcopenia, cachexia, muscle wasting syndromes, osteoporosis, osteoarthritis, metabolic bone diseases, and metabolic bone disorders (including low bone mass due to chronic glucocorticoid therapy, premature gonadal failure, androgen suppression, vitamin D deficiency, secondary hyperparathyroidism, nutritional deficiencies, and anorexia nervosa). In a preferred, but not limiting, embodiment, the invention provides safe and effective therapeutic methods for diagnosing, prognosing, monitoring, screening, treating, ameliorating, and/or preventing a GDF-8-associated disorder, e.g., a muscular disorder in vertebrates, particularly mammals, and more particularly humans. In a most preferred embodiment of the invention, the GDF-8-associated disorder, e.g., muscle disorder, diagnosed, prognosed, monitored, screened, treated, ameliorated, and/or prevented is ALS. In another embodiment, this invention provides methods of inhibiting GDF-8 function in vivo or in vitro. These methods are useful for treating GDF-8-associated disorders, e.g., muscle and bone degenerative disorders, particularly muscle disorders such as ALS, and for increasing muscle mass and/or bone strength and/or density. The methods are also useful for increasing muscle mass and bone density in normal animals including, but not limited to, humans. The subject methods can be used in vitro (e.g., in a cell-free system, in culture, etc.), ex vivo, or in vivo. For example, GDF-8 receptor-expressing cells can be cultured in vitro in culture medium and contacted with, e.g., one or more anti-GDF-8 antibodies, e.g., as described herein. Alternatively, the method can be performed on cells present within a subject, e.g., as part of an in vivo (e.g., therapeutic or prophylactic) protocol. Accordingly, in one aspect, the invention features a GDF-8 antagonist, e.g., an isolated antibody, that interacts with, e.g., binds to, and neutralizes and/or inhibits, GDF-8. In particular, the GDF-8 protein bound by the GDF-8 antibody is mammalian, e.g., human, sheep, nonhuman primate GDF-8. In another embodiment, the invention provides antibodies that bind GDF-8 with high affinity, e.g., with a Kd of at least 10-7M, preferably 10-8, 10-9, 10-10, more preferably, 10-11 M or higher. The affinity and binding kinetics of the anti-GDF-8 antibody can be tested using several well-known methods, e.g., biosensor technology (Biacore, Piscataway, NJ). In one embodiment, the anti-GDF-8 antibody (e.g., an intact antibody or an antibody fragment (e.g., a Fab, F(ab')2, Fv or a single chain Fv fragment)) is a monoclonal antibody. The antibody may be a human, humanized, chimeric, or an in vitro-generated antibody. In a preferred, but not limiting, embodiment, an anti-GDF-8 antibody of the invention is a humanized antibody. These anti-GDF-8 antibodies can be used to diagnose, prognose, monitor, screen, treat, ameliorate, and/or prevent muscle, bone, adipose and glucose metabolism-related disorders. A nonlimiling example of an anti-GDF-8 antibody is referred to herein as "RK35," and includes both mouse and modified antibodies, e.g., chimeric or humanized forms. The nucleotide and amino acid sequences for the heavy chain variable region of mouse RK35 are set forth herein as SEQ ID NO:2 and SEQ ID NO:3, respectively. The nucleotide and amino acid sequences for the heavy chain variable region of humanized RK35 are set forth herein as SEQ ID NO:6 and SEQ ID NO:7, respectively. The nucleotide and amino acid sequences for the light chain variable region of mouse RK35 are set forth herein as SEQ ID N0:4 and SEQ ID NO:5, respectively. The nucleotide and amino acid sequences for the light chain variable region of humanized RK35 are set forth herein as SEQ ID NO:8 and SEQ ID NO:9, respectively. In a preferred, but not limiting, embodiment of the invention, the antibody is a mouse or humanized antibody to GDF-8. In a more preferred embodiment of the invention, the antibody is comprised of the VH (variable heavy) domain set forth in SEQ ID NO:3 and the VL (variable light) domain set forth in SEQ ID NO:5. In another preferred embodiment of the invention, the antibody is comprised of the VH domain set forth in SEQ ID NO:7 and the VL domain set forth in SEQ ID N0:9. Additional embodiments of the invention comprise one or more VH or VL domains listed in Table 1. Other embodiments of the invention comprise an H3 fragment of RK35, i.e., the sequence set forth as SEQ ID NO: 12. In yet another embodiment, a GDF-8 antagonist comprises one, two, or three complementarity determining regions (CDRs) from a heavy chain variable region of an antibody disclosed herein with sequences selected from SEQ ID NOs:10-12 and 20-22. In yet another embodiment, an antagonist of the invention comprises one, two, or three CDRs from a light chain variable region of an antibody disclosed herein with sequences selected from SEQ ID NOs:13-15 and 23-25. In yet another embodiment, the antibody comprises one, two, three, four, five, or six CDRs with sequences selected from SEQ ID NOs:10-15 and 20-25. The heavy and light chains of an anti-GDF-8 antibody of the invention may be full-length (e.g., an antibody can include at least one, and preferably two, complete heavy chains, and at least one, and preferably two, complete light chains) or may include an antigen-binding fragment (e.g., a Fab, F(ab')2, Fv or a single chain Fv fragment ("scFv")). In other embodiments, the antibody heavy chain constant region is chosen from, e.g., IgGl, IgG2, IgG3, IgG4, IgM, IgAl, IgA2, IgD, and IgE, particularly chosen from, e.g., IgGl, IgG2, IgG3, and IgG4, more particularly, IgGl (e.g., human IgGl). In another embodiment, the antibody light chain constant region is chosen from, e.g., kappa or lambda, particularly kappa (e.g., human kappa). In one embodiment, the constant region is altered, e.g., mutated, to modify the properties of the antibody (e.g., to increase or decrease one or more of: Fc receptor binding, antibody glycosylation, the number of cysteine residues, effector cell function, or complement function). For example, the human IgGl constant region can be mutated at one or more residues, e.g., one or more of residues 117 and 120 of SEQ ID NO: 19. In one embodiment, the anti-GDF-8 antibody comprises the human IgGl constant region shown in SEQ ID NO: 19. In another embodiment, the anti-GDF-8 antibody comprises a human kappa sequence, e.g., the sequence shown as SEQ ID NO: 17. In another embodiment, the invention provides GDF-8 antibodies as novel antibody fragments that bind GDF-8 and retain the ability to neutralize or reduce GDF-8 activity. In a preferred, but not limiting, embodiment of the invention, the antibody fragment is selected from the group consisting of a dAb fragment, a diabody, an Fd fragment, an Fab fragment, an F(ab')2 fragment, an scFV fragment, and an Fv fragment. In a more preferred embodiment of the invention, the antibody fragment is encoded by a polynucleotide selected from SEQ ID NOs:2, 4, 6 or 8. In another preferred embodiment of the invention, the antibody fragment comprises an amino acid sequence selected from an amino acid sequence set forth in SEQ ID NOs:10-15 and 20-25. In another preferred embodiment, the invention provides novel antibody fragments that differ in sequence (e.g., due to the redundancy of the genetic code) from those sequences listed in Table 1, yet retain the ability to bind GDF-8 and neutralize or reduce GDF-8 activity. In another embodiment, the anti-GDF-8 antibody comprises at least one, two, three or four antigen-binding regions, e.g., variable regions, having an amino acid sequence as listed in Table 1 (SEQ ID NOs:3 or 7 for VH, and/or SEQ ID N0s:5 or 9 for VL), or a sequence substantially identical thereto (e.g., a sequence at least about 85%, 90%, 95%, 99% or more identical thereto, or which differs by no more than 1,2, 5,10 or 15 amino acid residues from SEQ ID NOs:3, 5, 7 or 9). In another embodiment, the antibody includes a VH and/or VL domain encoded by a nucleic acid having a nucleotide sequence as listed in Table 1 (SEQ ID NOs:2 or 6 for VH, and/or SEQ ID NOs:4 or 8 for VL), or a sequence substantially identical thereto (e.g., a sequence at least about 85%, 90%, 95%, 99% or more identical thereto, or which differs by no more than 3, 6, 15, 30 or 45 nucleotides from SEQ ID NOs:2,4,6, or 8). In yet another embodiment, the antibody comprises one, two, or three CDRs from a heavy chain variable region having amino acid sequences as listed in Table 1 (SEQ ID NOs:10-12 and 20-22), or a sequence substantially homologous thereto (e.g., a sequence at least about 85%, 90%, 95%, 99% or more identical thereto, and/or having one or more substitutions, e.g., conserved substitutions). In yet another embodiment, the antibody comprises at least one, two, or three CDRs from a light chain variable region having amino acid sequences as listed in Table 1 (SEQ ID NOs:13-15 and 23-25), or a sequence substantially homologous thereto (e.g., a sequence at least about 85%, 90%, 95%, 99% or more identical thereto, and/or having one or more substitutions, e.g., conserved substitutions). In yet another embodiment, the antibody comprises one, two, three, four, five or six CDRs from heavy and light chain variable regions having amino acid sequences as listed in Table 1 (SEQ ID NOs:10-12 and 20-22 for VH CDRs; and SEQ ID NOs:13-15 and 23-25 for VL CDRs), or a sequence substantially homologous thereto (e.g., a sequence at least about 85%, 90%, 95%, 99% or more identical thereto, and/or having one or more substitutions, e.g., conserved substitutions). In another embodiment, the anti-GDF-8 antibody comprises a human IgGl constant region having an amino acid sequence as set forth in SEQ ID NO: 19 or a sequence substantially homologous thereto (e.g., a sequence at least about 85%, 90%, 95%, 99% or more identical thereto, or which differs by no more than 1, 2, 5, 10, 50, or 100 amino acid residues from SEQ ID NO: 19). In another embodiment, the anti-GDF-8 antibody comprises a human kappa constant chain, e.g., a human kappa constant chain having an amino acid sequence as set forth in SEQ ID NO: 17 or a sequence substantially homologous thereto (e.g., a sequence at least about 85%, 90%, 95%, 99% or more identical thereto, or which differs by no more than 1,2, 5,10,20, or 50 amino acid residues from SEQ ID NO:17). In yet another embodiment, the antibody comprises a human IgGl constant region and a human kappa constant chain as described herein. In a preferred, but not limiting, embodiment, the invention provides antibodies encoded by polynucleotides set forth in SEQ ID NO:2,4, 6, or 8. In another preferred embodiment, the invention provides antibodies encoded by polynucleotide sequences that hybridize under stringent conditions to the polynucleotides set forth in SEQ ID NOs:2,4,6, or 8. In another preferred embodiment, the invention provides antibodies encoded by polynucleotides, which differ from those sequences set forth in SEQ ID N0s:2,4, 6, or 8, but due to the degeneracy of the genetic code, encode an amino acid sequence set forth in SEQ IDNOs:3,5,7,9,orlO-15. The GDF-8 antagonist, e.g., an anti-GDF-8 antibody, can be derivatized or linked to another functional molecule, e.g., another peptide or protein (e.g., an Fab fragment). For example, a fusion protein or an antibody, or antigen-binding portion, can be functionally linked (e.g., by chemical coupling, genetic fusion, noncovalent association or otherwise) to one or more other molecular entities, such as an antibody (e.g., a bispecific or a multispecific antibody), toxins, radioisotopes, cytotoxic or cytostatic agents, among others. A further aspect of the invention provides as GDF-8 antagonists purified and isolated nucleic acids that encode the GDF-8 antagonists, e.g., anti-GDF-8 antibodies, of the invention. In one embodiment, the invention provides polynucleotides comprised of a sequence encoding a VH (SEQ ID NO:3 or SEQ ID NO:7), VL (SEQ ID NO:5 or SEQ ID N0:9), and/or CDR (SEQ ID NOs:10-15 and 20-25) as listed in Table 1. In another embodiment, the invention provides polynucleotides that hybridize under stringent conditions to nucleic acids encoding a VH, VL, or CDR (SEQ ID NOs:3, 5, 7,9,10-15, or 20-25) as listed in Table 1. In another embodiment, the invention provides nucleic acids that comprise SEQ ID N0s:2,4, 6, or 8 or fragments of SEQ ID NOs:2,4,6, or 8. In yet a further embodiment, the invention provides polynucleotides that hybridize under stringent conditions to SEQ ID NOs:2,4, 6 or 8. Another aspect of the invention provides host cells and vectors comprising the polynucleotides of the invention as GDF-8 antagonists. The antibodies of the invention possess a number of useful properties. First, the antibodies are capable of binding mature GDF-8 with high affinity. Second, the disclosed antibodies inhibit GDF-8 activity in vitro and in vivo. Third, the disclosed antibodies inhibit GDF-8 activity associated with negative regulation of skeletal muscle mass and bone density. Fourth, the disclosed antibodies are an effective treatment for muscular disorders, particularly ALS. These antibodies have many additional uses, including diagnosing, prognosing, monitoring, screening, treating, ameliorating, and/or preventing GDF-8-associated disorders, e.g., muscle and/or bone-associated disorders. Other aspects of the invention provide compositions comprised of a GDF-8 antagonist of the invention, e.g., an anti-GDF-8 antibody of the invention, and the use of such compositions in inhibiting or neutralizing GDF-8 in animals, particularly in humans or other animals with muscular disorders such as ALS. The antibodies of the invention may also be used in a GDF-8-associated disorder, e.g., in a disorder in which it is desirable to increase muscle tissue or bone density. For example, anti-GDF-8 antibodies may be used in therapies and compositions to repair damaged muscle, e.g., myocardium, diaphragm, etc. Exemplary GDF-8-associated disorders and diseases treated by the disclosed methods and compositions include muscle and neuromuscular disorders such as muscular dystrophy (including Duchenne's muscular dystrophy); amyotrophic lateral sclerosis; muscle atrophy; organ atrophy; frailty; tunnel syndrome; congestive obstructive pulmonary disease (COPD); sarcopenia, cachexia, and other muscle wasting syndromes; adipose tissue disorders (e.g., obesity); type 2 diabetes; impaired glucose tolerance; metabolic syndromes (e.g., syndrome X); insulin resistance (including resistance induced by trauma, e.g., burns or nitrogen imbalance), and bone-degenerative diseases (e.g., osteoarthritis and osteoporosis). In a preferred, but not limiting, embodiment of the invention, a composition containing an anti-GDF-8 antibody is used in a method of treating, reducing, or ameliorating ALS. In another aspect, the invention provides compositions, e.g., pharmaceutical compositions, that include a pharmaceutically acceptable carrier and at least one GDF-8 antagonist, e.g., an anti-GDF-8 antibody described herein. In one embodiment, the compositions, e.g., pharmaceutical compositions, comprise a combination of two or more of the aforesaid GDF-8 antagonists, e.g., anti-GDF-8 antibodies or fragments thereof. Also within the scope of the invention are combinations of the GDF-8 antagonist, e.g., an anti-GDF-8 antibody, with a therapeutic agent, e.g., growth factor inhibitors, immunosuppressants, anti-inflammatory agents (e.g., systemic anti-inflammatory agents), metabolic inhibitors, enzyme inhibitors, and/or cytotoxic or cytostatic agents. In yet another embodiment, the GDF-8 antagonist, e.g., an anti-GDF-8 antibody, or a pharmaceutical composition thereof, is administered alone or in combination therapy, i.e., combined with other agents, e.g., therapeutic agents, which are useful for treating GDF-8-associated disorders. In addition to use in the treatment of various diseases or disorders, anti-GDF-8 antibodies may be used as diagnostic tools to quantitatively or qualitatively detect GDF-8 protein or protein fragments in a biological sample. The presence or amount of GDF-8 protein detected can be correlated with one or more of the medical conditions listed herein. Thus, in one embodiment, the invention provides methods to diagnose, prognose, monitor, and/or screen for GDF-8-associated disorders. In another aspect, the invention provides a method for detecting the presence of GDF-8 in a sample in vitro (e.g., a biological sample, such as serum, plasma, tissue, biopsy). The subject method can be used to diagnose a GDF-8-associated disorder, e.g., a bone, muscle, adipose or glucose metabolism-associated disorder. The method includes: (i) contacting the sample or a control sample with an anti-GDF-8 antibody as described herein; and (ii) detecting formation of a complex between the anti-GDF-8 antibody, and the sample or the control sample, wherein a substantially significant change in the formation of the complex in the sample relative to the control sample is indicative of the presence of GDF-8 in the sample. In yet another aspect, the invention provides a method for detecting the presence of GDF-8 in vivo in a subject (e.g., in vivo imaging in a subject). The subject method can be used to diagnose a GDF-8-associated disorder, e.g., ALS. The method includes: (i) administering an anti-GDF-8 antibody as described herein to a subject or a control subject under conditions that allow binding of the antibody to GDF-8; and (ii) detecting formation of a complex between the antibody and GDF-8, wherein a substantially significant difference in the formation of the complex in the subject relative to the control subject provides an indication related to the presence of GDF-8. Other embodiments of the invention provide a method of diagnosing or detecting whether a patient is suffering from a GDF-8-associated disorder (e.g., muscle disorder, neuromuscular disorder, bone degenerative disorder, metabolic or induced bone disorder, adipose disorder, glucose metabolism disorder, or insulin-related disorder) comprising the steps of: (a) obtaining a sample from a patient of interest; (b) contacting the sample with an anti-GDF-8 antibody as described herein; (c) determining the level of GDF-8 present in the patient sample; and (d) comparing the level of GDF-8 in the patient sample to the level of GDF-8 in a control sample, wherein a substantial increase, decrease, or similarity in the level of GDF-8 in the patient sample compared to the level of GDF-8 in the control sample indicates whether the patient is suffering from a GDF-8-associated disorder. Another further embodiment of a method for diagnosing or detecting whether a patient is suffering from a GDF-8-associated disorder described herein comprises the steps of: (a) obtaining a first sample taken from the patient of interest; (b) contacting the first sample with an anti-GDF-8 antibody as described herein; (c) determining the level of a GDF-8 in the first sample; (d) obtaining a second sample from an individual not afflicted with the GDF-8-associated disorder; (e) contacting the second sample with an anti-GDF-8 antibody as described herein; (f) determining the level of GDF-8 in the second sample; and (g) comparing the levels of GDF-8 in the first and second samples, wherein a substantial increase, decrease, or similarity in the level of first sample compared to the second sample indicates whether the patient is suffering from a GDF-8-associated disorder caused (in part or in full) by overexpression of GDF-8. For example, an increase in the level of GDF-8 in the first sample compared to the second sample may indicate that the patient is suffering from the GDF-8-associated disorder. In contrast, a decrease or similarity in the level of GDF-8 in the first sample compared to the second sample may indicate that the patient is not suffering from the GDF-8-associated disorder. Antibodies of the invention are also useful in methods of prognosing the likelihood that a patient will develop a GDF-8-associated disorder, e.g., a muscle disorder, neuromuscular disorder, bone degenerative disorder, metabolic or induced bone disorder, adipose disorder, glucose metabolism disorder, or insulin-related disorder. In a preferred, but nonlimiting, embodiment, the method comprises the steps of: (a) obtaining a first sample from a patient of interest; (b) contacting the first sample with an anti-GDF-8 antibody as described herein; (c) determining the level of GDF-8 in the first sample; (d) obtaining a second sample from an individual not afflicted with the GDF-8-associated disorder; (e) contacting the second sample with an anti-GDF-8 antibody as described herein; (f) determining the level of GDF-8 in the second sample; and (g) comparing the levels of GDF-8 in the first and second samples, wherein an increase, decrease, or similarity in the level of GDF-8 in the first sample as compared with the second sample indicates the likelihood that the patient will develop the GDF-8-associated disorder. For example, for a GDF-8-associated disorder caused (in part or in full) by overexpression of GDF-8, it is likely that the patient will develop the GDF-8-associated disorder if the first sample has an increased level of GDF-8 compared to second sample. In contrast, for a GDF-8-associated disorder caused (in part or in full) by overexpression of GDF-8, it is unlikely that the patient will develop the GDF-8-associated disorder if the first sample has a similar or decreased level of GDF-8 compared to second sample. Antibodies of the invention are also useful in methods of monitoring the severity of a GDF-8-associated disorder, e.g., muscle disorder, neuromuscular disorder, bone degenerative disorder, metabolic or induced bone disorder, adipose disorder, glucose metabolism disorder, or insulin-related disorder. In a preferred, but not limiting, embodiment, the method comprises the steps of: (a) obtaining a first sample taken from a patient of interest at a first time point; (b) contacting the first sample with an anti-GDF-8 antibody as described herein; (c) determining the level of GDF-8 in the first sample; (d) obtaining a second sample taken from the patient at a second time point; (e) contacting the second sample with an anti-GDF-8 antibody as described herein; (f) determining the level of GDF-8 in the second sample; and (g) comparing the levels of GDF-8 in the first and second samples, wherein an increase, decrease, or similarity in the level of GDF-8 in the second sample indicates whether the GDF-8-associated disorder has changed in severity. In one embodiment, a method of monitoring of the invention is used to monitor ALS, and a decrease in the level of GDF-8 in the second sample indicates that ALS has decreased in severity. An additional method of monitoring a disorder as described herein comprises the steps of: (a) obtaining a first sample from a patient of interest; (b) contacting the first sample with an anti-GDF-8 antibody as described herein; (c) determining the level of GDF-8 in the first sample; (d) obtaining a second sample from an individual not afflicted with a muscle disorder, neuromuscular disorder, bone degenerative disorder, metabolic or induced bone disorder, adipose disorder, glucose metabolism disorder, or insulin-related disorder; (e) contacting the second sample with an anti-GDF-8 antibody as described herein; (f) determining the level of GDF-8 in the second sample; and (g) comparing the levels of GDF-8 in the first and second samples, wherein an increase, decrease, or similarity in the level of GDF-8 in the first sample compared to the second sample indicates the severity of the GDF-8 disorder at that point. In one embodiment, a method of monitoring of the invention is used to monitor ALS, and a decrease or similarity in the level of GDF-8 in the first sample compared to the second sample indicates that ALS has low severity. Preferably, the antibody is directly or indirectly labeled with a detectable substance to facilitate detection of the bound or unbound antibody. Suitable detectable substances include various enzymes, prosthetic groups, fluorescent materials, luminescent materials and radioactive materials. [0044] Methods for delivering or targeting an antagonist of the invention, e.g., an antibody, to a GDF-8-expressing cell in vivo are also disclosed herein and are within the scope of the invention. Kits comprising the GDF-8 antagonists, e.g., the anti-GDF-8 antibodies, of the invention for therapeutic and diagnostic uses are also within the scope of the invention Additional objects of the invention will be set forth in the following description. Various objects, aspects, and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1. Characterization of the RK35 anti-GDF-8 antibody. A. Direct binding of RK35 antibody to GDF-8, as measured in an ELISA assay with biotinylated GDF-8. The binding affinity of RK35 antibody (circles) for GDF-8 was determined to be 4 nM. Control IgG shows no appreciable binding (squares). B. Effect of RK35 antibody on GDF-8 binding to its high affinity receptor. In a competition ELISA using the high affinity GDF-8 receptor, ActRIIB was used to measure the GDF-8 inhibitory activity of RK35. The binding of biotinylated GDF-8 to immobilized human chimeric protein ActRIIB fused to the human IgG constant region (Fc) was evaluated in the absence (diamonds) or presence of various concentrations of RK35 mAb, soluble ActRIIB or control IgG. Soluble ActRIIB-Fc receptor (squares) and irrelevant mouse IgG (triangles) were used as positive and negative controls, respectively. RK35 (circles) blocked the binding of biotinylated GDF-8 to immobilized ActRIIB with an IC5o ~ 2.5 nM. C. Inhibition of GDF-8-induced signal transduction. Rhabdomyosarcoma cells expressing a TGF-P promoter-luciferase fusion gene were treated with 10 ng/ml of GDF-8 in the absence (squares) or presence (circles) of varying concentrations of RK35 antibody. RK35 reduced the GDF-8 induction of luciferase activity in a dose-responsive manner, with an IC5o of 0.2 nM. Background (diamonds) signal was measured with no GDF-8 added. FIG. 2. Inhibition of myostatin leads to increased body weight and increased muscle mass in both SODG93A mice and rats. A. Body weights of RK35-treated (squares) (n=l 1) and PBS-treated (diamonds) (n=l 1) transgenic SODG93A mice and PBS-treated littermate control (wild type) mice (triangles) (n=9). B. Body weights of male (circles) and female (triangles) RK35-treated and male (squares) and female (diamonds) PBS-treated transgenic SODG93 A rats (n=10 per group). C. Muscle mass of RK35- and PBS-treated SODG93A and PBS-treated littermate control mice (n=9-12) during early-stage disease. Wet weights were determined for the gastrocnemius (gastroc), cranial tibialis (tibialis), quadriceps (quad) and diaphragm (diaphragm) muscles from 88-day old wild type mice treated with PBS (black bars), SODG93 A mice treated with PBS (white bars), and SODG93A mice treated with RK35 (grey bars). D. Muscle mass from SODG93A rats, treated with PBS (white bars) or RK35 (grey bars), at early-stage disease (~95 days) (n=7 per group). E. Muscle mass of wild type mice treated with PBS (black bars), SODG93A mice treated with PBS (white bars), and SODG93A mice treated with RK35 (grey bars) at end-stage disease (~134 days). F. Muscle mass from SODG93A rats, treated with PBS (white bars) or RK35 (grey bars) at end-stage disease (~128 days). Asterisks (*) denote statistically (p < 0.05) differences between indicated groups. FIG. 3. Myostatin inhibition enhances muscle strength in SODG93A mice and rats. A. Hindlimb grip strength in PBS-treated wild type mice (triangles), and SODG93A mice treated with either PBS (diamonds) or RK35 (squares) as a function of age. Hind limb grip strength is expressed as compression in kilograms (kg). B. Forelimb grip strength in PBS-treated wild type mice (triangles), and SODG93A mice treated with either PBS (diamonds) or RK35 (squares) as a function of age. C. Forelimb grip strength in wild type rats treated with PBS (WT+PBS), or SODG93 A rats treated with PBS (SOD+PBS) or RK35 (SOD+RK35). For rats, measurements were taken during a 4-week interval corresponding to early disease phase, between 95-110 days in age. Forelimb grip strength is expressed as tension in kilograms (kg). Asterisks (*) denote a statistically significant difference (p < 0.0001) between indicated groups. FIG. 4. Effects of myostatin inhibition on muscle structure and function in SODG93A rodents. Hematoxylin and eosin staining of medial gastrocnemius muscle from mice at 88 days indicates significant atrophy in (B) PBS-treated SODG93A mice, in comparison to either (A) wild type or (C) RK35-treated SODG93A mice. Hematoxylin and eosin staining of medial gastrocnemius muscle from both (E) PBS-treated and (F) RK35-treated SODG93 A mice at end-stage indicates both significant muscle atrophy and centrally placed nuclei (arrowheads) compared to (D) wild type mouse gastrocnemius. Hematoxylin and eosin staining of diaphragm from (G) PBS-treated wild type mice and (H) PBS- or (I) RK35-treated SODG93A mice, respectively, at end-stage. Examples of atrophic myofibers are marked ("a"). The asterisk in panel H denotes fiber splitting. Bars shown denote 50 µm in scale in panels A-F and 25 µm in panels G-I. Panel J: EMG interference pattern showing inspiratory bursts, recorded from the diaphragm muscles of wild type rats treated with PBS (WT+PBS) or SODG93A rats treated with PBS (SOD+PBS) or RK35 (SOD+RK35). Panel K: EMG burst spike rates (in Hz) from the diaphragm muscles of wild type rats (n=4), and from SODG93A rats treated either with vehicle (PBS, n=9) or RK35 (n=8). Asterisks (*) denote a statistically significant difference (p < 0.05) between the indicated groups. FIG. 5. RK35 treatment slows the decrease in muscle fiber diameter in gastrocnemius muscle through early-stage disease in SODG93 A mice, and in diaphragm through end-stage disease. Fiber diameters were measured by morphometry on gastrocnemius muscle from PBS-treated (A) and RK35-treated (B) SODG93A mice and PBS-treated wild type mice (C) at 88 days. Means were significantly different by ANOVA (p < 0.0001); pairwise comparisons by Tukey's multiple comparison post-test were also significant (p < 0.001). By end-stage, no significant differences in fiber distribution were observed in the gastrocnemius muscle of PBS-treated and RK35-treated SODG93A mice (data not shown). D. Analysis of fiber diameters of diaphragm muscle from end-stage PBS-treated and RK35-treated SODG93A mice in comparison to age-matched wild type control mice. Diaphragm muscle from RK35-treated SODG93A mice shows a fiber diameter distribution intermediate between PBS-treated SODG93 A mice and wild type control mice at end-stage. Means were significantly different by ANOVA (p < 0.0001); pairwise comparisons by Tukey's multiple comparison post-test were also significant (p < 0.01). Three muscles per group were analyzed; linear measurements of the maximum diameter of the minor axis of at least two hundred fibers were taken, using Zeiss Axiovision software. Fiber diameters were binned in 20 urn intervals, and frequency histograms were generated for each muscle group. FIG. 6. Effect of anti-myostatin treatment on motor neuron loss in the ventral horn of the spinal cord. Shown are stereological analyses of large motor neurons (area greater than 300 µm2) from the L3-5 regions of the ventral horn from SODG93A mice treated with either PBS (SOD + PBS) or RK35 (SOD + RK35) at early-stage (A) and end-stage (C) disease in comparison to age-matched wild type mice (WT + PBS). RK35 treatment showed a trend towards reversing the motor neuron loss (p=0.08) in early-stage disease (A). Individual counts of large healthy motor neurons with visible nucleoli were performed on NISSL-stained sections L3-5 from SODG93A mice treated with either PBS or RK35 at (B) early-stage and (D) end-stage disease in comparison to age-matched wild type mice. For each section, both ventral horns were counted (total of 20 ventral horns per animal) and data are represented as average number of large motor neurons per ventral horm. Asterisks (*) denote statistically significant differences (P < 0.001) between the indicated groups. Representative images from NISSL-stained ventral horn sections (20x magnification) are shown for (E and H) wild type mice treated with PBS, (F and I) SODG93A mice treated with PBS, and (G and J) SODG93A mice treated with RK35 analyzed at early (88 days) (E-G) and end-stage disease (134 days; H-J). Bar denotes scale of 200 urn. FIG. 7. Epitope mapping of GDF-8 for the RK35 antibody. The binding sites on GDF-8 for RK35 were identified using overlapping 13 amino acid peptides of human GDF-8. RK35 contact sites with GDF-8 are in bold. FIG. 8. Alignment of the light chain and heavy chain variable regions of RK35 (VL and VH, respectively) with the human germline frameworks DPK9 and DP-47, respectively. The amino acids of the murine RK35 (MRK35) variable chains that are changed in the humanized RK35 (HuRK35) regions are designated with an asterisk (*) and are in bold; complementarity determining regions of RK35 are boxed and underlined. DETAILED DESCRIPTION OF THE INVENTION I. Definitions "Antibody," as used herein, refers to an immunoglobulin or a part thereof, and encompasses any polypeptide comprising an antigen-binding site regardless of the source, species of origin, method of production, and characteristics. For the purposes of the present invention, it also includes, unless otherwise stated, antibody fragments such as Fab, F(ab')2, Fv, scFv, Fd, dAb, diabodies, and other antibody fragments that retain antigen-binding function. Antibodies can be made, for example, via traditional hybridoma techniques, recombinant DNA methods, or phage display techniques using antibody libraries. For various other antibody production techniques, see Antibodies: A Laboratory Manual, eds. Harlow et al., Cold Spring Harbor Laboratory, 1988. The term "antigen-binding domain" refers to the part of an antibody molecule that comprises the area specifically binding to or complementary to a part or all of an antigen. Where an antigen is large, an antibody may only bind to a particular part of the antigen. The "epitope" or "antigenic determinant" is a portion of an antigen molecule that is responsible for interactions with the antigen-binding domain of an antibody. An antigen-binding domain may be provided by one or more antibody variable domains (e.g., a so-called Fd antibody fragment consisting of a VH domain). An antigen-binding domain may comprise an antibody light chain variable region (VL) and an antibody heavy chain variable region (VH). The term "GDF-8" refers to a specific growth and differentiation factor-8, but not other factors that are structurally or functionally related to GDF-8, for example, BMP-11 and other factors belonging to the TGF-P superfamily. The term refers to the full-length unprocessed precursor form of GDF-8 as well as the mature and propeptide forms resulting from post-translational cleavage. The term also refers to any fragments and variants of GDF-8 that maintain at least some biological activities associated with mature GDF-8, as discussed herein, including sequences that have been modified. The amino acid sequence of mature human GDF-8 is provided in SEQ ID NO:1. The present invention relates to GDF-8 from all vertebrate species, including, but not limited to, human, bovine, chicken, mouse, rat, porcine, ovine, turkey, baboon, and fish (for sequence information, see, e.g., McPherron et al., supra). The term "GDF-8 activity" refers to one or more of physiologically growth-regulatory or morphogenetic activities associated with active GDF-8 protein. For example, active GDF-8 is a negative regulator of skeletal muscle mass. Active GDF-8 can also modulate the production of muscle-specific enzymes (e.g., creatine kinase), stimulate myoblast proliferation, and modulate preadipocyte differentiation to adipocytes. Exemplary procedures for measuring GDF-8 activity in vivo and in vitro are set forth in the Examples. The term "GDF-8 antagonist" or "GDF-8 inhibitor" includes any agent capable of inhibiting activity, expression, processing, or secretion of GDF-8. Such inhibitors include macromolecules and small molecules, e.g., proteins, antibodies, peptides, peptidomimetics, siRNA, ribozymes, antisense oligonucleotides, double-stranded RNA, and other small molecules, that inhibit GDF-8. A GDF-8 antagonist includes, in addition to the antibodies provided herein, any antibody that efficiently inhibits GDF-8, including antibodies with high specificity for binding to GDF-8 (e.g., antibodies with a low affinity for other members of the TGF-ß superfamily (e.g., BMP-11)). Variants, including humanized variants, of these antibodies are contemplated in the methods of diagnosing, prognosing, monitoring, treating, ameliorating, and preventing of the invention. Such inhibitors are said to "inhibit," "decrease," or "reduce" the biological activity of GDF-8. The terms "neutralize," "neutralizing," and their cognates refer to a dramatic reduction or abrogation of GDF-8 activity relative to the activity of GDF-8 in the absence of the same inhibitor. For example, a reduction of 75-100% of activity may be said to "neutralize" GDF-8 activity. The term "treatment" is used interchangeably herein with the term "therapeutic method" and refers to both therapeutic treatment and prophylactic/ preventative measures. Those in need of treatment may include individuals already having a particular medical disorder as well as those who may ultimately acquire the disorder (i.e., those needing preventive measures). The term "isolated" refers to a molecule that is substantially separated from its natural environment. For instance, an isolated protein is one that is substantially separated from the cell or tissue source from which it is derived. The term "purified" refers to a molecule that is substantially free of other material that associates with the molecule in its natural environment. For instance, a purified protein is substantially free of the cellular material or other proteins from the cell or tissue from which it is derived. The term refers to preparations where the isolated protein is sufficiently pure to be administered as a therapeutic composition, or at least 70% to 80% (w/w) pure, more preferably, at least 80%-90% (w/w) pure, even more preferably, 90-95% pure; and, most preferably, at least 95%, 96%, 97%, 98%, 99%, or 100% (w/w) pure. The term "effective dose," "therapeutically effective dose," "effective amount," or the like refers to that amount of the compound that results in either amelioration of symptoms in a patient or a desired biological outcome (e.g., increasing skeletal muscle mass and/or bone density). Such amount should be sufficient to reduce the activity of GDF-8 associated with negative regulation of skeletal muscle mass and bone density or with glucose homeostasis and adipose metabolism. The effective amount can be determined as described herein. A "disorder associated with GDF-8 activity," "disorder associated with GDF-8," "GDF-8-associated disorder" or the like refers to disorders that may be caused, in full or in part, by dysregulation of (e.g., abnormally increased or decreased) GDF-8 (and/or GDF-8 activity), and/or disorders that may be treated, ameliorated, prevented, diagnosed, prognosed, or monitored by regulating and/or monitoring GDF-8 (and/or GDF-8 activity). GDF-8-associated disorders include muscle disorders, neuromuscular disorders, bone degenerative disorders, metabolic or induced bone disorders, adipose disorders, glucose metabolism disorders or insulin-related disorders. A preferred GDF-8-associated disorder of the invention is amyotrophic lateral sclerosis (ALS). The term "small molecule" refers to compounds that are not macromolecules (see, e.g., Karp (2000) Bioinformatics Ontology 16:269-85; Verkman (2004) AJP-Cell Physiol. 286:465-74). Thus, small molecules are often considered those compounds that are less than one thousand daltons (e.g., Voet and Voet, Biochemistry, 2nd, ed. N. Rose, Wiley and Sons, New York, 14 (1995)). For example, Davis et al. ((2005) Proc. Natl. Acad. Sci. USA 102:5981-86) use the phrase small molecule to indicate folates, methotrexate, and neuropeptides, while Halpin and Harbury ((2004) PLos Biology 2:1022-30) use the phrase to indicate small molecule gene products, e.g., DNAs, RNAs and peptides. Examples of natural small molecules include, but are not limited to, cholesterols, neurotransmitters, and siRNAs; synthesized small molecules include, but are not limited to, various chemicals listed in numerous commercially available small molecule databases, e.g., FCD (Fine Chemicals Database), SMID (Small Molecule Interaction Database), ChEBI (Chemical Entities of Biological Interest), and CSD (Cambridge Structural Database) (see, e.g., Alfarano et al. (2005) Nuc. Acids Res. Database Issue 33:D416-24). II. Antibodies Against GDF-8 and Antibody Fragments A. Mouse and Humanized Antibody RK35 The present disclosure provides novel antibodies (e.g., intact antibodies and antibody fragments) that efficiently bind GDF-8. A nonlimiting illustrative embodiment of such an antibody is termed RK35. This exemplary embodiment is provided in the form of mouse and humanized antibodies, and antibody fragments thereof. The exemplary antibody of the invention, referred to herein as "RK35," possesses unique and beneficial characteristics. First, this antibody and antibody fragments are capable of binding mature GDF-8 with high affinity. Second, the antibody and antibody fragments of the invention inhibit GDF-8 activity in vitro and in vivo as demonstrated, for example, by inhibition of ActRIIB binding and reporter gene assays. Third, the disclosed antibody and antibody fragments are useful to treat symptoms associated with a GDF-8-associated disorder, e.g., muscular disorders, particularly ALS, as demonstrated, e.g., by increasing muscle mass in treated mutant SOD mice. In an exemplary embodiment, GDF-8 antagonists are antibodies that efficiently bind to GDF-8 and inhibit one or more GDF-8 associated activities. One of ordinary skill in the art will recognize that the antibodies of the invention may be used to detect, measure, and inhibit GDF proteins derived from various species, e.g., those described in the present specification. The percent identity is determined by standard alignment algorithms such as, for example, Basic Local Alignment Tool (BLAST) described in Altschul et al. (1990) J. Mol. Biol 215:403-10, the algorithm of Needleman et al. (1970) J. Mol. Biol, 48:444-53, or the algorithm of Meyers et al. (1988) Comput. Appl. Biosci. 4:11-17. In general, the antibody and antibody fragments of the invention can be used with any protein that retains substantial GDF-8 biological activity and comprises an amino acid sequence which is at least about 70%, 80%, 90%, 95%, or more identical to any sequence of at least 100, 80,60,40,20, or 15 contiguous amino acids of the mature form of GDF-8 set forth in SEQ ID NO:1. B. Antibody Variable Domains Intact antibodies, also known as immunoglobulins, are typically tetrameric glycosylated proteins composed of two light (L) chains of approximately 25 kDa each, and two heavy (H) chains of approximately 50 kDa each. Two types of light chain, termed lambda and kappa, exist in antibodies. Depending on the amino acid sequence of the constant domain of heavy chains, immunoglobulins are assigned to five major classes: A, D, E, G, and M, and several of these may be further divided into subclasses (isotypes), e.g., IgG1, IgG2, IgG3, IgG4, IgA1, and IgA2. Each light chain is composed of an N-terminal variable (V) domain (VL) and a constant (C) domain (CL). Each heavy chain is composed of an N-terminal V domain (VH), three or four C domains (CHs), and a hinge region. The CH domain most proximal to VH is designated CH1. The VH and VL domains consist of four regions of relatively conserved sequences named framework regions (FR1, FR2, FR3, and FR4), which form a scaffold for three regions of hypervariable sequences (complementarity determining regions, CDRs). The CDRs contain most of the residues responsible for interactions of the antibody with the antigen. CDRs are referred to as CDR1, CDR2, and CDR3. Accordingly, CDR constituents on the heavy chain are referred to as H1, H2, and H3, while CDR constituents on the light chain are referred to as L1, L2, and L3. CDR3 is the greatest source of molecular diversity within the antibody-binding site. H3, for example, can he as short as two amino acid residues or greater than 26 amino acids. The subunit structures and three-dimensional configurations of different classes of immunoglobulins are well known in the art. For a review of the antibody structure, see Antibodies: A Laboratory Manual, Cold Spring Harbor Laboratory, eds. Harlow et al., 1988. One of skill in the art will recognize that each subunit structure, e.g., a CH, VH, CL, VL, CDR, and/or FR structure, comprises active fragments. For example, active fragments may consist of the portion of the VH, VL, or CDR subunit that binds the antigen, i.e., the antigen-binding fragment, or the portion of the CH subunit that binds to and/or activates an Fc receptor and/or complement. Nonlimiting examples of binding fragments encompassed within the term "antibody fragment" used herein include: (i) an Fab fragment, a monovalent fragment consisting of the VL, VH, CL and CH1 domains; (ii) an F(ab')2 fragment, a bivalent fragment comprising two Fab fragments linked by a disulfide bridge at the hinge region; (iii) an Fd fragment consisting of the VH and CH1 domains; (iv) an Fv fragment consisting of the VL and VH domains of a single arm of an antibody, (v) a dAb fragment, which consists of a VH domain; and (vi) an isolated CDR. Furthermore, although the two domains of the Fv fragment, VL and VH, are coded for by separate genes, they may be recombinantly joined by a synthetic linker, creating a single protein chain in which the VL and VH regions pair to form monovalent molecules (known as single chain Fv (scFv). The most commonly used linker is a 15-residue (Gly4Ser)3 peptide, but other linkers are also known in the art. Single chain antibodies are also intended to be encompassed within the term "antibody" or "antigen-binding fragment" of an antibody. These antibodies are obtained using conventional techniques known to those skilled in the art, and the fragments are screened for utility in the same manner as intact antibodies. Antibody diversity is created by multiple germline genes encoding variable regions and a variety of somatic events. The somatic events include recombination of variable gene segments with diversity (D) and joining (J) gene segments to make a complete VH region, and the recombination of variable and joining gene segments to make a complete VL region. The recombination process itself is imprecise, resulting in the loss or addition of amino acids at the V(D)J junctions. These mechanisms of diversity occur in the developing B cell prior to antigen exposure. After antigenic stimulation, the expressed antibody genes in B cells undergo somatic mutation. Based on the estimated number of germline gene segments, the random recombination of these segments, and random VH-VL pairing, up to 1.6xl07 different antibodies may be produced (Fundamental Immunology, 3rd ed. (1993), ed. Paul, Raven Press, New York, NY). When other processes that contribute to antibody diversity (such as somatic mutation) are taken into account, it is thought that upwards of IxlO10 different antibodies may be generated (Immunoglobulin Genes, 2nd ed. (1995), eds. Jonio et al., Academic Press, San Diego, CA). Because of the many processes involved in generating antibody diversity, it is unlikely that independently derived monoclonal antibodies with the same antigen specificity will have identical amino acid sequences. Thus, the present invention provides novel antibodies that bind GDF-8. The antibody fragments of the invention, e.g., structures containing a CDR, will generally be an antibody heavy or light chain sequence, or an active fragment thereof, in which the CDR is placed at a location corresponding to the CDR of naturally occurring VH and VL. The structures and locations of immunoglobulin variable domains, e.g., CDRs, may be defined using well-known numbering schemes, e.g., the Kabat numbering scheme, the Chothia numbering scheme, a combination of Kabat and Chothia (AbM), etc. (see, e.g., Sequences of Proteins of Immunological Interest, U.S. Department of Health and Human Services (1991), eds. Kabat et al.; Al-Lazikani et al. (1997) J. Mol. Biol. 273:927-48). Thus, the present invention further provides novel CDRs. The structure for carrying a CDR of the invention will generally be a polypeptide, e.g., an antibody heavy or light chain sequence or a substantial portion thereof, in which the CDR is located at a position corresponding to the CDR of naturally occurring VH and VL regions. The structures and locations of immunoglobulin variable domains may be determined as described in, e.g., Kabat et al., supra and Al-Lazikani et al., supra. Antibody molecules (including antibody fragments) of the present invention, i.e., antibody molecules that antagonize GDF-8, include, but are not limited to, murine monoclonal antibody RK35 and its variants, specifically the humanized variant. GDF-8 antagonists of the invention include, in addition to RK35, other antibodies that bind efficiently to GDF-8, including antibodies with high specificity for binding to GDF-8 (e.g., antibodies with a lower affinity for other members of the TGF-P superfamily (e.g., BMP-11)). Variants, including humanized variants, of these antibodies are contemplated in the methods of diagnosing, prognosing, monitoring, treating, ameliorating, and preventing of the invention. These antibody molecules may be useful in preventing or treating a GDF-8-associated disorder, e.g., bone, muscle, adipose and glucose metabolism-related pathologies. The ammo acid sequences of the light chain variable regions of murine and humanized RK35 are set forth in SEQ ID NOs:5 and 9, respectively. The amino acid sequences of the heavy chain variable regions of murine and humanized RK35 are set forth in SEQ ID NOs:3 and 7, respectively. The amino acid sequences of the three complementarity determining regions (CDRs) in the variable light chains of murine and humanized RK35 are set forth in SEQ ID NOs:13,14,15,23,24, and 25. The amino acid sequences of the three CDRs in the variable heavy chains of murine and humanized RK35 are set forth in SEQ ID NOs:10,11,12, 20,21, and 22. As described above, the CDRs contain most of the residues responsible for interactions with an antigen, and are contained within the VH and VL domains, i.e., the heavy chain variable region and the light chain variable region, respectively. Consequently, provided that an antibody comprises at least one CDR comprising an amino acid sequence selected from the amino acid sequences set forth in SEQ ID NOs:10-15 and 20-25, or an active antibody fragment thereof, it is an antibody of the invention, i.e., one that binds to GDF-8 and interferes with GDF-8 signaling. Therefore, an embodiment of the invention includes polypeptides, e.g., antibodies, that contain one or more CDRs that comprise an amino acid sequence selected from the amino acid sequences set forth in SEQ ID NOs:10-15 and 20-25, or an active fragment thereof. Consequently, one of skill in the art will recognize that the antibodies of the invention include an antibody in which the CDRs of the VL chain are those set forth in SEQ ID NOs:13-15 and 23-25, or the CDRs of the VH chain are those set forth in SEQ ID N0s:10-12 and 20-22. An antigen-binding fragment may be an Fv fragment, which consists of VH and VL domains. Thus, an Fv fragment of RK35 may constitute an antibody of the invention, provided that it binds to GDF-8 and interferes with GDF-8 signaling. One of skill in the art will recognize that any antibody fragment containing the Fv fragment of, e.g., RK35, may also be an antibody of the invention. Additionally, any Fv fragment, scFv fragment, Fab fragment, or F(ab')2 fragment, that contains one or more CDRs having an amino acid sequence selected from the amino acid sequences set forth in SEQ ID NOs:10-15 and 20-25, may also be an antibody of the invention. Such antibody molecules may be produced by methods well known to those skilled in the art. For example, monoclonal antibodies may be produced by generation of hybridomas in accordance with known methods. Hybridomas formed in this manner are then screened using standard methods, such as enzyme-linked immunosorbent assay (ELISA) and Biacore analysis, to identify one or more hybridomas that produce an antibody that binds GDF-8, interferes with GDF-8 signaling, and neutralizes or inhibits one or more GDF-8-associated activities. Recombinant GDF-8, naturally occurring GDF-8, any variants thereof, and antigenic peptide fragments of GDF-8 may be used as the immunogen. An antigenic peptide fragment of GDF-8 comprises at least seven continuous amino acid residues and encompasses an epitope such that an antibody raised against the peptide forms an immune complex with GDF-8. Preferably, the antigenic peptide comprises at least 10 amino acid residues, more preferably at least 15 amino acid residues, even more preferably at least 20 amino acid residues, and most preferably at least 30 amino acid residues. Additionally, it is preferable that the antigenic peptide fragment of GDF-8 comprises the GDF-8 receptor-binding site. Polyclonal sera and antibodies of the invention may be produced by immunizing a suitable subject with GDF-8, its variants, and/or portions thereof. The antibody titer in the immunized subject may be monitored over time by standard techniques, such as an ELISA, or by using immobilized GDF-8 or other arker proteins (e.g., FLAG). If desired, the antibody molecules of the present invention may be isolated from the subject or culture media and further purified by well-known techniques, such as protein A chromatography, to obtain an IgG fraction. Certain embodiments of the invention comprise the VH and/or VL domain of the Fv fragment of RK35. Fragments of antibodies of the present invention, e.g., Fab, F(ab')2, Fd, and dAb fragments, may be produced by cleavage of the antibodies in accordance with methods well known in the art. For example, immunologically active Fab and F(ab')2 fragments may be generated by treating the antibodies with an enzyme such as papain and pepsin. Further embodiments comprise one or more CDRs of any of these VH and VL domains, as set forth in SEQ ID NOs:10-15 and 20-25. One embodiment comprises an H3 fragment of the VH domain of RK35 as set forth in SEQ ID N0:12. DNA and arnino acid (AA) sequences of VH and VL domains, and CDRs of the presently disclosed antibodies are enumerated as listed in Table 1. For convenience, the approximate positions of each CDR within the VH and VL domains are listed in Table 2. (Table Removed) Anti-GDF-8 antibodies may further comprise antibody constant regions or parts thereof. For example, a VL domain of the invention may be attached at its C-terminal end to an antibody light chain constant domain, e.g., a human CK or Cλ chain, preferably a Cλ chain. Similarly, an antigen-binding fragment based on a VH domain may be attached at its C-terminal end to all or part of an immunoglobulin heavy chain derived from any antibody isotype, e.g., IgG, IgA, IgE, and IgM, and any of the isotype subclasses, particularly IgG1 and IgG4. In exemplary embodiments, antibodies comprise C-terminal fragments of heavy and light chains of human IgG1λ. Preferred DNA and amino acid sequences for the C-terminal constant fragment of the light X chain are set forth in SEQ ID NO: 16 and SEQ ID NO: 17, respectively. Preferred DNA and amino acid sequences for the C-terminal constant fragment of IgG1 heavy chain are set forth in SEQ ID NO: 18 and SEQ ID NO: 19, respectively. It is understood that, due to the degeneracy of the genetic code, DNA sequences listed in Table 1 are merely representative of nucleic acids that encode the amino acid sequences, peptides, and antibodies of interest, and are not to be construed as limiting. . Certain embodiments of the invention comprise the VH and/or VL domain of the Fv fragment of RK35. Further embodiments comprise one or more complementarity determining regions (CDRs) of any of these VH and VL domains. One embodiment comprises an H3 fragment of the VH domain of RK35. The VH and VL domains of the invention, in certain embodiments, are germlined, i.e., the framework regions (FRs) of these domains are changed using conventional molecular biology techniques to match the consensus amino acid sequences of human germline gene products. This is also known as a humanized or germlined antibody. In other embodiments, the framework sequences remain diverged from the germline. Humanized antibodies may be produced using transgenic mice that are incapable of expressing endogenous immunoglobulin heavy and light chain genes, but are capable of expressing human heavy and light chain genes. C. Modified Antibodies and Their Fragments A further aspect of the invention provides methods for obtaining an antibody antigen-binding domain directed against GDF-8. The skilled artisan will appreciate that the antibodies of the invention are not limited to the specific sequences of VH and VL as listed in Table 1 but also include variants of these sequences that retain antigen-binding ability. Such variants may be derived from the provided sequences using techniques known in the art. Amino acid substitutions, deletions, or additions, can be made in either the FRs or in the CDRs. While changes in the framework regions are usually designed to improve stability and reduce immunogenicity of the antibody, changes in the CDRs are usually designed to increase affinity of the antibody for its target. Such affinity-increasing changes are typically determined empirically by altering the CDR and testing the antibody. Such alterations can be made according to the methods described in, e.g., Antibody Engineering, 2nd. ed., Borrebaeck, ed., Oxford University Press, 1995. k Thus, the antibodies of the invention also include those that bind to GDF-8, interfere with GDF-8 signaling, and have mutations in the constant regions of the heavy and light chains. It is sometimes desirable to mutate and inactivate certain fragments of the constant region. For example, mutations in the heavy constant region are sometimes desirable to produce antibodies with reduced binding to the Fc receptor (FcR) and/or complement; such mutations are well known in the art. One of skill in the art will also recognize that the determination of which active fragments of the CL and CH subunits are necessary will depend on the application to which an antibody of the invention is applied. For example, the active fragments of the CL and CH subunits that are involved with their covalent link to each other will be important in the generation of an intact antibody. The method for making a VH domain that is an amino acid sequence variant of a VH domain set out herein comprises a step of adding, deleting, substituting or inserting one or more amino acids in the amino acid sequence of the presently disclosed VH domain, optionally combining the VH domain thus provided with one or more VL domains, and testing the VH domain or VH/VL combination or combinations for binding to GDF-8, and (preferably) testing the ability of such antigen-binding domain to modulate one or more GDF-8-associated activities. The VL domain may have an amino acid sequence that is substantially as set out herein. An analogous method may be employed in which one or more sequence variants of a VL domain disclosed herein are combined with one or more VH domains. A further aspect of the invention provides a method of preparing an antigen-binding fragment that interacts with GDF-8. The method comprises: (a) providing a starting repertoire of nucleic acids encoding a VH domain that either includes a CDR, e.g., CDR3, to be replaced or a VH domain that lacks a CDR, e.g., CDR3, encoding region; (b) combining the repertoire with a donor nucleic acid encoding a donor CDR comprising an active fragment of SEQ ID NO:2 or 6, e.g., a donor nucleic acid encoding the amino acid sequence set forth in SEQ ID NO:3 or 7, such that the donor nucleic acid is inserted into the CDR, e.g., CDR3, region in the repertoire so as to provide a product repertoire of nucleic acids encoding a VH domain; (c) expressing the nucleic acids of the product repertoire; (d) selecting an antigen-binding fragment that interacts with GDF-8; and (e) recovering the selected antigen-binding fragment or nucleic acid encoding it. Again, an analogous method may be employed in which a VL CDR (e.g., L3) of the invention is combined with a repertoire of nucleic acids encoding a VL domain, which either includes a CDR to be replaced or lacks a CDR encoding region. A coding sequence of a CDR of the invention (e.g., CDR3) may be introduced into a repertoire of variable domains lacking a CDR (e.g., CDR3), using recombinant DNA technology. For example, Marks et al. (1992) Bio/Technology 10:779-83, describes methods of producing repertoires of antibody variable domains in which consensus primers directed at or adjacent to the 5' end of the variable domain area are used in conjunction with co nsensus primers to the third framework region of human VH genes to provide a repertoire of VH variable domains lacking a CDR3. The repertoire may be combined with a CDR3 of a particular antibody. Using analogous techniques, the CDR3-derived sequences of the present invention may be shuffled with repertoires of VH or VL domains lacking a CDR3, and the shuffled complete VH or VL domains combined with a cognate VL or VH domain to provide antigen-binding fragments of the invention. The repertoire may then be displayed in a suitable host system, such as the phage display system of, e.g., WO 92/01047, so that suitable antigen-binding fragments can be selected. Analogous shuffling or combinatorial techniques are also disclosed by Stemmer (1994) Nature 370:389-91, which describes a technique in relation to a ß-lactamase gene but observes that the approach may be used for the generation of antibodies. [0093] A further alternative is to generate novel VH or VL regions carrying a CDR-derived sequence of the invention using random mutagenesis of one or more selected VH and/or VL genes to generate mutations within the entire variable domain. Such a technique is described in Gram et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89:3576-80 by using error-prone PCR. Another method that may be used to generate novel antibodies or fragments thereof is to direct mutagenesis to CDRs of VH or VL genes. Such techniques are disclosed in Barbas et al. (1994) Proc. Natl. Acad Sci. U.S.A. 91:3809-13 and Schier et al. (1996) J. Mol. Biol. 263:551-67. [0095] Similarly, one, two, or all three CDRs, may be grafted into a repertoire of VH or VL domains which are then screened for a binding partner or binding fragments for GDF-8. A substantial portion of an immunoglobulin variable domain will comprise at least the CDRs and, optionally, their intervening framework regions from the antibody fragments as set out herein. The portion will also include at least about 50% of either or both of FR1 and FR4, the 50% being the C-terminal 50% of FR1 and the N-terminal 50% of FR4. Additional residues at the N-terminal cr C-terminal end of the substantial part of the variable domain may be those not normally associated with naturally occurring variable domain regions. For example, construction of antibody fragments of the present invention made by recombinant DNA techniques may result in the introduction of N- or C-terminal residues encoded by linkers introduced to facilitate cloning or other manipulation steps. Other manipulation steps include the introduction of linkers to join variable domains of the invention to further protein sequences including immunoglobulin heavy chains, other variable domains (for example, in the production of diabodies) or protein labels as discussed in more details below. Although the embodiments illustrated in the Examples comprise a "matching" pair of VH and VL domains, the invention also encompasses binding fragments containing a single variable domain, e.g., a dAb fragment, derived from either VH or VL domain sequences, especially VH domains. In the case of either of the single chain binding domains, these domains may be used to screen for complementary domains capable of forming a two-domain antigen-binding domain capable of binding GDF-8. This may be achieved by phage display screening methods using the so-called hierarchical dual combinatorial approach as disclosed in, e.g., WO 92/01047. In this technique, an individual colony containing either an H or L chain clone is used to infect a complete library of clones encoding the other chain (L or H) and the resulting two-chain antigen-binding domain is selected in accordance with phage display techniques, such as those described in that reference. This technique is also disclosed in Marks et ah, supra. [0098] Antibodies can be conjugated by chemical methods with radionuclides, drugs, macromolecules, or other agents, and may be made as fusion proteins comprising one or more CDRs of the invention. An antibody fusion protein contains a VH-VL pair in which one of these chains (usually VH) and another protein are synthesized as a single polypeptide chain. These types of products differ from antibodies in that they generally have an additional functional element - the active moiety of a small molecule or the principal molecular structural feature of the conjugated or fused macromolecule. In addition to the changes to the amino acid sequence outlined above, the antibodies can be glycosylated, pegylated, or linked to albumin or a nonproteinaceous polymer. For instance, anti-GDF-8 antibodies may be linked to one of a variety of nonproteinaceous polymers, e.g., polyethylene glycol, polypropylene glycol, or polyoxyalkylenes. The antibodies may be chemically modified, e.g., to increase their circulating half-life by covalent conjugation to a polymer. Exemplary polymers, and methods to attach them to peptides are known in the art. In other embodiments, the antibody may be modified to have an altered glycosylation pattern (i.e., relative to the original or native glycosylation pattern). As used herein, "altered" means having one or more carbohydrate moieties deleted, and/or having one or more glycosylation sites added to the original antibody. Addition of glycosylation sites to an anti-GDF-8 antibody is accomplished by well-known methods of altering the amino acid sequence to contain glycosylation site consensus sequences. Another means of increasing the number of carbohydrate moieties on the antibodies is by chemical or enzymatic coupling of glycosides to the amino acid residues of the antibody. Removal of any carbohydrate moieties present on the antibodies may be accomplished chemically or enzymatically as known in the art. Antibodies of the invention may also be tagged with a detectable or functional label such as 131Ior 99 Tc, which may be attached to antibodies of the invention using conventional chemistry known in the art. Labels also include enzyme labels such as horseradish peroxidase or alkaline phosphatase. Labels further include chemical moieties such as biotin, which may be detected via binding to a specific cognate detectable moiety, e.g., labeled avidin. Antibodies, in which CDR sequences differ only insubstantially from those listed in Table 1, are encompassed within the scope of the invention. Insubstantial differences include minor amino acid changes, e.g., substitutions of one or two out of any five amino acids in the sequence of a CDR. Typically, an amino acid is substituted by a related amino acid having similar charge, hydrophobicity, or stereochemical characteristics. Such substitutions would be within the ordinary skills of an artisan. The structure framework regions (FRs) can be modified more substantially than CDRs without adversely affecting the binding properties of an antibody. Changes to FRs include, but are not limited to, humanizing a nonhuman derived framework or engineering certain framework residues that are important for antigen contact or for stabilizing the binding site, e.g., changing the class or subclass of the constant region, changing specific amino acid residues which might alter an effector function such as Fc receptor binding (e.g., Lund etal. (1991) J. Immunol. 147:2657-62; Morgan et al. (1995) Immunology 86:319-24), or changing the species from which the constant region is derived. Antibodies may have mutations in the CH2 region of the heavy chain that reduce or alter effector function, e.g., Fc receptor binding and complement activation. For example, antibodies may have mutations such as those described in U.S. Patent Nos. 5,624,821 and 5,648,260. In the IgG1 or IgG2 heavy chain, for example, such mutations may be made at amino acid residues 117 and 120 of SEQ ID NO: 19, which represents the Fc portion of IgG1 (these residues correspond to amino acids 234 and 237 in the full-length sequence of IgG1 or IgG2). Antibodies may also have mutations that stabilize the disulfide bond between the two heavy chains of an immunoglobulin, such as mutations in the hinge region of IgG4, as disclosed in, e.g., Angal et al. (1993) Mol. Immunol. 30:105-08. The polypeptides and antibodies of the present invention also encompass proteins that are structurally different from the disclosed polypeptides and antibodies, e.g., which have an altered sequence but substantially the same biochemical properties as the disclosed polypeptides and antibodies, e.g., have changes only in functionally nonessential amino acids. Such molecules include naturally occurring allelic variants and deliberately engineered variants containing alterations, substitutions, replacements, insertions, or deletions. Techniques for such alterations, substitutions, replacements, insertions, or deletions are well known to those skilled in the art. Antibodies of the invention may additionally be produced using transgenic nonhuman animals that are modified so as to produce fully human antibodies rather than the animal's endogenous antibodies in response to challenge by an antigen. See, e.g., PCT publication WO 94/02602. The endogenous genes encoding the heavy and light immunoglobulin chains in the nonhuman host have been incapacitated, and active loci encoding human heavy and light chain immunoglobulins are inserted into the host's genome. The human genes are incorporated, for example, using yeast artificial chromosomes containing the requisite human DNA segments. An animal which provides all the desired modifications is then obtained as progeny by crossbreeding intermediate transgenic animals containing fewer than the full complement of the modifications. One embodiment of such a nonhuman animal is a mouse, and is termed the XENOMOUSE™ as disclosed in PCT publications WO 96/33735 and WO 96/34096. This animal produces B cells that secrete fully human immunoglobulins. The antibodies can be obtained directly from the animal after immunization with an immunogen of interest, as, for example, a preparation of a polyclonal antibody, or alternatively from immortalized B cells derived from the animal, such as hybridomas producing monoclonal antibodies. Additionally, the genes encoding the immunoglobulins with human variable regions can be recovered and expressed to obtain the antibodies directly, or can be further modified to obtain analogs of antibodies such as, for example, single chain Fv molecules. Consequently, the term antibody as used herein includes intact antibodies, fragments of antibodies, e.g., Fab, F(ab')a Fd, dAb and scFv fragments, and intact antibodies and fragments that have been mutated either in their constant and/or variable regions (e.g., mutations to produce chimeric, partially humanized, or fully humanized antibodies, as well as to produce antibodies with a desired trait, e.g., enhanced GDF-8 binding and/or reduced FcR binding). As such these antibodies are included in the scope of the invention, provided that the antibody binds specifically to GDF-8, interferes with GDF-8 signaling, and/or neutralizes or inhibits one or more GDF-8-associated activities. Other protein-binding molecules may also be employed to modulate the activity of GDF-8. Such protein-binding molecules include small modular immunopharmaceutical (SMIP™) drags (Trabion Pharmaceuticals, Seattle, WA). SMIPs are single-chain polypeptides composed of a binding domain for a cognate structure such as an antigen, a counterreceptor or the like, a hinge-region polypeptide having either one or no cysteine residues, and immunoglobulin CH2 and CHS domains (see also www.trubion.com). SMIPs and their uses and applications are disclosed in, e.g., U.S. Published Patent Appln. Nos. 2003/0118592,2003/0133939,2004/0058445, 2005/0136049,2005/0175614, 2005/0180970,2005/0186216,2005/0202012, 2005/0202023,2005/0202028, 2005/0202534, and 2005/0238646, and related patent family members thereof, all of which are hereby incorporated by reference herein in their entireties. The binding capacity of an antibody of the invention may be measured by the following methods: Biacore analysis, enzyme linked immunosorbent assay (ELISA), X-ray crystallography, sequence analysis and scanning mutagenesis as described in the Examples below, and other methods that are well known in the art. The ability of an antibody of the invention to neutralize and/or inhibit one or more GDF-8-associated activities may be measured by the following nonlimiting list of methods: assays for measuring the proliferation of a GDF-8-dependent cell line; assays for measuring the expression of GDF-8-mediated polypeptides; assays measuring the activity of downstream signaling molecules; assays testing the efficiency of an antibody of the invention to prevent muscle disorders in a relevant animal model; assays as described in the Examples below; and other assays that are well known in the art. A further aspect of the invention provides a method of selecting antibodies capable of binding GDF-8 and neutralizing and/or inhibiting one or more GDF-8-associated activities. The method comprises: a) contracting a plurality of antibodies with GDF-8; b) choosing antibodies that bind to GDF-8; c) testing the ability of chosen antibodies to prevent GDF-8 from binding to the GDF-8 receptor; and d) selecting antibodies capable of preventing GDF-8 from binding to its receptor. The anti-GDF-8 antibodies of the invention are also useful for isolating, purifying, and/or detecting GDF-8 in supernatants, cellular lysates, or on a cell surface. Antibodies disclosed in this invention can be used diagnostically to monitor GDF-8 protein levels as part of a clinical testing procedure. Additionally, antibodies of the invention can be used in treatments requiring the neutralization and/or inhibition of one or more GDF-8-associated activities, e.g., treatments for ALS and other muscle-related pathologies. The present invention also provides novel isolated and purified polynucleotides and polypeptides related to novel antibodies directed against human GDF-8. The genes, polynucleotides, proteins, and polypeptides of the present invention include, but are not limited to, murine and humanized antibodies to GDF-8 (e.g., RK35) and variants thereof. D. Nucleic Acids, Cloning, and Expression Systems The present invention further provides isolated and purified nucleic acids encoding antibodies of the present invention. Nucleic acids according to the present invention may comprise DNA or RNA and may be wholly or partially synthetic. Reference to nucleotide sequences as set out herein encompass DNA molecules with the specified sequences or genomic equivalents, as well as RNA molecules with the specified sequences in which U is substituted for T, unless context requires otherwise. For example, the invention provides purified and isolated polynucleotides encoding the variable region of a murine antibody to GDF-8 that modulates one or more GDF-8-associated activities (e.g., neutralizes GDF-8 bioactivity) (RK35), and a humanized version of RK35. Preferred DNA sequences of the invention include genomic, cDNA, and chemically synthesized DNA sequences. The nucleotide sequences of the invention include those that encode the light chain variable regions of mouse RK35 set forth in SEQ ID NO:4, including those that encode a leader sequence preceding the light chain variable region sequence, e.g., the nucleotide sequence set forth as SEQ ID NO:30 (nucleotides 1-60 correspond to the leader sequence, and nucleotides 61-381 correspond to SEQ ID NO:4). The nucleotide sequences of the invention also include those that encode the heavy chain variable region of RK35 set forth in SEQ ID N0:2, including those that encode a leader sequence preceding the heavy chain variable region, e.g., the nucleotide sequence set forth as SEQ ID N0:28 (nucleotides 1-57 correspond to the leader sequence, and nucleotides 58-405 correspond to SEQ ID NO:2). The nucleotide sequences of the invention also include humanized sequences of the heavy and light chain variable regions, such as those set forth in SEQ ID NOs:6 and 8, respectively. Polynucleotides of the present invention also include polynucleotides that hybridize under stringent conditions to the nucleic acid sequences set forth in SEQ ID NOs:2,4, 6, and 8, and complements thereof, and/or encode polypeptides that retain substantial biological activity (i.e., active fragments) in the variable regions. Polynucleotides of the present invention also include continuous portions of the sequences set forth in SEQ ID NOs:2,4, 6, and 8, comprising at least 15 consecutive nucleotides. ' The arnino acid sequence of the variable light chains of mouse RK35 is set forth in SEQ ID NO:5. An example of an amino acid sequence of the variable light chain domain of mouse RK35 preceded by a leader sequence is set forth as SEQ ID NO:31. The amino acid sequence of the variable heavy chains of RK35 is set forth in SEQ ID N0:3. An example of an amino acid sequence of the variable heavy chain domain of mouse RK35 preceded by a leader sequence is set forth as SEQ ID NO:29. The amino acid sequences of humanized variable heavy and light chains are set out in SEQ ID NOs:7 and 9, respectively. The amino acid sequences of the CDRs contained within the heavy chains of mouse RK35 are set forth in SEQ ID NOs:10-12 and 20-22. The amino acid sequences of the CDRs contained within the light chains of mouse RK35 are set forth in SEQ ID NOs:13-15 and 23-25. Polypeptides of the present invention also include continuous portions of any of the sequences substantially set forth in SEQ ID NOs:3, 5, 7, 9,10-15, and 20-25 comprising at least 5 consecutive amino acids. A preferred polypeptide of the present invention includes any continuous portion of any sequence substantially set forth in SEQ ID NOs:3, 5, 7, 9, and 10-15 retaining substantial biological activity of an antibody of the invention. In addition to those polynucleotides described above, the present invention also includes polynucleotides that encode the arnino acid sequences substantially set forth in SEQ ID NOs:3, 5,7, 9, and 10-15 or a continuous portion thereof, and that differ from the polynucleotides described above only due to the well-known degeneracy of the genetic code. The isolated polynucleotides of the present invention may be used as hybridization probes and primers to identify and isolate nucleic acids having sequences identical to or similar to those encoding the disclosed polynucleotides. Polynucleotides isolated in this fashion may be used, for example, to produce antibodies against GDF-8 or other TGF-(ß family members or to identify cells expressing such antibodies. Hybridization methods for identifying and isolating nucleic acids include polymerase chain reaction (PCR), Southern hybridizations, in situ hybridization and Northern hybridization, and are well known to those skilled in the art. Hybridization reactions can be performed under conditions of different stringencies. The stringency of a hybridization reaction includes the difficulty with which any two nucleic acid molecules will hybridize to one another. Preferably, each hybridizing polynucleotide hybridizes to its corresponding polynucleotide under reduced stringency conditions, more preferably stringent conditions, and most preferably highly stringent conditions. Examples of stringency conditions are shown in Table 3 below: highly stringent conditions are those that are at least as stringent as, for example, conditions A-F; stringent conditions are at least as stringent as, for example, conditions G-L; and reduced stringency conditions are at least as stringent as, for example, conditions M-R. Table 3 (Table Removed) 1 The hybrid length is that anticipated for the hybridized region(s) of the hybridizing polynucleotides. When hybridizing a polynucleotide to a target polynucleotide of unknown sequence, the hybrid length is assumed to be that of the hybridizing polynucleotide. When polynucleotides of known sequence are hybridized, the hybrid length can be determined by aligning the sequences of the polynucleotides and identifying the region or regions of optimal sequence complementarity. 2 SSPE (IxSSPE is 0.15M NaCl, lOmM NaH2PO4, and 1.25mM EDTA, pH 7.4) can be substituted for SSC (IxSSC is 0.15M NaCl and 15mM sodium citrate) in the hybridization and wash buffers; washes are performed for 15 minutes after hybridization is complete. TB* - TR*: The hybridization temperature for hybrids anticipated to be less than 50 base pairs in length should be 5-10°C less than the melting temperature (Tm) of the hybrid, where Tm is determined according to the following equations. For hybrids less than 18 base pairs in length, Tm(°C) = 2(# of A + T bases) + 4(# of G + C bases). For hybrids between 18 and 49 base pairs in length, Tm(°C) = 81.5 + 16.6(log,10Na+) + 0.41(%G + C) - (600/N), where N is the number of bases in the hybrid, and Na+ is the concentration of sodium ions in the hybridization buffer (Na+ for IX SSC = 0.165 M). Additional examples of stringency conditions for polynucleotide hybridization are provided in Sambrook et al., Molecular Cloning: A Laboratory Manual, Chs. 9 & 11, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY (1989), and Ausubel et al, eds., Current Protocols in Molecular Biology, Sects. 2.10 & 6.3-6.4, John Wiley & Sons, Inc. (1995), herein incorporated by reference. The isolated polynucleotides of the present invention may be used as hybridization probes and primers to identify and isolate DNAs having sequences encoding allelic variants of the disclosed polynucleotides. Allelic variants are naturally occurring alternative forms of the disclosed polynucleotides that encode polypeptides that are identical to or have significant similarity to the polypeptides encoded by the disclosed polynucleotides. Preferably, allelic variants have at least 90% sequence identity (more preferably, at least 95% identity; most preferably, at least 99% identity) with the disclosed polynucleotides. ; , The isolated polynucleotides of the present invention may also be used as hybridization probes and primers to identify and isolate DNAs having sequences encoding polypeptides homologous to the disclosed polynucleotides. These homologs are polynucleotides and polypeptides isolated from a different species than that of the disclosed polypeptides and polynucleotides, or within the same species, but with significant sequence similarity to the disclosed polynucleotides and polypeptides. Preferably, polynucleotide homologs have at least 50% sequence identity (more preferably, at least 75% identity; most preferably, at least 90% identity) with the disclosed polynucleotides, whereas polypeptide homologs have at least 30% sequence identity (more preferably, at least 45% identity; most preferably, at least 60% identity) with the disclosed antibodies/polypeptides. Preferably, homologs of the disclosed polynucleotides and polypeptides are those isolated from mammalian species. [0119] The isolated polynucleotides of the present invention may also be used as hybridization probes and primers to identify cells and tissues that express the antibodies of the present invention and the conditions under which they are expressed. Additionally, the isolated polynucleotides of the present invention may be used to alter (i.e., enhance, reduce, or modify) the expression of the genes corresponding to the polynucleotides of the present invention in a cell or organism. These "corresponding genes" are the genomic DNA sequences of the present invention that are transcribed to produce the mRNAs from which the polynucleotides of the present invention are derived. The present invention also provides constructs in the form of plasmids, vectors, transcription or expression cassettes which comprise at least one nucleic acid of the invention as above. The isolated polynucleotides of the present invention may be operably linked to an expression control sequence for recombinant production of the polypeptides of the present invention. Additionally one of skill in the art will recognize that the polynucleotides of the invention may be operably linked to well-known nucleotide sequences encoding the constant region for various antibody isotypes. For example, a polynucleotide of the invention that encodes a light chain variable region(s) of the invention (e.g., the sequence set forth in SEQ ID NOs:4 or 8) may be operably linked to a nucleotide sequence that encodes the constant region (or derivatives thereof) of either a K light chain or λ light chain, such that the expression of the linked nucleotides will result in a full kappa or lambda light chain with a variable region that specifically binds to and neutralizes GDF-8. Similarly, a polynucleotide of the invention that encodes a heavy chain variable region of the invention (e.g., the sequence set forth in SEQ ID N0s:2 or 6) may be operably linked to a nucleotide sequence that encodes the constant region of a heavy chain isotype (or derivatives thereof), e.g., IgM, IgD, IgE, IgG and IgA. General methods of expressing recombinant proteins are well known in the art. Such recombinant proteins may be expressed in soluble form for use in treatment of disorders related to GDF-8 activity, e.g., muscle and bone degenerative disorders. The recombinant expression vectors of the invention may carry additional sequences, such as sequences that regulate replication of the vector in host cells (e.g., origins of replication), tag sequences such as histidine, and selectable marker genes. The selectable marker gene facilitates selection of host cells into which the vector has been introduced. For example, typically the selectable marker gene confers resistance to drugs, such as G418, hygromycin or methotrexate, on a host cell into which the vector has been introduced. Preferred selectable marker genes include the dihydrofolate reductase (DHFR) gene (for use in dhfr host cells with methotrexate selection/amplification) and the neo gene (for G418 selection). Suitable vectors, containing appropriate regulatory sequences, including promoter sequences, terminator sequences, polyadenylation sequences, enhancer sequences, marker genes and other sequences as appropriate, may be either chosen or constructed. Vectors may be plasmids or viral, e.g., phage, or phagemid, as appropriate. For further details see, for example, Molecular Cloning: a Laboratory Manual: 2nd ed., Sambrook et al., Cold Spring Harbor Laboratory Press, 1989. Many known techniques and protocols for manipulation of nucleic acid, for example, in preparation of nucleic acid constructs, mutagenesis, sequencing, introduction of DNA into cells and gene expression, and analysis of proteins, are described in detail in Current Protocols in Molecular Biology, 2nd ed., Ausubel et al. eds., John Wiley & Sons, 1992. The present invention also provides a host cell that comprises one or more constructs as above. A nucleic acid encoding any CDR (HI, H2, H3, LI, L2, or L3), VH or VL domain, or antigen-binding fragment as provided herein, forms an aspect of the present invention. The present invention also includes a method of producing a peptide by expressing the protein from the encoding nucleic acid in a host cell. Expression may be achieved by culturing recombinant host cells containing the nucleic acid under appropriate conditions. Specific antibody fragments, VH and/or VL domains, and encoding nucleic acid molecules and vectors according to the present invention may be provided isolated and purified, e.g., from their natural environment, in substantially pure or homogeneous form, or, in the case of nucleic acids, free or substantially free of nucleic acids or genes of origin other than the sequence encoding a polypeptide with the required function. A number of cell lines are suitable host cells for recombinant expression of the polypeptides and antibodies of the present invention. Mammalian host cell lines include, for example, COS cells, CHO cells, 293T cells, A431 cells, 3T3 cells, CV-1 cells, HeLa cells, L cells, BHK21 cells, HL-60 cells, U937 cells, HaK cells, Jurkat cells, as well as cell strains derived from in vitro culture of primary tissue and primary explants. Such host cells also allow splicing of the polynucleotides of the invention that consist of genomic DNA. Alternatively, it may be possible to recombinantly produce the polypeptides and antibodies of the present invention in lower eukaryotes such as yeast or in prokaryotes. Potentially suitable yeast strains include Saccharomyces cerevisiae, Schizosaccharomyces pombe, Kluyveromyces strains, and Candida strains. Potentially suitable bacterial strains include Escherichia coli, Bacillus subtilis, and Salmonella typhimurium. If the polypeptides of the present invention are made in yeast or bacteria, it may be necessary to modify them by, for example, phosphorylation or glycosylation of appropriate sites, in order to obtain functional proteins. Such covalent attachments may be accomplished using well-known chemical or enzymatic methods. The polypeptides and antibodies of the present invention may also be recombinantly produced by operably linking the isolated polynucleotides of the present invention to suitable control sequences in one or more insect expression vectors, such as baculovirus vectors, and employing an insect cell expression system. Materials and methods for baculovirus/Sf9 expression systems are commercially available in kit form (e.g., the MAXBAC® kit, Invitrogen, Carlsbad, CA). Following recombinant expression in the appropriate host cells, the polypeptides and antibodies of the present invention may be purified from culture medium or cell extracts using known purification processes, such as gel filtration and ion exchange chromatography. Purification may also include affinity chromatography with agents known to bind the polypeptides and antibodies of the present invention. These purification processes may also be used to purify the polypeptides and antibodies of the present invention from natural sources. Alternatively, the polypeptides and antibodies of the present invention may be recombinantly expressed in a form that facilitates purification. For example, the polypeptides may be expressed as fusions with proteins such as maltose-binding protein (MBP), glutathione-S-transferase (GST), or thioredoxin (TRX). Kits for expression and purification of such fusion proteins are commercially available from New England BioLabs (Beverly, MA), Pharmacia (Piscataway, NJ), and Invitrogen, respectively. The polypeptides and antibodies of the present invention can also be tagged with a small epitope and subsequently identified or purified using a specific antibody to the epitope. A preferred epitope is the FLAG epitope, which is commercially available from Eastman Kodak (New Haven, CT). The polypeptides and antibodies of the present invention may also be produced by known conventional chemical synthesis. Methods for chemically synthesizing the polypeptides and antibodies of the present invention are well known to those skilled in the art. Such chemically synthetic polypeptides and antibodies may possess biological properties in common with the natural purified polypeptides and antibodies, and thus may be employed as biologically active or immunological substitutes for the natural polypeptides and antibodies. A further aspect of the present invention provides a host cell comprising nucleic acids, polypeptides, vectors, or antibodies and fragments thereof as disclosed herein. A still further aspect provides a method comprising introducing a nucleic acid of the invention into a host cell. The introduction may employ any available technique. For eukaryotic cells, suitable techniques may include calcium phosphate transfection, DEAE-Dextran, electroporation, liposome-mediated transfection and transduction using a retrovirus or another virus, e.g., vaccinia or, for insect cells, baculovirus. For bacterial cells, suitable techniques may include calcium chloride transformation, electroporation and infection using bacteriophage. The introduction of nucleic acids may be followed by causing or allowing protein production from the nucleic acid, e.g., by culturing the host cells under conditions suitable for gene expression. Such conditions are well known in the art. III. Methods of Treating, Ameliorating, Preventing, and Inhibiting the Progress of Bone, Adipose, Glucose Metabolism, Insulin and Muscle Disorders The involvement of GDF-8 in ALS, and the discovery of the novel antibodies of the invention, enables methods for treating, alleviating, and ameliorating GDF-8-associated disorders, e.g., muscle disorders, neuromuscular disorders, bone degenerative disorders, metabolic or induced bone disorders, glucose metabolism disorders, adipose disorders, and insulin-related disorders. In addition, the antibodies allow for diagnosing, prognosing and monitoring the progress of bone, muscle, adipose or insulin disorders by measuring the level of GDF-8 in a biological sample. In particular, the antibodies of the invention can be used to treat an individual with ALS or other muscle disorder, or in a method of distinguishing whether a patient is suffering from ALS or another muscle disorder. The antibodies and other molecules of the present invention are useful to prevent, diagnose, or treat various medical disorders in humans or animals. The antibodies can be used to inhibit or reduce one or more activities associated with GDF-8. Most preferably, the antibodies inhibit or reduce one or more of the activities of GDF-8 relative to unbound GDF-8 activities. In certain embodiments, the activity of GDF-8, when bound by one or more anti-GDF-8 antibody is inhibited at least 50%, preferably at least 60, 62, 64, 66, 68, 70,72, 72, 76, 78, 80, 82, 84, 86, or 88%, more preferably at least 90, 91, 92, 93, or 94%, and even more preferably at least 95% to 100% relative to a mature GDF-8 protein that is not bound by one or more of the anti-GDF-8 antibodies. Inhibition or neutralization of GDF-8 activity can be measured, e.g., in pGL3(CAGA)12 reporter gene assays (RGA) as described in Thies et ah, supra, and in ActRIIB receptor assays as illustrated in the Examples. The medical disorders diagnosed, prognosed, monitored, treated, ameliorated or prevented by GDF-8 antibodies are GDF-8-associated disorders, e.g., muscle or neuromuscular disorders including, e.g., muscular dystrophy (MD; including Duchenne's muscular dystrophy), amyotrophic lateral sclerosis (ALS), muscle atrophy, organ atrophy, frailty, carpal tunnel syndrome, congestive obstructive pulmonary disease, sarcopenia, cachexia, and other muscle wasting syndromes (e.g., caused by other diseases and conditions), In addition, other medical disorders that may be diagnosed, prognosed, monitored, treated, ameliorated or prevented by the GDF-8 antibodies are adipose tissue disorders such as obesity, type 2 diabetes, impaired glucose tolerance, metabolic syndromes (e.g., syndrome X), insulin resistance induced by trauma (such as burns or nitrogen imbalance), or bone degenerative diseases (e.g., osteoarthritis, osteoporosis, etc.). In preferred embodiments, the disorders that are diagnosed, prognosed, monitored, treated, ameliorated or prevented by GDF-8 antibodies are muscular or neuromuscular disorders. In a more preferred embodiment, the muscular or neuromuscular disorder that is diagnosed, prognosed, monitored, treated, ameliorated or prevented by anti-GDF-8 antibodies is either MD or ALS. In the most preferred embodiment of the invention, the muscular or neuromuscular disorder that is diagnosed, prognosed, monitored, treated, ameliorated or prevented by GDF-8 antagonists of the present invention, e.g., antibodies that inhibit GDF-8 activity, is ALS. Other medical disorders that may be diagnosed, prognosed, monitored, treated, ameliorated or prevented by GDF-8 antagonists are those associated with a loss of bone, which include osteoporosis, especially in the elderly and/or postmenopausal women, glucocorticoid-induced osteoporosis, osteopenia, osteoarthritis, and osteoporosis-related fractures. Other target metabolic bone diseases and disorders include low bone mass due to chronic glucocorticoid therapy, premature gonadal failure, androgen suppression, vitamin D deficiency, secondary hyperparathyroidism, nutritional deficiencies, and anorexia nervosa. The antibodies are preferably used to diagnose, prognose, monitor, treat, ameliorate or prevent such disorders in mammals, particularly in humans. The antibodies of the present invention are administered in therapeutically effective amounts. Generally, a therapeutically effective amount may vary with the subject's age, condition, and sex, as well as the severity of the medical condition in the subject. The dosage may be determined by a physician and adjusted, as necessary, to suit observed effects of the treatment. Toxicity and therapeutic efficacy of such compounds can be determined by standard pharmaceutical procedures in cell cultures or experimental animals, e.g., for determining the LD50 (the dose lethal to 50% of a population) and the ED50 (the dose therapeutically effective in 50% of a population). The dose ratio between toxic and therapeutic effects, i.e., the LD50/ED50. is the therapeutic index, and antibodies exhibiting large therapeutic indices are preferred. The data obtained from cell culture assays and animal studies can be used in formulating a range of dosage for use in humans. The dosage of such compounds lies preferably within a range of circulating concentrations that includes the EDso with little or no toxicity. The dosage may vary within this range depending upon the form of dosage and the route of administration. For any antibody used in the present invention, the therapeutically effective dose can be estimated initially from cell culture assays. A dose may be formulated in animal models to achieve a circulating plasma concentration range that includes the IC50 (e.g., the concentration of the test antibody which achieves a half-maximal inhibition of symptoms or half-maximal inhibition of inhibition of biological activity) as determined in cell culture. Levels in plasma may be measured, for example, by high performance liquid chromatography. The effects of any particular dosage can be monitored by a suitable bioassay. Examples of suitable bioassays include, but are not limited to, DNA replication assays, transcription-based assays, GDF-8 protein/receptor binding assays, creatine kinase assays, assays based on the differentiation of preadipocytes, assays based on glucose uptake in adipocytes, and immunological assays. Generally, the compositions are administered so that antibodies or their binding fragments are given at a dose from 1 µg/kg to 150 mg/kg, 1 µg/kg to 100 mg/kg, 1 µg/kg to 50 mg/kg, 1 µg/kg to 20 mg/kg, 1 µg/kg to 10 mg/kg, 1 µg/kg to 1 mg/kg, 10 µg/kg to 1 mg/kg, 10 µg/kg to 100 µg/kg, 100 µg to 1 mg/kg, and 500 µg/kg to 1 mg/kg. Preferably, the antibodies are given as a bolus dose to maximize the circulating levels of antibodies for the greatest length of time after the dose. Continuous infusion may also be used before, after, or in place of the bolus dose. IV. Methods of Identifying Therapeutic Agents Yet another aspect of the invention provides a method of identifying therapeutic agents useful in treatment of muscle, e.g., glucose metabolism, adipose, and bone disorders. Appropriate screening assays, e.g., ELISA-based assays, are known in the art. In such a screening assay, a first binding mixture is formed by combining an antibody of the invention and its ligand, GDF-8, and the amount of binding between the ligand and the antibody in the first binding mixture (Mo) is measured. A second binding mixture is also formed by combining the antibody, the ligand, and a compound or agent to be screened, and the amount of binding between the ligand and the antibody in the second binding mixture (M1) is measured. The amounts of binding in the first and second binding mixtures are then compared, for example, by calculating the M1/Mo ratio. The compound or agent is considered to be capable of inhibiting GDF-8 activity if a decrease in binding in the second binding mixture as compared to the first binding mixture is observed (i.e., M1/Mo100 nM for JA16; Whittemore et al. (2003) Biochem. Biophys. Res. Commun. 300:965-71; Bogdanovich et al. (2002) Nature 420:418-21) was used, resulting in greater increases in muscle mass in wild type mice treated with RK35 (data not shown). [0222] In SODG93A mouse and rat models of familial ALS, treatment with RK35 resulted in increased body weight and increased muscle mass and strength during the early phases of motor neuron disease. This early phase of disease is defined as the age (56-88 days after birth) at which SODG93A mice show a significant loss of muscle strength, as measured by grip strength assessment (Ligon et al. (2005) Neuroreport 16:533-36) and gait abnormalities (Wooley et al. (2005) Muscle Nerve 32:43-50), and which coincides with the denervation of neuromuscular junctions (Frey et al. (2000) J. Neurosci. 20:2534-42; Fischer et al (2004) Experimental Neurology 185:232-40). Muscle mass increases resulting from GDF-8 inhibition by RK35 were most evident in the quadriceps muscles, but were also pronounced in the gastrocnemius, cranial tibialis and the diaphragm in both rodent models tested. These increases correlated well with increased strength, as hindlimb and forelimb strength declined more slowly in RK35-treated mice in comparison to controls. The extent of muscle mass increase induced by treatment with the RK35 anti-GDF-8 antibody was similar in magnitude to a 25% increase in muscle mass observed in mice heterozygous for disruption of the GDF-8 gene; muscle mass from mice that are homozygous null for GDF-8 is about two-fold that of wild type mice (McPherron (1997) Nature 387:83-90). At approximately 84-88 days after birth in SODG93 A mice, and approximately 110 days for SODG93A rate, overt signs of disease including body weight decreases, gait abnormalities and paralysis become evident. RK35 treatment did not extend survival in either SODG93A mice or rats. The increased muscle mass and strength induced by anti-GDF-8 treatment in the early phase of disease did not delay the appearance of gait abnormalities and limb paralysis in both SODG93A mice and rats (data not shown), nor were gains in leg muscle mass maintained. However significant increases in diaphragm mass in RK35-treated SODG93A mice and rats as compared to vehicle-treated SODG93A controls in both early and end-stage disease phases were observed. Diaphragm muscle in RK35-treated SODG93A mice at end-stage was comparable to that of age-matched wild type controls in both mass and histological evaluation. RK35-treated SODG93 A rats also maintained a significant increase in diaphragm muscle mass at end-stage. Consistent with these morphological changes, electrophysiological analysis of diaphragms from untreated SODG93A rats indicates that expression of mutant SOD1 results in significant inhibition of muscle function, and that treatment with RK35 effectively preserved muscle function in the diaphragm. In this study a defined endpoint (failure of the right reflex test, indicating significant limb paralysis) was used, as the criteria for "end-stage" and euthanasia. Therefore it is not clear whether the enhanced diaphragm muscle mass, decreased atrophy, and improved electrophysiological function induced by RK35 treatment would have resulted in prolonged lifespan in rodents provided with nutritional supplementation. However, these findings are potentially important given the fact that respiratory dysfunction is the leading cause of death in patients with ALS (Lechtzin et al. (2002) Amyotroph. Lateral Scler. Other Motor Neuron Disord 3:5-13). Treatments such as RK35 designed to enhance diaphragm function may have the potential to delay the necessity for mechanically assisted breathing in ALS patients. GDF-8 inhibition by RK35 \may influence motor neuron loss in the lumbar spinal cord in the early disease phase, although many therapeutic benefits apparently were lost by end-stage disease. These data indicate that therapies acting directly on muscle can have a benefit on motor neurons innervating muscle, possibly by modulating the trophic microenvironment, although this approach is apparently not sufficient to delay disease. These observations are therefore consistent with the results of Dobrowolny et al. ((2005) J. Cell. Biol. 168:193-99), in which expression of a muscle-specific isoform of IGF led to slowed loss of motor neurons in the SODG93 A mouse model, and that of Kaspar et al. ((2003) Science 301:839-42), where viral delivery of IGF1 to muscle resulted in reduced motor neuron loss in early-stage disease. Similar to the observations of Kaspar et al. ((2003), supra), beneficial effects of treatment on motor neuron survival were not maintained through end-stage disease. Improved trophic factor support from muscle is therefore likely to be insufficient to prevent motor neuron loss in the SODG93 A model. In summary, in both mouse and rat models of familial ALS, inhibition of myostatin results in enhanced muscle mass and strength, which is maintained through the early stages of disease but lost by end-stage. Myostatin inhibition slowed degenerative changes in skeletal muscle in early-stage disease, but did not delay onset of paralysis nor extend survival, as defined by right reflex failure, nor did myostatin inhibition significantly slow motor neuron loss. However, both morphological and functional differences through late-stage disease were observed in the diaphragm muscle of animals treated with anti-myostatin antibody, in comparison to untreated controls. Overall, the data provided herein support the potential for a beneficial effect of muscle building by treatment with RK35 which may contribute to an enhanced "quality of patient life" early in the disease process. Given that anti-GDF-8 antibodies are currently in clinical development, use of such clinical reagents in ALS for the maintenance of limb and diaphragm muscle mass warrants further investigation as a component of a multi-pronged approach to the treatment of ALS. The combination of an antiGDF-8 therapy with existing drugs such as the glutamate-antagonist riluzole, or newer agents entering clinical development, might not only improve the level of efficacy by helping maintain muscle mass but also have significant impact on overall patient quality of life. Example 5 Mapping of Epitopes for RK35 In order to map the exact antibody epitopes to GDF-8,48 overlapping 13-residue peptides representing the entire sequence of mature GDF-8 set forth in SEQ ID NO:1 were synthesized directly on cellulose paper using the spot synthesis technique (e.g., Molina et al. (1996) Peptide Res. 9:151-55; Frank et al. (1992) Tetrahedron 48:9217-32). The overlap of the peptides was 11 amino acids. In this array, cysteine residues were replaced with serine in order to reduce the chemical complications caused by cysteines. Cellulose membranes modified with polyethylene glycol and Fmoc-protected amino acids were purchased from Abimed (Lagenfeld, Germany). The array was defined on the membrane by coupling a p-alanine spacer, and peptides were synthesized using standard DIC (diisopropylcarbodiimide)/HOBt (hydroxybenzotriazolej coupling chemistry as described previously (Molina et al., supra; Frank et al., supra). Activated amino acids were spotted using an Abimed ASP 222 robot. Washing and deprotection steps were done manually, and the peptides were N-terminally acetylated after the final synthesis cycle. Following peptide synthesis, the membrane was washed in methanol for 10 minutes and in blocker (TBST (Tris-buffered saline with 0.1% (v/v) Tween™ 20) and 1% (w/v) casein) for 10 minutes. The membrane was then incubated with 2.5µg/ml of an anti-GDF-8 antibody in blocker for 1 hr with gentle shaking. After washing in blocker 3 times for 10 min, the membrane was incubated with HRP-labeled secondary antibody (0.25 µg/ml in blocker) for 30 min. The membrane was then washed three times for 10 min each with blocker and 2 times for 10 minutes each with TBST. Bound antibody was visualized using SuperSignal™ West reagent (Pierce) and a digital camera (Alphananotech Fluoromager). As shown in FIG. 7, the RK35 epitope maps to a region of GDF-8 between amino acids 30-40 and 84-97, that putatively interacts with the GDF-8 Type II receptor as predicted by GDF-8 receptor sequence comparison with homologous TGF-P family receptors with characterized domains. Example 6 Characterizing the RK35 Antibody The variable heavy (VH) and variable light (VL) genes encoding RK35 were cloned from hybridoma cells producing the antibody, and the amino acid sequences were determined. Sequence data for the RK35 antibody was used to identify the nearest germline sequence for the heavy and light chain. A comparison of RK35 light and heavy variable regions with the closest human germline sequences is shown in FIG. 8. Appropriate mutations may be made using standard site-directed mutagenesis techniques with the appropriate mutagenic primers. Mutation of the antibody is then confirmed by sequence analysis. Exemplary amino acid sequences for humanized RK35 are set forth in SEQ ID NOs:7 (VH) and 9 (VL), both of which may be encoded by a nucleic acid sequence readily determined by a skilled artisan, e.g., the nucleic acid sequences set forth as SEQ ID NO:6 (VH) and 8 (VL). A skilled artisan will recognize that any and/or all amino acids in the framework of the humanized antibody may be mutated back to the original murine amino acid, e.g., to maintain the conformation of the antigen binding fragment. Nonlimiting examples of SEQ ID N0:7 (VH) and SEQ ID N0:9 (VL) with some back-mutations that may help to maintain the affinity of the antibody to GDF-8 are set forth as SEQ ID NO:26 and SEQ ID NO:27, respectively. To create chimeric antibodies, the VH sequence is subcloned into a pED6 hulgG1 mut expression vector, which encodes human IgG1 containing two point mutations (L234A and G237A) to reduce binding to human Fc receptors and complement components (Morgan et al. (1995) Immunology 86:319-24; Shields et al (2001) J. Biol. Chem. 276:6591-604). The VL sequence of RK35 may be subcloned into the pED6 Kappa expression vector. The expression vectors containing the RK.35 VH and VL sequences are then cotransfected into COS-1 cells and a chimeric RK35 antibody is purified from conditioned medium. Example 7 Treatment of Muscle Disorders Inhibitors of GDF-8, i.e., GDF-8 antagonists, such as, for example, inhibitory antibodies, are useful for treatment of metabolic disorders associated with GDF-8 such as type 2 diabetes, impaired glucose tolerance, metabolic syndrome (e.g., syndrome X), insulin resistance (e.g., induced by trauma such as bums or nitrogen imbalance), and adipose tissue disorders (e.g., obesity). Inhibitors of GDF-8 are also useful for the treatment of bone and muscle disorders associated with GDF-8, such as ALS, muscular dystrophy and osteoarthritis. The anti-GDF-8 antibodies and antibody fragments of the invention may be used to treat a subject, e.g., a human subject, preferably a subject suffering from ALS, at disease onset, or a subject having an established metabolic or bone/muscular disease. The inhibitory antibodies against GDF-8 may also be used to prevent and/or to reduce the severity and/or the symptoms of the disease. It is anticipated that the anti-GDF-8 antibodies and antibody fragments are administered, e.g., subcutaneously, as frequently as once per day and as infrequently as once per month. Treatment durations range from about one month (or less) to several years. (0233] To test the clinical efficacy of anti-GDF-8 in humans, subjects suffering from or at risk for ALS are identified and randomized to treatment groups. Treatment groups include a placebo group and one to three or more groups receiving antibody (at different doses when there are two or more groups). Individuals are followed prospectively for, e.g., one month to three years to assess changes in weight, muscle mass, and grip strength. It is anticipated that individuals receiving treatment will exhibit an improvement. A GDF-8 antagonist, preferably in the form of an antibody or antibodies, is administered as the sole active compound or in combination with another compound or composition. When administered as the sole active compound or in combination with another compound or composition, the dosage is preferably from approximately 1 µg/kg to 100 mg/kg, depending on the severity of the symptoms and/or the progression of the disease. The appropriate effective dose may be selected by a treating clinician from the following nonlimiting list of ranges: 1 µg/kg to 100 mg/kg, 1 µg/kg to 50 mg/kg, 1 µg/kg to 20 mg/kg, 1 µg/kg to 10 mg/kg, 1 µg/kg to 1 mg/kg, 10 µg/kg to 1 mg/kg, 10 µg/kg to 100 µg/kg, 100 µg to 1 mg/kg, and 500 µg/kg to 1 mg/kg. Exemplary nonlimiting treatment regimens and potential outcomes are summarized in Table 5. Table 5: Examples of Potential Clinical Cases (Table Removed) The specification is most thoroughly understood in light of the teachings of the references cited within the specification, all of which are hereby incorporated by reference herein in their entireties. The embodiments within the specification provide illustrations of the invention and should not be construed to limit the scope of the invention. The skilled artisan recognizes that many other embodiments are encompassed by the claimed invention, and that the specification and examples should be considered as exemplary only. WHAT IS CLAIMED IS: 1. An isolated polypeptide capable of binding GDF-8, wherein the polypeptide comprises at least one complementarity determining region comprising an amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 10, the amino acid sequence of SEQ ID NO:11, the amino acid sequence of SEQ ID NO: 12, the amino acid sequence of SEQ ID NO: 13, the amino acid sequence of SEQ ID NO: 14, the amino acid sequence of SEQ ID NO: I5, the amino acid sequence of SEQ ID NO:20, the amino acid sequence of SEQ ID NO:21, the amino acid sequence of SEQ ID NO:22, the amino acid sequence of SEQ ID NO:23, the amino acid sequence of SEQ ID NO:24, the amino acid sequence of SEQ ID NO:25, and the amino acid sequences of the active fragments thereof. 2. The isolated polypeptide of claim 1, wherein the polypeptide is an isolated antibody. 3. The isolated polypeptide of claim 2, wherein the antibody comprises a heavy chain, and wherein the heavy chain comprises a first, second, and third complementarity determining region, wherein the first complementarity determining region comprises an amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 10, the amino acid sequence of an active fragment of SEQ ID NO: 10, the amino acid sequence of SEQ ID NO:20, and the amino acid sequence of an active fragment of SEQ ID NO:20, wherein the second complementarity determining region comprises an amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 11, the amino acid sequence of an active fragment of SEQ ID N0:l 1, the amino acid sequence of SEQ ID NO:21, and the amino acid sequence of an active fragment of SEQ ID NO:21, and wherein the third complementarity determining region comprises an amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO:12, the amino acid sequence of an active fragment of SEQ ID NO:12, the amino acid sequence of SEQ ID NO:22, and the amino acid sequence of an active fragment of SEQ ID NO:22. 4. The isolated polypeptide of claim 3, wherein the heavy chain comprises an amino acid sequence selected from group consisting of the amino acid sequence of SEQ ID NO:3, the amino acid sequence of an active fragment of SEQ ID NO:3, the amino acid sequence of SEQ ID NO:7, and the amino acid sequence of an active fragment of SEQ ID NO:7. 5. The isolated polypeptide of claim 2, wherein the antibody comprises a light chain, and wherein the light chain comprises a first, second, and third complementarity determining region, wherein the first complementarity determining region comprises an amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO:13, the amino acid sequence of an active fragment of SEQ ID NO:13, the amino acid sequence of SEQ ED NO:23, and the amino acid sequence of an active fragment of SEQ ID NO:23, wherein the second complementarity determining region comprises an amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 14, the amino acid sequence of an active fragment of SEQ ID NO: 14, the amino acid sequence of SEQ ID NO:24, and the amino acid sequence of an active fragment of SEQ ID NO:24, and wherein the third complementarity determining region comprises an amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 15, the amino acid sequence of an active fragment of SEQ ID NO:15, the amino acid sequence of SEQ ID NO:25, and the amino acid sequence of an active fragment of SEQ ID NO:25. 6. The isolated polypeptide of claim 5, wherein the light chain comprises an amino acid sequence selected from group consisting of the amino acid sequence of SEQ ID NO:5, the amino acid sequence of an active fragment of SEQ ID N0:5, the amino acid sequence of SEQ ID NO:9, and the amino acid sequence of an active fragment of SEQ ID NO:9. 7. The isolated polypeptide of claim 2, wherein the antibody comprises a light chain comprising the amino acid sequence of SEQ ID NO:5, and wherein the antibody further comprises a heavy chain comprising the amino acid sequence of SEQ ID NO:3. 8. The isolated polypeptide of claim 2, wherein the antibody comprises a light chain comprising the amino acid sequence of SEQ ID NO:9, and wherein the antibody further comprises a heavy chain comprising the amino acid sequence of SEQ ID N0:7. 9. The isolated polypeptide as in any one of claims 2-8, wherein the antibody is selected from the group consisting of an intact antibody, a diabody, an Fd fragment, an Fab fragment, an F(ab')2 fragment, a scFV fragment, and an Fv fragment. 10. The isolated polypeptide as in any one of claims 2-4, wherein the antibody is a dAb fragment. 11. An isolated polynucleotide encoding the polypeptide of claim 1. 12. A host cell comprising a polynucleotide as in claim 11, wherein the polynucleotide is operably linked to a regulatory sequence. 13. A vector comprising a polynucleotide as in claim 11. 14. A method for producing an antibody comprising culturing a host cell comprising a polynucleotide as in claim 11, and recovering the antibody expressed by the host cell. 15. The isolated and purified antibody produced by the method of claim 14. 16. A pharmaceutical composition for treating, ameliorating, and/or preventing a GDF-8-associated disorder comprising a pharmaceutically acceptable carrier and at least one GDF-8 antagonist selected from the group consisting of the polypeptide of claim 1 and a polynucleotide encoding the polypeptide of claim 1. 17. The pharmaceutical composition of claim 16, wherein the GDF-8 associated disorder is selected from the group consisting of a muscle disorder, neuromuscular disorder, bone degenerative disorder, metabolic or induced bone disorder, adipose disorder, glucose metabolism disorder, and insulin-related disorder in a mammalian patient. 18. The pharmaceutical composition of claim 17, wherein the GDF-8 associated disorder is selected from the group consisting of muscular dystrophy, ALS, muscle atrophy, organ atrophy, frailty, congestive obstructive pulmonary disease, sarcopenia, cachexia, muscle wasting syndromes, obesity, type-2 diabetes, impaired glucose tolerance, metabolic syndromes, insulin resistance, nutritional disorders, premature gonadal failure, androgen suppression, secondary hvperparathyroidism, osteoporosis, osteopenia, osteoarthritis, low bone mass, vitamin D deficiency, and anorexia nervosa. 19. The pharmaceutical composition of claim 18, wherein the GDF-8- associated disorder is ALS. 20. A method of treating, ameliorating and/or preventing a GDF-8-associated disorder comprising administering a therapeutically effective amount of a GDF-8 antagonist to a mammalian patient in need of such therapy. 21. The method of claim 20, wherein the therapeutically effective amount of the GDF-8 antagonist has little or no toxicity. 22. The method of claim 21, wherein the GDF-8 antagonist is selected from the group consisting of the polypeptide of claim 1 and a polynucleotide encoding the polypeptide of claim 1. 23. The method of claim 22, wherein the GDF-8-associated disorder is selected from the group consisting of a muscle disorder, neuromuscular disorder, bone degenerative disorder, metabolic or induced bone disorder, adipose disorder, glucose metabolism disorder, and insulin-related disorder in the mammalian patient. 24. The method of claim 23, wherein the GDF-8-associated disorder is selected from the group consisting of muscular dystrophy, ALS, muscle atrophy, organ atrophy, frailty, congestive obstructive pulmonary disease, sarcopenia, cachexia, muscle wasting syndromes, obesity, type-2 diabetes, impaired glucose tolerance, metabolic syndromes, insulin resistance, nutritional disorders, premature gonadal failure, androgen suppression, secondary hyperparathyroidism, osteoporosis, osteopenia, osteoarthritis, low bone mass, vitamin D deficiency, and anorexia nervosa. 25. The method of claim 24, wherein the GDF-8-associated disorder is ALS. 26. A method of diagnosing or detecting whether a patient is suffering from a GDF-8-associated disorder comprising the steps of: (a) obtaining a first sample from the patient of interest; (b) contacting the first sample with the antibody of claim 2; (c) determining the level of GDF-8 in the first sample; (d) obtaining a second sample from an individual not afflicted with the GDF-8-associated disorder; (e) contacting the second sample with the antibody; (f) determining the level of GDF-8 in the second sample; (g) comparing the levels of GDF-8 in the first and second samples, wherein an increase, decrease, or similarity in the level of GDF-8 in the first sample compared to the second sample indicates whether the patient is suffering from the GDF-8-associated disorder. 27 A method of monitoring the severity of a GDF-8-associated disorder comprising the steps of: (a) obtaining a first sample taken from a patient afflicted with the GDF-8- associated disorder at a first time point; (b) contacting the first sample with the antibody of claim 2; (c) determining die level of GDF-8 in the first sample; (d) obtaining a second sample taken from the patient at a second time point; (e) contacting the second sample with the antibody; (f) determining the level of GDF-8 in the second sample; and (g) comparing the levels of GDF-8 in the first and second samples, wherein an increase, decrease, or similarity in the level of GDF-8 in the second sample compared to the first sample indicates whether the GDF-8-associated disorder has changed in severity. 28. The method of monitoring of claim 27, wherein the GDF-8-associated disorder is ALS, and wherein a decrease in the level of GDF-8 in the second sample indicates that ALS has decreased in severity. 29. A method of monitoring the severity of a GDF-8-associated disorder comprising the steps of: (a) obtaining a first sample from a patient afflicted with the GDF-8- associated disorder; (b) contacting the first sample with the antibody of claim 2; (c) determining the level of GDF-8 in the first sample; (d) obtaining a second sample from an individual not afflicted with the GDF-8-associated disorder; (e) contacting the second sample with the antibody; (f) determining the level of GDF-8 in the second sample; and (g) comparing the levels of GDF-8 in the first and second samples, wherein an increase, decrease, or similarity in the level of GDF-8 in the first sample compared to the second sample indicates whether the GDF-8-associated disorder has changed in severity. 30. The method of monitoring of claim 29, wherein the GDF-8-associated disorder is ALS, and wherein a decrease or similarity in the level of GDF-8 in the first sample compared to the second sample indicates that ALS is decreased in severity. 31. A method of prognosing the likelihood that a patient will develop a GDF-8- associated disorder comprising the steps of: (a) obtaining a first sample taken from the patient of interest at a first time point; (b) contacting the first sample with the antibody of claim 2; (c) determining the level of GDF-8 in the first sample; (d) obtaining a second sample taken from the patient at a second time point; (e) contacting the second sample with the antibody; (f) determining the level of GDF-8 in the second sample; and (g) comparing the levels of GDF-8 in the first and second samples, wherein an increase, decrease, or similarity in the level of GDF-8 in the second sample indicates the likelihood that the patient will develop a GDF-8-associated disorder. 32. The method of prognosing of claim 31, wherein the GDF-8-associated disorder is ALS, and wherein an increase in the level of GDF-8 in the second sample indicates an increased likelihood that the patient will develop ALS. 33. A method of prognosing the likelihood that a patient will develop a GDF-8- associated disorder comprising the steps of: (a) obtaining a first sample from the patient of interest; (b) contacting the first sample with the antibody of claim 2; (c) determining the level of GDF-8 in the first sample; (d) obtaining a second sample from an individual not afflicted with the GDF-8-associated disorder; (e) contacting the second sample with the antibody; (f) determining the level of GDF-8 in the second sample; and (g) comparing the levels of GDF-8 in the first and second samples, wherein an increase, decrease, or similarity in the level of GDF-8 in first sample compared to the second sample indicates the likelihood that the patient will develop a GDF-8-associated disorder. 34. The method of prognosing of claim 33, wherein the GDF-8-associated disorder is ALS, and wherein a decrease or similarity in the level of GDF-8 in the first sample compared to the second sample indicates a decreased likelihood that the patient will develop ALS. 35. The Invention substantially such as herein before described.