The assistance to the effort exoskeletons are mechanical structures that duplicate the human skeletal structure that improve the physical capabilities of the human body. There are different exoskeletons support to the effort, the shape and structure depend on the tasks by the user. In general, each exoskeleton is designed to assist the user to accomplish a specific task. There are in particular structures exoskeleton intended to be fixed on the upper body of a user to assist the user in his arm movements, in particular for lifting and handling loads. However, these structures generally do not allow to adapt to all the movements of the articular complex of the shoulder. For example, these structures can allow flexion and extension of the shoulder, but does not permit movement of abduction and adduction of the shoulder. Or the complex shoulder joint is a double joint complex having six degrees of freedom including three degrees of freedom provided by the shoulder itself, and three degrees of freedom provided by the shoulder belt supporting shoulder. Other structures have also been proposed to better reproduce the movements of the shoulder. However, these structures generally have a bulky or do not coincide with the logic of mobilization of the shoulder, hampering the mobility of the user. SUMMARY OF THE INVENTION An object of the invention to provide a shoulder module for an exoskeleton structure that provides greater freedom of movement of the articular complex of the shoulder of the user while being compact and light compared to existing technologies . This object is achieved in the context of the present invention by a shoulder module for an exoskeletal structure, the shoulder module being intended to connect an elbow module mounted on a arm of a user to a module back attached to the back of the user, the shoulder module comprising: - a plurality of connecting pieces, and - a first pivot connecting two connecting pieces allowing a rotation of one of the connecting parts relative to the other connecting part according to a first axis of rotation, upon rotation of the bend module in relation to Rear module corresponding to a movement of abduction or adduction of the shoulder of the user, wherein the connecting elements include a first connecting part comprising two parts mounted for sliding relative to each other through a first slide, the sliding of the parts relative to each other allowing a shortening the connecting member upon rotation of the elbow module relative to the back of the module caused by an abduction movement of the user's shoulder and allowing an extension of the first connecting member upon rotation of the elbow module relative to the back of the module caused by an adduction movement of the shoulder of the user, and wherein the first slide comprises an elastic return element tending to oppose a parts spacing one compared to theother leading to an elongation of the first connecting part. In such a module, the first rotation axis does not necessarily coincide with the real axis of rotation of the shoulder. Indeed, the lengthening and shortening of the connecting piece possible to vary the length of the connecting piece in order to compensate the offset of the axes of rotation. Furthermore, the elastic return element is used to assist the user in his movement of shoulder abduction by compensating the weight of the bend module. In this way, it is possible to design a shoulder module in which the connecting parts are positioned as close to the body of the user, which allows to design a particularly compact module. The module can also have the following characteristics: - the module further comprises a second pivot connecting the two connecting parts permitting rotation of one of the connecting parts relative to the other connecting part according to a second axis of rotation, during a rotational movement of the elbow module relative to the back of module corresponding to an internal or external rotation of the shoulder of the user, - the first axis of rotation intersects the second axis of rotation, - the module further comprises a third pivot connecting the two connecting parts permitting rotation of one of the connecting parts relative to the other connecting part according to a third axis of rotation, upon rotation of the module shoulder elbow relative to the back of module corresponding to a flexion or extension of the shoulder of the user, and wherein the third axis of rotation intersects the second axis of rotation, - the third axis of rotation intersects the second axis of rotation, - the connecting parts include a second part of own connection to connect the shoulder modulus at an elbow joint of the elbow module, the second connecting piece comprising two parts mounted for sliding relative to each other by the via a second slide, the sliding of the parts relative to each other allowing a shortening or elongation of the second connecting part during a rotation of the bend module relative to the back module corresponding to a flexion or extension of the user's elbow, - the second slide comprises an elastic return element tending to oppose a parts spacing relative to each other resulting in an elongation of the second connecting part, - the four-bar module further comprises a mechanism connecting the module back to the first pivot, the four-bar mechanism comprising four bars and four joints connecting the rods together, forming a deformable parallelogram, - the four-bar linkage comprises a resilient member extending along a diagonal of the parallelogram and connecting two opposite joints between them, - the module equipped with quatrieme pivot reliant headings des barres du mécanisme quatre barres à une pièce du module de dos et a pivot reliant Cinquième a bar parallèle du mécanisme quatre barres in the visit rooms liaison. The invention further relates to an exoskeletal structure comprising: - a clean back module to be fixed on the back of a user, - a bend own module to be fixed on the arm of a user, and - a shoulder module as defined above. PRESENTATION OF DRAWINGS Other characteristics and advantages will emerge from the following description which is purely illustrative and non-limiting and should be read with reference to the appended figures, in which: - Figure 1 shows schematically, in front view, a user equipped with an exoskeleton structure according to a possible embodiment of the invention, - Figures 2 and 3 show schematically, in back view and in side view, the user equipped with the exoskeletal structure, according to a first possible configuration of the invention, - Figures 4 and 5 show schematically, in back view and in side view, the user equipped with the exoskeletal structure, according to a second possible configuration of the invention, - Figures 6 and 7 schematically show, in rear view and in side view, the user equipped with the exoskeletal structure, according to a third possible configuration of the invention, - Figures 8A and 8B schematically illustrate a hip joint connecting a hip module to the basic module, - Figure 9 shows schematically a fastening device for attaching the hip module to the basic module, - Figures 10A and 10B show schematically the attachment device in the unlocked configuration and respectively locked configuration, - Figure 1 1 shows schematically the lower modules of the exoskeletal structure, - Figures 12A-12E schematically illustrate a boot equipped with a foot module during different from the user walks phases, - Figure 13 shows schematically an enlarged view of the upper modules of the exoskeletal structure, - Figure 14 shows schematically a spine segment forming part of the back of the module, - Figure 15 schematically shows a vertebra of the spine member segment, - Figures 16 to 18 show schematically a shoulder module, - Figure 19 shows schematically a fastening device for attaching the shoulder module to bend module, - Figures 20 and 21 schematically show a backpack carrier module, - Figure 22 shows schematically a fastening device for attaching the support module backpack hip each module. DETAILED DESCRIPTION OF AN EMBODIMENT modular structure In Figures 1 to 7, the exoskeletal structure shown comprises a base module 1, a back module 2, two shoulder modules 3, two elbow modules 4, two hip modules 5, two knee modules 6 , two foot modules 7 and a bag support module 14 back. The exoskeleton shown in these figures structure can be used in different configurations in order to obtain different exoskeletons adapted to different uses. In a first configuration illustrated in Figures 1 to 3, the exoskeleton is formed by the assembly of the basic module 1, module 2 back on both shoulder modules 3, of the two elbow modules 4, the two hip modules 5, the two knee modules 6 and 7 foot two modules. In a second configuration illustrated in Figures 4 and 5, the exoskeleton is formed by the assembly of the basic module 1, module 2 back on both shoulder modules 3 and two elbow module 4 only. In a third configuration illustrated in Figures 6 and 7, the exoskeleton is formed by the assembly of the basic module 1, two hip modules 5, the two knee modules 6, the two leg modules 7, and bag support module back 14 only. The three examples of configurations illustrated in these figures are obtained from three different assemblies of the modular structure of exoskeleton. However, other configurations are of course possible. In these various configurations, the exoskeleton is formed from one or more modules assembled together. As illustrated in Figures 1 to 3, the basic module 1 comprises a waist belt 1 one is able to surround the bottom of the wearer's trunk. Lumbar buckle 1 1 is arranged around the waist of the user, resting on the hips of the user. The basic module 1 also includes a first battery 12 for powering the various actuators of the electric power structure, and a control unit 13 programmed to control the various actuators. The first battery 12 and the control unit 13 are fixed to the waist belt 1 January. The back module 2 is adapted to be fixed to the upper body of the user, above the basic module 1, along the back of the user. bend 4 of the modules are adapted to be secured on the user's arm, respectively in the right arm and left arm. Each shoulder module 5 is adapted to connect the back of module 2 to a respective elbow module 4. The back module 2, the shoulder and the elbow 3 modules 4 modules form a set of upper modules whose function is to assist the user in much it exercises with his upper body, e.g. when performing repetitive tasks with his upper body. 5 hip modules are adapted to be secured over the thighs of the user, respectively in the right thigh and the left thigh of the user. The knee modules 6 are adapted to be secured to the calf of the user, respectively in the calf of the right leg and calf of the left leg of the user. 7 foot modules are adapted to be secured on the user's feet, respectively, right foot and left foot. Hip modules 5, the knee modules 6 and 7 foot modules form a set of lower modules whose function is to assist the user in his efforts produced with the lower body, in particular when walking or when carrying or moving loads. Note that the hip modules 5 are symmetrical to one another. Hip modules 5 therefore comprise identical or similar parts. Similarly, knee modules 6 are symmetrical with each other, and include identical or similar parts. It is the same foot 7 modules, shoulder modules 3 and 4 elbow modules. hip module As illustrated in Figures 1 to 3, each hip module 5 comprises a femoral portion 51 suitable for being fixed on the thigh of the user and a hip joint 52. The femoral part 51 comprises a femoral segment 1 51 provided to extend along the thigh of the user and fastening straps 512 adapted to surround the user's thigh to fix the femoral segment 51 1 to the thigh. Each hip module 5 is connected to base module 1 through a respective hip joint 52. Specifically, the hip joint 52 connects the portion 51 of the femoral hip module 5 to the belt 1 1 of the basic module 1. Hip joint As illustrated in Figures 8A and 8B, the hip joint 52 comprises a hip actuator 521 to assist the user during a flexion movement or extension of the hip of the user. The actuator 521 includes a stator 522 and a rotor 523 adapted to be rotated relative to the stator 522 when the stator 522 is supplied with electric power for rotating the hip module 5 relative to the base module 1 during a flexion or extension of the hip. The hip joint 52 further comprises an elastic member 524 arranged to exert a return spring force that supports the rotor 523 when the user gets up from a sitting or crouched position. The elastic return element 524 may comprise a pretensioned spring arranged between the stator 522 and the rotor 523, in a guide groove 525 provided between the stator 522 and the rotor 523. More specifically, the elastic return element 524 is arranged so that: - in a first angular range of movement CH rotor 523 relative to stator 522, corresponding to an angular range in which is located the rotor 523 when the user walks or runs (Figure 8A), the elastic member return 524 n 'exerts no spring force on the rotor 523, and - in a second angular range