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 types of support to the effort exoskeletons, dependent tasks by the user. Known especially exoskeletons for the upper body to relieve the upper limbs when transporting heavy loads and / or during repeated movements. These exoskeletons help reduce the risk of musculoskeletal disorders. However, these exoskeletons generally have a non-articulated architecture in the back with a single point of mobility at the hip joint. These exoskeletons do not allow a bending of the back of the user. In other exoskeletons, mass raised by the upper body is supported by an ad-hoc mechanism of direct transfer to soil. These exoskeletons will not allow the user to move easily in space and does not respect its biomechanics. In other words, in both cases, these exoskeletons tend to reduce the mobility of the user. SUMMARY OF THE INVENTION An object of the invention is to provide a solution to assist the upper body of a user to carry loads, while providing greater freedom of movement. This object is achieved in the context of the present invention by a back module for an exoskeletal structure comprising a segment spine intended to extend along a spinal column of a user, the spine segment comprising a plurality of vertebra members, stacked on each other, and a flexible member connecting the connecting elements vertebra with each other, the spine segment having a stable equilibrium position in which the flexible connection member holds against the vertebra support elements each other, and the flexible connecting member being resilient so that, upon movement of the back of the user, the flexible connection element allows a displacement of the vertebral elements relative to each other,while exerting a restoring force tending to return the spine segment in the position of stable equilibrium. The vertebral column segment being formed by a plurality of support elements vertebrae against each other, it can transmit a vertical load on the back of the module and to accommodate movements of the upper body the user. Indeed, the flexible element allows a certain degree of freedom of the vertebrae of the elements relative to each other, which gives a certain freedom of movement of the spine. Moreover, the number of vertebra elements can be adjusted depending on the size of the user, allowing to easily adapt the exoskeleton structure to the morphology of the user. The module can also have the following characteristics: - the flexible connecting element exerts a compressive force on the elements of vertebrae to maintain the vertebrae support elements against each other in the position of stable equilibrium, - the flexible connecting element extends within the spinal segment through each of the vertebrae members, the flexible element connection being kept tensioned so as to exert a compressive force on the vertebra members, - each vertebra element has a recess and a protrusion, each protrusion being adapted to be received in a recess of another element vertebra located immediately above or below it in the stack, - each vertebra element is connected to a next vertebra member by a connection allowing a bending movement and / or radial rotation and / or lateral inclination of the back of the user, - each vertebra element has an arcuate shape with a concavity oriented towards the bottom of the spine when the spine segment extends along the spine of the user, - the module further comprises one or more cable (s) of electric transmission or data transmission extending into the spine segment through each of the vertebrae elements for connecting a battery and / or actuators and / or sensors to a control module of the exoskeletal structure or to connect two control modules of the exoskeletal structure, - or the cable (s) of electric transmission or transmission data has a length greater than a length of the spine segment such that they allow deformation of the spinal column segment without undergoing stretching, - the module comprises a fastening device comprising an attachment part attached to a lower end of the spinal segment, the attachment piece being adapted to be fixed to a complementary attachment part attached to a waist belt of a base module of the exoskeleton structure for attaching the module back to the base module. The invention further relates to an exoskeleton structure for assistance to the effort of a user, comprising: - a base module comprising an own hip belt to surround the waist of the wearer and an attachment part fixed to the belt, and - a back module as defined above, comprising an attachment part attached to a lower end of spine segment, the attachment part of the back of module being adapted to be attached to the attachment part of the belt for attaching the module back to the base module, so that a weight applied to the spine member is transferred to the base module. In one embodiment of the invention, the base module includes a control unit and a battery secured to the belt, and back module comprises an add-on battery and / or actuators, and the fasteners each comprise clean electrical contacts to electrically connect the battery and the control unit of the base module to the battery and / or back module actuators when the attachment part back module is fastened to the attachment part of the base module. 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