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 various types of assistance to the effort exoskeletons dependent tasks by the user. Each type of exoskeleton can limit or reduce the effort made by the user when performing certain tasks. However, the user must always carry the structure of the exoskeleton, which has the consequence of limiting the freedom of movement of the user and to generate an additional charge which is necessarily transferred to the lower body of the user. To relieve the user are known exoskeleton structures in which the load is transferred to the ground via the plates placed under the feet of the user. In these structures, the user's feet are not in contact with the ground, which makes uncomfortable structure. In addition, the mobility of the user is reduced. structures are also known exoskeleton in which the load is transferred to the ground via a support structure on the floor beside the feet of the user. However, this structure does not provide support on the ground in all market phases and / or any type of soil, especially when the user walks on sloping ground. SUMMARY OF THE INVENTION An object of the invention is to provide a solution which allows to relieve the user loads it carries, whether the load generated by the exoskeleton structure itself, by external elements can be associated the structure of the exoskeleton (eg backpack) or the same weight of the user, and while having a better comfort and better mobility. This object is achieved in the context of the present invention by a foot module for an exoskeletal structure, comprising an ankle joint, a first plate of the support intended to come into support on top of a shoe of user, when the foot unit is attached to the foot of the user, a second plate own support to be placed under the sole of the boot, a first segment connecting the ankle joint to the first plate support and a second segment connecting the first support plate to the second support plate, the first segment and the second segment forming an own spring to be compressed when the shoe is in contact with the ground and to be relaxed when the shoe n is not in contact with the ground. The proposed foot module can transfer the load exerted on the exoskeleton to the ground during walking phases: during the running, the load is transferred to the ground successively via the second support plate, and then via the first bearing plate and the shoe of the user. Furthermore, by relaxing the spring assists the user to lift the foot while walking. In this way, the user's shoe is in contact with the ground, which improves the comfort of the user and allows the user to move on any terrain. The base module is particularly suitable for use in combination with a shoe type "store" or "combat boot" for example. These shoes are in fact designed to carry a load is applied on top of the shoe up to 50 kilograms. These shoes can also include protective shells of the forefoot and / or metal reinforcing elements. Moreover, the proposed foot module can be used with a standard shoe and does not need to modify or adapt the shoe. The module can also have the following characteristics: - the second support plate is adapted to be disposed in a recess formed in the sole between the heel and the forefoot, - the module further comprises a third rod connecting the second segment to the second support plate, the third bar forms an angle with the second segment, - the module comprises its own connecting bar to be inserted into a segment of a knee module secured to the calf of the user, for attaching the foot module knee module, and the ankle joint connecting the first segment to the connecting rod allowing rotation of the first segment relative to the connecting rod at a flexion or extension of the ankle of the user, - the ankle joint comprises a first plate, a second own armature to rotate relative to the first plate in a flexion movement or extension of the ankle of the user, and an elastic member disposed between the plates the resilient member being adapted to exert a return force opposing the relative rotation of the second frame relative to the first armature, - the elastic member comprises a ring formed of an elastomeric material, precompressed between the first armature and the second armature, - the second bearing plate has a lower surface intended to be in contact with the ground, provided with a slip coating. 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 - two hip modules adapted to be secured to the user's thighs and connected to the base module through hip joints, two modules own knees to be attached to the user's legs, and connected to the hip modules by knee joints, and two leg modules as defined above, adapted to be connected to the modules by knee ankle joints. 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 module 2 to a respective elbow module 4. The back module 2, the shoulder and the elbow modules 3 modules 4 form a set of upper modules whose function to assist the user in the efforts he produced with his upper body, for example 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. More precisely, 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