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The multibody systems have had a really important development during the last decade and this has allowed the study of very complex systems in movement as it can be the human body.The human body is composed by a large amount of bones and muscles that generate a biomechanical model with a redundancy problem, in other words, not enough equilibrium equations are available to determine all the muscle forces.Because of that it is necessary use softwares as Anybody that make it possible to deduce which muscles are going to work during a specific movement.The Anybody software make the analyses through the method of the inverse dynamic.For that purpose, it is necessary to introduce as an initial data the movement that is going to perform the model and the reaction generated usually with the floor during the movement due to the inherent weight (ground reaction force).To obtain these data it is necessary to perform really complex experimental analyses as the ones performed in this work.The experimental analyses, is based on obtain on the one hand the movement and on the other hand the ground reaction force.To obtain the human movement it has been necessary ten infrared cameras located around a stairs-machine and a large amount of markers fixed in the person that is going to be the responsible of perform the movement.The characteristics of this person (weight, age and height), are very important parameters that have to be taken in account in the design of the model in order to fix as best as possible the real movement with the movement of the model.On the other hand, to obtain the ground reaction force in this work a pressure insoles has been used instead of using force plates due to the more accurate results that provide the pressure insoles.Once the experimental analyses have been performed the model has been designed and programed in the Anybody software.Only has been modeled the lower limbs of the human body in order to reduce computational cost because in this paper is only necessary obtain the results of the muscles of the lower limbs of the human body.In this paper the loads acting on the human lower limbs during walking up and down stairs is studied.This is because estimating the muscles conditions is one of the most important factors for patients undergoing rehabilitation or with walking difficulties during his usual life.Furthermore, knowledge of the forces is essential in the design of artificial joints or prostheses with an adequate strength in order to avoid the possible failure.To conclude, the study shows that the supported loads when walking up and down stairs are relatively low from the point of view of the static failure so it can be stated that the static failure is not going to be a critical point when a prostheses is designed.However, the fatigue failure is a very critical factor so be familiar with these loads in the legs prostheses design phase is extremely important for a proper design, because theseprostheses must be designed for an unlimited life from the point of view of fatigue due to the serious consequences that involve the failure of a prostheses.