The normal daily human activities include different levels of energy and strength needs, and this applies also for walking such that walking at high speeds needs more stiff muscles compared with walking slowly, and walking during carrying heavy loads or on inclined surfaces needs more energy than during doing the light housekeeping activities. For a healthy individual the human foot has already the ability to change its stiffness and to store and return a part of the elastic energy in a compliant structure of muscle fibres and series elastic elements. In the case of amputations in the lower limbs the amputated limbs are replaced with artificial limbs. A problem appears by this replacement that the new artificial limbs are designed to satisfy some described tasks and any change in these tasks or in the boundary conditions leads to an incompliance and unsatisfactory motion of the amputees. For example, the use of soft foot for walking at high speeds leads to a larger motion of the body centre of mass compared with walking at the preferred walking speed. In order to overcome these problems or reduce their effects, the need arises to design adaptive prostheses that have the ability to change their properties according to the surrounding conditions. This work has the goal to evaluate the usefulness of using adaptive elastic foot prosthesis, through the numerical modelling and simulation of the human gait of an above-knee amputee with an adaptive elastic foot. The body of the amputee is divided into a limited number of rigid bodies (segments) connected together by hinge joints. A two dimensional model of the human body is built from these segments using a commercial multibody simulation program. The initial conditions of the system, some of the body segments relative motions and forces are given as inputs in the model. In order to integrate the adaptive elastic foot in the rigid bodies system a numerical model of the foot is built using the finite element method and then reduced by static condensation. The reduced elastic model is then integrated in the rigid bodies’ model of the human gait. This model is used to simulate the stance period of the human gait. Four parameters, the vertical ground reaction force (GRF), the body centre of mass (BCoM), the ankle joint moment and the hip joint rotation are considered as defining characteristics of the human gait. These characteristics are used in the evaluation of the model results and latterly in the evaluation of the adaptive foot usefulness. The simulation model is validated through comparison with experimental results of the human gait. The model shows good consistency with experimental results and can be further used in simulating the human gait using prosthetic feet with different mechanical properties and positions. Different prosthetic foot properties and walking conditions are studied for the adaptive elastic prosthetic foot. The stiffness of the foot sole is changed for normal walking on level and inclined surfaces and for fast walking on level surfaces. The changes in stiffness show changes in the vertical BCoM motion which improve the gait form for walking faster than the normal walking speed but show no significant changes on the other parameters. The ankle joint inclination is also changed for walking on uphill inclined surfaces; the results show that increasing the inclination angle reduces the vertical GRF and increases the horizontal motion of the BCoM and relatively the step size, which improves the uphill motion on inclined surfaces. Also designs of beam elements with changeable stiffness that could be used in an adaptive prosthetic foot’s sole are considered. In this part two concepts are developed and studied theoretically then two models are manufactured and proved experimentally. The results show good changes in the stiffness of the models, then the first model consisting of two plates sliding one in the other shows experimentally a change of ± 8.5% in the stiffness and the second model consisting of two plates screwed together gives experimentally an average change of ±18% in the stiffness. Since these models are designed to be used as replacements for human limbs (where the available external energy sources are limited) attention was given to model a light weight system with minimum energy consumption for controlling and driving it. Also attention was given to design a system that could be used with the different commercially available prosthetic feet without the need to make large changes in the original models designs and sizes.