The knee is the largest joint in the body and is vulnerable to injury during athletic activities or to musculoskeletal conditions such as arthrosis. Knee orthotic devices are widely used by physicians as preventive and therapeutic adjuncts for both musculoskeletal conditions and sport injuries. Their goal is to stabilize or restrict non-physiological knee ranges of motion. The efficiency of these devices has been studied both from clinical and biomechanical perspectives, leading to controversial results from questionable methods. As for now, the mechanisms of force transfer from the device to the joint bones have never been characterized and both device manufacturers and clinicians still expect a standard procedure to compare and grade the efficiency of different knee braces. The objectives of this work are: 1. to quantify the mechanical reactions of knee braces against non-physiological movements; 2. to relate these mechanical reactions to the pressure applied by the braces onto the skin. The latter is particularly important because it refers to comfort issues, which play a key role in a patient's compliance to the orthopedic treatment. A Finite Element Model of a braced human leg is developed. The model is first applied for characterizing the behavior of different kinds of knee braces, focusing on the mechanical reactions against non-physiological movements. In the model, a special attention is paid to the interfaces between knee-braces and the skin and between the skin and the muscles. The interface properties of the model are calibrated against experimental data measured by full-field measurements of 3D displacement over the surface of a patient's leg. The results show that the mechanical action of knee braces is essentially limited by skin/fabric and skin/muscles sliding. Finally, the model leads to a better understanding of the knee/brace interaction, and of the role of the brace components on the stability of the injured knee. Thanks to this computational tool, novel brace designs can be tested and evaluated for an optimal mechanical efficiency of the devices. Future work consists in considering the patient's comfort in the approach.