This study is devoted to investigate the influence of a knee orthosis for human walking by using a mathematical model for a 7-links planar biped-composed with two identical legs, two feet and one trunk-with an orthosis attached to both thigh and calf, during walking. In the first part, we design a cyclic walking gait in the sagittal plane for human without the orthosis. The second part we consider that the human looses his muscular possibilities in one of his knees. To overcome his handicap the human is equipped of a knee orthosis. We analyse the positive effect of the orthosis over the assisted knee to track the previous designed reference trajectory. By using at each time an optimisation algorithm, we minimise the torques provided by the human. The numerical tests confirm the possibility to reduce the torque produced in the disabled knee. The next step is to take into account explicitly an information from EMG signal during a walking to modulate the power of the orthosis.