This paper proposes a mathematical model for a 7-links an-thropomorphic biped-composed with two identical legs, two feet and one trunk-with an orthosis attached to both thigh and calf, during walking. We consider a cyclic walking gait in the sagittal plane. In the first part, we obtain trajectories of the motion for different walking speeds. It confirmed the fact that much more torque is recquired in the stance leg than in the swing leg. The second part analyses the impact of an ortho-sis, which fully assists the knee over the walking. Simulations point out the fact that valid joints have to compensate the weight of the orthotic system and show the interest to assist the knee of the stance leg.