This paper deals with a concept of a lower limbs closed loop rehabilitation device. The human force applied to this device is considered as an external disturbance to the system dynamics. A control structure was proposed to track desired trajectories and to attenuate the external disturbances. Stability conditions are given in terms of Linear Matrix Inequalities (LMI) using a quadratic Lyapunov function and an H-infinity criterion. Finally, the simulation of the designed device on an isokinetic movement illustrates the efficiency of the proposed control structure.