Functional electrical stimulation (FES) can help in regaining limited locomotor activities in humans with paraplegia through electrical stimulation of the lower extremity muscles, mainly the quadriceps and hamstrings. Closed loop position control of a flexion-extension knee under (FES) based on a high order sliding mode techniques has been presented. Stimulation currents amplitude were assessed via a variable structure control systems (VSCS). Special attention is given to a 2-sliding (Prescribed convergence law) control algorithm. The controller was used to control shank movement and has shown a robustness against force perturbation as well as high capability of tracking a pre-defined reference trajectory. Cocontraction of the antagonistic muscles, basically quadriceps and hamstrings, may yield an increasing joint stiffness and a stable movement. The redundancy of two muscles spanning the knee joint in co-contraction, is solved by a linear minimization of the sum of the stress in the antagonistic muscles.