This paper concerns the synthesis of Functional Electrical Stimulation (FES) patterns for efficient functional movement. We propose an approach based on a nonlinear optimization formulation. The study considers a biomechanical knee model and the associated agonist/antagonist muscles. The goal of this method is to synthesize optimal patterns which minimize the muscular activities in order to reduce the fatigue. The approach is illustrated with a sinusoidal desired knee joint trajectory. Moreover, The applied optimal FES patterns allow the muscles co-contraction during the movement.