The work presented in this paper, concerns the synthesis of Functional Electrical Stimulation (FES) patterns which generate a movement of paralysed limbs in spinal cord injury patient. We propose an approach based on a nonlinear optimization formulation that may encounter physiological and technological constraints. 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 muscular fatigue while verify an efficient movement. Different tests have been performed and the results compared with respect to the energetic balance. The approach is illustrated in simulation with: 1) sinusoidal desired knee joint trajectory, 2) optimal reference knee joint trajectory and 3) without explicit reference knee joint trajectory. The simulations have been performed from model parameters estimated from real subject data. We show that the tracking of trajectory present a high energy consumption which demonstrate the inappropriateness of robotics methods for musculoskeletal system.