This paper describes a cooperative control approach that combines the use of a powered knee joint orthosis along with Functional Electrical Stimulation (FES) for knee joint flexion-extension movement restoration. A closed-loop adaptive control and an open-loop FES of the quadriceps muscle group are combined together to track a desired knee joint angle trajectory of flexion/extension movements. A nonlinear disturbance observer is used to estimate the torque provided by the subject’s muscles through the FES. Simulations and experiments with a healthy subject show the feasibility of the proposed approach. Experiments show the repeatability of motion and the complementarity between the torque provided by the quadriceps muscle through FES and the one delivered by the orthosis actuator to ensure satisfactory tracking of the desired trajectory.