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Introduction: Utilizing Functional Electrical Stimulation (FES) and rehabilitation robots for motion control is an open research problem. In this paper, a new control algorithm has been proposed which was de-signed based on a combination of FES and an active mechanical actuator to control the knee joint movement. Methods: An adaptive controller and a Proportional-Derivative (PD) controller have adjusted the mo-tor torque and stimulation intensity, respectively. The FES controller was activated whenever a dis-turbance observer detected the presence of the external disturbance. In this manner, the occurrence of the muscle fatigue arises from the FES can be postponed. Results: The simulation studies were carried out on a model of muscle-joint system along with a model of a servo-motor. The computed RMS of the tracking errors compared to the range of knee motion show that the tracking performance is acceptable. In this research, the trajectories envisioned as the knee joint reference trajectory were designed using the recorded human data. Conclusion: The achieved results prove the ability of the proposed control strategy to not only reject the external disturbance but also compensate the muscle fatigue.