The assistive rehabilitation devices have been studied for over half-century. However; Assistive as needed (AAN) devices are emergent robots to aid the patients based on their muscle capability. Exposure of the patients to these devices demands more understanding of the human impedance and torque modulation. This paper investigates the impedance modulation of human's elbow joint based on optimal control and Computed Muscle Control (CMC) of the human forearm musculoskeletal systems. The human elbow is modeled as a single degree-of-freedom (DOF) system with an intrinsic passive stiffness and damping. The OpenSim program is exploited to derive the muscle activation and generate the resultant torque of the effective muscles around the elbow joint. Forearm tracks a sinusoidal pattern in the simulation. The consequent torque interpreted by the optimal control solution of the system which opens a new horizon in the assimilation of the human intention angle and elbow impedance in a trade-off for accurate tracking and energy consumption.