Biological systems are highly nonlinear, have at least one pair of muscles actuating every degree of freedom at any joint, and possess neural transmission delays in their feedforward (efferent) and feedback (afferent) paths. This nonlinear time delay system, involved in movement and continuous interaction with the environment, is precise, stable, and very adaptive. The problem of trajectory tracking control of a three-link sagittal model of the shank, thigh and trunk is considered in this study. The controller is synthesized using the Q-parameterization method of controller design, for the class of stable nonlinear systems. The "free design" parameter is chosen to achieve certain performance and robustness objectives, the standard approach in H/sup /spl infin// control design. Performance of the system to a class of reference inputs and robustness issues pertaining to neglected nonlinearities, unmodeled dynamics and uncertainties in time delays, are addressed in the problem. Simulations are conducted to test the performance and feasibility of the controller for the task of squatting.