In this paper, a new posture prediction method is proposed and evaluated on human leg, as being a physiologically constrained three-link arm. The main posture prediction solution is focused on optimizing the manipulability and the human performance of the leg. The forward kinematics is used to define the feasible workspace of the human leg in sagittal plane. Using an effective optimization-based human performance measure that incorporates a new objective function of musculoskeletal discomfort, the optimal posture is obtained.