This paper focuses on the feedforward position control for a musculoskeletal system which has two links, two joints and six muscles. This feedforward control method utilizes the resultant torque generated by inputting the internal force among muscles balancing at a desired posture. Therefore, any sensory feedback is not necessary for positioning. The motion convergence is extremely sensitive, namely it strongly depends on muscular arrangement of a musculoskeletal system. Focusing on the potential filed generated by the input of internal force, this paper analyzes the motion convergence from the quasistatic viewpoint. In this analysis, the muscular lengths along with the joint angles are approximated by Taylor expansion. After demonstration of the reliability of the approximation, Hessian matrix of the potential field is assessed to clarify the quasistatic condition of the motion convergence at a desired posture.