This paper proposes the position tracking control for a 2-link manipulator with a bi-articular muscle. The 2-link manipulator with the bi-articular muscle is designed by using the epicycle gear. This 2-link manipulator has the redundancy, since this has the not only the mono-articular muscle but also the bi-articular muscle. Therefore, it is necessary to implement the particular control method for its redundancy. In order to solve this redundancy, phase different control (PDC), which is based on electromyogram of each muscle, has been reported. This paper proposes PDC combined with the workspace observer. By using the proposed method, the error between the force reference and the force response due to the linearization is compensated. In addition, the modeling errors are also compensated. As a result, the high precision motion control based on the musculoskeletal model of the upper arm is achieved. The effectiveness of the proposed method was confirmed by the simulation and experimental results.