We have developed a new power assist arm for handling heavy loads using large pneumatic muscles. We chose to use the pneumatic muscles from a viewpoint of safety, but large actuators were required for heavy loads. As a result, it was expected that control becomes difficult due to the increased air flow requirement. Therefore, to optimize the hardware, we used silicon rubber filled pneumatic muscles in order to reduce a flow of air. And we increase the total cross-sectional area of a flow. On the control side, we developed an original pneumatic muscle model and the torque control based on it. Furthermore, we developed an impedance control system and successfully implemented it as part of a working power assist arm for handling heavy loads.