Recently, a number of studies have focused on the musculoskeletal systems of the human arm and foot with the aid of human-machine systems. The studies aim to evaluate the spec of manipulators or analyze the characteristic of the human. We focus on the fact that some postures reduce the effort necessary for the application of force from the musculoskeletal systems. In this research, we aim to apply the systems to the power assist system. Using a musculoskeletal model, we propose the Compensation Considering Posture (CCP) method, which adjusts the assist ratio, and consequently the applied force, in accordance with the arm position of the operator. Then we applied it to a standard valve used in fire fighting operations as the one link manipulator. In this regard, operation responsiveness changes when the output force is reduced with a correct arm position. CCP improves valve maneuverability by providing additional assistance in such cases, and ensures that operation responsiveness is uniform regardless on arm position. In conducting an experimental verification of the proposed method, first we measured the arm position of the operator in real time by using a musculoskeletal simulator. Next, we quantitatively defined the reduction of the output force according to the arm position as an output force level. Finally, we determined the appropriate amount of compensation by normalizing the applied force with the maximum output force level. The effectiveness of this method was experimentally confirmed using visual tracking.