Muscle synergy is defined as a combination of the limited number of muscle activities, which has been considered useful for controlling a large number of degrees of freedom in the musculoskeletal system. In previous studies, the robustness of muscle synergy recruitment across biomechanical tasks has been reported, i.e., the central nervous system controls a few pre-organized muscle synergies corresponding to a specific motor behavior. In contrast, the present study considers a different hypothesis wherein muscle synergy recruitment can be affected by muscle contraction. In our experiment, the subjects were instructed to perform an additional muscle contraction during the eight directional point-to-point reaching movement, on the basis of visual feedback of four levels of contraction. The result of extracted muscle synergies from electromyogram across contraction levels that as the level of contraction increases, so does the number of recruited muscle synergy. Moreover, we found a high similarity between shared muscle synergies-the first synergy under different levels of contraction. The result revealed the hypothesis that pre-organized synergies may be responding to the specific motor task but this coordination of existing muscle synergies can be adjusted by muscle contraction. This adjustment can be useful to develop the more complex myoelectric interfaces efficiently which could control not only arm motion but also arm force.