Muscle synergy is the theory that movements are controlled by a module of coordinated combined muscles. This theory is thought to solve the degrees-of-freedom problem in the musculoskeletal system. Previous studies have investigated the robustness of muscle synergies under conditions such as varying speeds and required degrees of accuracy. One of the principles of human movement is that when movement becomes faster, spatial accuracy is reduced. This is called the "speed-accuracy trade-off" (SAT), and many models have been proposed to explain this phenomenon. Studies on muscle synergies have shown that muscle synergy modules are robust against changes in speed; however, the relationship between SAT and motor control by muscle synergies remains unclear. Therefore, we investigated the relationship between changes in spatial accuracy and changes in speed and muscle synergies from measured behavioral data and surface electromyography. This was achieved by performing an isometric contraction task in which subjects exerted a horizontal force with various movement speeds. The results showed that the module structures of muscle synergies were robust against speed changes, and that the neural commands to muscle synergies changed in response to speed changes. In addition, changes in spatial accuracy with variations in speed tended to increase when movement was performed with a single muscle synergy. These results suggest that the number of muscle synergies used for movement may affect movement accuracy.Clinical Relevance-The results of this study suggest that the number of muscle synergies used for movement affects spatial accuracy.