It has been hypothesized that the central nervous system simplifies the production of movement by limiting motor commands to a smalls et of modules known as muscle synergies. Recently, investigators have questioned whether a low-dimensional controller can produce the rich and flexible behaviors seen in everyday movements. We implemented muscle synergies in a biomechanically realistic human upper extremity model and performed computational experiments to determine whether synergies introduced task performance deficits, generalized to different movements, and facilitated the learning of new movements as occurs during motor skill acquisition.