Evidence for basic units of upper limb muscle synergies underlying a variety of complex human manipulations

Manipulations require complex upper limb movements in which the central nervous system (CNS) must deal with many degrees of freedom. Evidence suggests that the CNS utilizes motor primitives called muscle synergies to simplify the production of movements. However, the exact neural mechanism underlying muscle synergies to control a wide array of manipulations is not fully understood. Here, we tested whether there are basic units of muscle synergies that can explain a diverse range of manipulations. We measured the electromyographic activities of 20 muscles across the shoulder, elbow, and wrist and fingers during 24 manipulation tasks. As a result, nonnegative matrix factorization identified nine basic units of muscle synergies derived from the upper limb muscles that are shared across all tasks. The high similarity between muscle synergies of each of the 24 tasks and various combinations of nine basic unit muscle synergies in a single and/or merging state provides evidence that the CNS flexibly selects and modifies the degree of contribution of the nine basic units of muscle synergies to overcome different mechanical demands of tasks.NEW & NOTEWORTHY We expanded upon experiments that investigated motor modularity in upper limb movements in humans. The identification of modular features, including distinct functional muscle weightings, during highly variable manipulation tasks supports a hypothetical neural mechanism in which the CNS combines preexisting basic patterns of muscle synergies rather than framing new patterns to deal with behavioral diversity in the upper limb.