Intramuscle Synergies: Their Place in the Neural Control Hierarchy
in Motor ControlClick name to view affiliation
Search for other papers by Mark L. Latash in
Current site
Google Scholar
PubMed
Search for other papers by Shirin Madarshahian in
Current site
Google Scholar
PubMed
Clinical and Translational Science Institute, Penn State College of Medicine, Hershey, PA, USA
Search for other papers by Joseph M. Ricotta in
Current site
Google Scholar
PubMed
We accept a definition of synergy introduced by Nikolai Bernstein and develop it for various actions, from those involving the whole body to those involving a single muscle. Furthermore, we use two major theoretical developments in the field of motor control—the idea of hierarchical control with spatial referent coordinates and the uncontrolled manifold hypothesis—to discuss recent studies of synergies within spaces of individual motor units (MUs) recorded within a single muscle. During the accurate finger force production tasks, MUs within hand extrinsic muscles form robust groups, with parallel scaling of the firing frequencies. The loading factors at individual MUs within each of the two main groups link them to the reciprocal and coactivation commands. Furthermore, groups are recruited in a task-specific way with gains that covary to stabilize muscle force. Such force-stabilizing synergies are seen in MUs recorded in the agonist and antagonist muscles but not in the spaces of MUs combined over the two muscles. These observations reflect inherent trade-offs between synergies at different levels of a control hierarchy. MU-based synergies do not show effects of hand dominance, whereas such effects are seen in multifinger synergies. Involuntary, reflex-based, force changes are stabilized by intramuscle synergies but not by multifinger synergies. These observations suggest that multifinger (multimuscle synergies) are based primarily on supraspinal circuitry, whereas intramuscle synergies reflect spinal circuitry. Studies of intra- and multimuscle synergies promise a powerful tool for exploring changes in spinal and supraspinal circuitry across patient populations.
Ricotta https://orcid.org/0000-0001-5902-5964
Latash (mll11@psu.edu) is corresponding author, https://orcid.org/0000-0003-3102-2571
© 2023 Human Kinetics