Intramuscle Synergies: Their Place in the Neural Control Hierarchy

Abstract

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.

Electromyographic signature of isometric squat in the highest refuge in Europe

“…Further studies investigating intra-muscle synergies will allow to depict whether the motor adjustments during isometric squat in hypoxic conditions selectively affect, if any, either supraspinal or spinal circuitry. 25 Another interpretation can be based on the fact that humans at high altitude could not support the same muscle activation as at low altitude, after a repeated stress (i.e., trekking) on muscles. Following these interpretations, further studies should focus on measuring EMG for comparing 1) exercises on single isolated muscle groups vs whole body exercises, and 2) exercises after a trek in normoxia vs hypoxia.…”

Mini-review: Biological Movements as Objects of Natural Science

“…In particular, during multi-finger tasks, MU-mode synergies stabilized reflex changes in muscle force, while analysis in the space of finger forces did not show such synergies 21 . On the other hand, effects of dominance have been described for multi-finger synergies but not for multi-MU-mode synergies 22 . At the other end of the spectrum, analysis of performancestabilizing synergies at the task level using mechanical variables reflecting the R and C commands has documented such synergies over a range of tasks, from single-finger pressing to whole-body standing 23,24 .…”