Search

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 syncrgies 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 (inultimuscle synergies) are based primarily on supraspinal circuitry, whereas intramuscle synergies reftect spinal circuitry. Studies of intra- and multimuscle synergies promise a powerful tool for exploring changes in spinal and supraspinal circuitry across patient populations. [ABSTRACT FROM AUTHOR]

Key points: We used the idea of synergic control and the framework of the uncontrolled manifold hypothesis to explore the synergic control of a single muscle.Individual motor units in flexor digitorum superficialis formed two‐three groups (MU‐modes) with parallel changes in firing frequency, robust over force‐up and force‐down segments.There were strong force‐stabilizing synergies in the MU‐mode space during accurate cyclical force production.The results show, for the first time, that the idea of synergic control is applicable to individual muscles.The results suggest that segmental spinal mechanisms, such as recurrent inhibition and stretch reflex, probably play a major role in the synergic control of action. In the present study, for the first time, we have used the idea of synergic control and the framework of the uncontrolled manifold (UCM) hypothesis to test two hypotheses: (i) individual motor units are organized into stable groups (MU‐modes) with parallel scaling of firing rates with changes in the muscle force, and (ii) the gains of MU‐mode involvement co‐vary to stabilize the desired force magnitude. Young healthy subjects performed accurate cyclical force tracing tasks at 1 Hz by pressing with intermediate phalanges; only one finger was instructed to produce force at a time. Surface electromyographic signal from flexor digitorum superficialis was recorded and used to identify individual motor units and their firing frequencies. Principal component analysis with rotation and factor extraction was used to identify MU‐modes, which showed similar compositions over the force‐up and force‐down task segments. Inter‐cycle variance analysis in the MU‐mode space confirmed the existence of strong synergies stabilizing finger force. There were no synergies stabilizing MU‐mode magnitude in the space of individual motor units. This is the first application of the UCM framework to the neural control of a single muscle. It extends the applicability of this approach to analysis of individual muscles. We discuss the importance of the findings for the idea of hierarchical control and the notion of muscle compartments. The results suggest that segmental spinal mechanisms, such as recurrent inhibition and stretch reflex, probably play a major role in the synergic control of action. Key points: We used the idea of synergic control and the framework of the uncontrolled manifold hypothesis to explore the synergic control of a single muscle.Individual motor units in flexor digitorum superficialis formed two‐three groups (MU‐modes) with parallel changes in firing frequency, robust over force‐up and force‐down segments.There were strong force‐stabilizing synergies in the MU‐mode space during accurate cyclical force production.The results show, for the first time, that the idea of synergic control is applicable to individual muscles.The results suggest that segmental spinal mechanisms, such as recurrent inhibition and stretch reflex, probably play a major role in the synergic control of action. [ABSTRACT FROM AUTHOR]

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.