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: 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.
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