Synergic control of a single muscle: The example of flexor digitorum superficialis.

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.

Contribution from motor unit firing adaptations and muscle coactivation during fatigue.

The control of motor unit firing behavior during fatigue is still debated in the literature. Most studies agree that the central nervous system increases the excitation to the motoneuron pool to compensate for decreased force contributions of individual motor units and sustain muscle force output during fatigue. However, some studies claim that motor units may decrease their firing rates despite increased excitation, contradicting the direct relationship between firing rates and excitation that governs the voluntary control of motor units. To investigate whether the control of motor units in fact changes with fatigue, we measured motor unit firing behavior during repeated contractions of the first dorsal interosseous (FDI) muscle while concurrently monitoring the activation of surrounding muscles, including the flexor carpi radialis, extensor carpi radialis, and pronator teres. Across all subjects, we observed an overall increase in FDI activation and motor unit firing rates by the end of the fatigue task. However, in some subjects we observed increases in FDI activation and motor unit firing rates only during the initial phase of the fatigue task, followed by subsequent decreases during the late phase of the fatigue task while the coactivation of surrounding muscles increased. These findings indicate that the strategy for sustaining force output may occasionally change, leading to increases in the relative activation of surrounding muscles while the excitation to the fatiguing muscle decreases. Importantly, irrespective of changes in the strategy for sustaining force output, the control properties regulating motor unit firing behavior remain unchanged during fatigue. NEW & NOTEWORTHY This work addresses sources of debate surrounding the manner in which motor unit firing behavior is controlled during fatigue. We found that decreases in the motor unit firing rates of the fatiguing muscle may occasionally be observed when the contribution of coactive muscles increases. Despite changes in the strategy employed to sustain the force output, the underlying control properties regulating motor unit firing behavior remain unchanged during muscle fatigue.