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.

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.