The Forces Generated by Agonist Muscles during Isometric Contractions Arise from Motor Unit Synergies

Area of Science:

• Neuroscience
• Motor Control
• Computational Biology

Background:

• The nervous system coordinates synergistic muscles through common synaptic inputs to motor neurons.
• Understanding these inputs is crucial for deciphering motor control strategies.

Purpose of the Study:

• To identify low-dimensional latent components, termed motor unit modes, in motor unit discharge rates.
• To investigate the distribution of these modes in knee extensor and hand muscles during isometric contractions.

Main Methods:

• Factor analysis was applied to motor unit discharge rates from vastus medialis, vastus lateralis, first dorsal interosseus, and thenar muscles.
• Simulations using integrate-and-fire neurons validated the accuracy of factor analysis in identifying motor unit modes.

Main Results:

• Two independent motor unit modes were identified, explaining most of the discharge rate covariance.
• Hand muscles exhibited distinct motor unit modes (75% muscle-specific), while vastii muscles showed a continuous distribution with shared inputs (45-60% muscle-specific).
• Coherence in discharge rates between motor unit pools was linked to shared synaptic inputs.

Conclusions:

• Correlated motor unit discharge rates arise from at least two independent sources of common input to motor neurons innervating synergistic muscles.
• The nervous system employs distinct control strategies for hand and leg muscles, with greater integration observed in knee extensors.