Intra-muscle Synergies Stabilizing Reflex-mediated Force Changes.

Area of Science:

• Neuroscience
• Motor Control
• Biomechanics

Background:

• Motor control research often focuses on how the nervous system coordinates muscles to produce desired movements.
• Understanding motor synergies is crucial for diagnosing and treating neurological disorders affecting movement.

Purpose of the Study:

• To investigate force-stabilizing synergies and motor equivalence using motor unit (MU) firing frequencies.
• To differentiate between spinal and supraspinal contributions to motor synergies.

Main Methods:

• Utilized the uncontrolled manifold hypothesis framework.
• Recorded surface electromyography (EMG) to identify individual MU action potentials during steady force production tasks (one-finger and three-finger presses).
• Analyzed MU firing frequencies and their groupings (MU-modes) in flexor digitorum superficialis (FDS) and extensor digitorum communis (EDC).

Main Results:

• Identified stable MU-modes with parallel firing frequency scaling in FDS and EDC, linked to reciprocal and coactivation commands.
• Observed greater motion in MU-modes that maintained force (motor equivalence) compared to those that changed force.
• Demonstrated that force changes were stabilized by co-varying MU-mode contributions in single-finger tasks.
• Found no force-stabilizing synergies in the three-finger task when analyzing individual finger forces.
• Noted hand dominance effects on multi-finger synergies but not intra-muscle synergies.

Conclusions:

• Spinal mechanisms (recurrent inhibition, proprioceptive reflex loops) significantly contribute to intra-muscle synergies.
• Multi-finger synergies are primarily mediated by supraspinal processes.
• The study provides a framework for examining altered motor synergies in neurological disorders.