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Motor unit spike counts before and after maximal voluntary contraction.
J L Gooch1, B Y Newton, J H Petajan
1University of Utah Health Sciences Center, Physical Medicine and Rehabilitation, Salt Lake City 84132.
Muscle & Nerve
|December 1, 1990
Summary
After maximal voluntary contraction (MVC), the brain reduces motor unit activation by about 50% to maintain posture. This finding reveals a neural feedback mechanism during muscle fatigue.
Area of Science:
- Neuroscience
- Human Physiology
- Motor Control
Background:
- Sustaining a posture requires continuous motor unit activation.
- Muscle fatigue is known to alter motor control strategies.
- The precise neural adaptations to fatigue during postural tasks are not fully understood.
Purpose of the Study:
- To investigate changes in motor unit discharge rates during sustained posture after maximal voluntary contraction (MVC).
- To examine the relationship between motor unit activity and electromyographic (EMG) signals post-fatigue.
- To explore potential compensatory mechanisms in synergistic muscles.
Main Methods:
- Monopolar needle electrodes recorded motor unit spike counts in the biceps brachii.
- Measurements were taken during antigravity posture maintenance (45-degree elbow flexion) before and after MVC.
- Surface EMG recorded activity in synergistic muscles simultaneously.
Main Results:
- Motor unit spike counts decreased by approximately 50% after MVC.
- Surface rectified integrated EMG activity showed a parallel decline.
- Synergistic muscles exhibited similar reductions in motor unit activity.
Conclusions:
- Muscle fatigue leads to a significant reduction in motor unit discharge rates for postural maintenance.
- Reduced motor unit activation post-MVC is likely due to decreased muscle fiber contraction-relaxation rates.
- A neural feedback system from the muscle to the central nervous system (CNS) may regulate this adaptive response.