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Fatigue effects on motor unit activity during submaximal contractions.
R A Conwit1, D Stashuk, H Suzuki
1Department of Neurology, Johns Hopkins Bayview Medical Center, Baltimore, MD 21224, USA.
Archives of Physical Medicine and Rehabilitation
|September 15, 2000
Summary
During submaximal contractions, the body recruits additional motor units to maintain strength as fatigue develops. These motor unit changes, including increased size and activation, occur early, especially after prior exertion.
Area of Science:
- Neuromuscular Physiology
- Exercise Science
- Motor Control
Background:
- Understanding motor unit behavior is crucial for comprehending muscle fatigue.
- Fatigue impacts motor unit recruitment and firing patterns.
- Previous research has explored changes in motor unit firing rates during fatigue.
Purpose of the Study:
- To investigate alterations in motor unit characteristics during the onset of fatigue.
- To analyze changes in motor unit size and firing rate during sustained submaximal contractions.
Main Methods:
- Utilized automated decomposition-enhanced spike-triggered averaging.
- Characterized motor unit size (S-MUAP amplitude) and firing rate.
- Measured force, including maximal voluntary contraction (MVC), in the vastus medialis.
Main Results:
- Increased surface electromyogram and S-MUAP amplitudes were observed during 10% and 30% MVC contractions.
- Mean firing rates decreased, while the motor unit index (indicating pool activation) increased.
- These changes suggest recruitment of new and larger motor units to sustain contractions, with earlier onset after repeated efforts.
Conclusions:
- Submaximal fatiguing contractions lead to the activation of additional motor units to preserve muscle strength.
- Motor unit recruitment and size changes initiate early in the fatigue process, within the first minute.
- Prior fatiguing episodes can accelerate these neuromuscular adaptations.