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Discharge patterns in human motor units during fatiguing arm movements
L Griffin1, S J Garland, T Ivanova
1Graduate Program in Neuroscience, University of Western Ontario, London, Canada N6G 1H1.
Journal of Applied Physiology (Bethesda, Md. : 1985)
|November 6, 1998
Summary
Short interspike intervals (ISIs) occur during voluntary movement and increase significantly with muscle fatigue. This study investigated their prevalence and correlation with motor unit recruitment during elbow movements.
Area of Science:
- Neuroscience
- Human Physiology
- Motor Control
Background:
- Motor unit discharge patterns are crucial for understanding muscle force generation.
- Short interspike intervals (ISIs) are indicative of high motor unit firing rates.
- The occurrence and significance of short ISIs during voluntary contractions, especially under fatigue, require further investigation.
Purpose of the Study:
- To determine if short ISIs (<20 ms) naturally occur during voluntary arm movements.
- To assess whether the number of short ISIs increases with muscle fatigue.
- To explore the relationship between short ISIs and motor unit recruitment thresholds.
Main Methods:
- Electromyography was used to record activity from 34 triceps brachii motor units in nine subjects.
- A fatiguing task involving repeated elbow flexion and extension movements against a constant load was performed.
- Analysis focused on the percentage of short ISIs relative to total ISIs during unfatigued and fatigued states.
Main Results:
- Short ISIs were observed during voluntary movement, particularly at movement initiation.
- The percentage of short ISIs increased from 7.1% in the unfatigued state to 11.8% in the fatigued state for initially recruited motor units.
- For motor units recruited during fatigue, 16.4% of ISIs were short.
- A strong positive correlation (r² = 0.85) was found between the number of short ISIs and recruitment threshold torque.
Conclusions:
- Short interspike intervals are a natural phenomenon during voluntary muscle activation.
- Muscle fatigue leads to a significant increase in the occurrence of short ISIs.
- The findings suggest that short ISIs are related to motor unit recruitment properties and may reflect altered motor control strategies under fatigue.