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Human motor unit activity during induced muscle cramp

B H Ross¹, C K Thomas

¹School of Physical Education, University of Otago, Dunedin, New Zealand.

Brain : a Journal of Neurology

|August 1, 1995

Summary

Muscle cramps involve increased and variable motor unit firing rates in specific calf muscles. Stretching the cramped muscle boosts activity, and a depressed tonic vibration reflex suggests altered neural control.

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Area of Science:

Neurology
Exercise Physiology
Muscle Physiology

Background:

Muscle cramps are common, involuntary contractions.
The precise physiological mechanisms underlying muscle cramps remain incompletely understood.
Previous research has not fully elucidated the role of motor unit activity during cramps.

Purpose of the Study:

To investigate motor unit activity during experimentally induced muscle cramps.
To explore the neural responses to stretching a cramped muscle.
To assess the impact of muscle cramps on the tonic vibration reflex (TVR).

Main Methods:

Muscle cramps were induced in the medial gastrocnemius using maximal voluntary contraction.
Surface and intramuscular electromyography recorded motor unit activity.
Muscle stretching and Achilles tendon vibration were employed to assess neural responses.

Main Results:

During cramps, motor unit firing rates were significantly higher and more variable compared to voluntary contractions.
Stretching the cramped muscle led to increased motor unit activity across triceps surae muscles.
The tonic vibration reflex was depressed or absent following muscle cramps, but not voluntary contractions.

Conclusions:

Motor units are actively involved in the generation of ordinary muscle cramps.
A positive feedback loop involving peripheral afferents and alpha motor neurons is a potential mechanism for muscle cramps.
Muscle cramps may involve altered neural excitability and reflex responses.

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