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Related Experiment Videos

Servo action in the human thumb.

C D Marsden, P A Merton, H B Morton

    The Journal of Physiology
    |May 1, 1976
    PubMed
    Summary
    This summary is machine-generated.

    Human thumb muscles exhibit servo-like properties, automatically adjusting activity within 50ms to counteract unexpected movements. This automatic muscle response, driven by the stretch reflex, scales with load and effort, compensating for fatigue.

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    Area of Science:

    • Neuroscience
    • Human Physiology
    • Biomechanics

    Background:

    • Muscle control involves complex feedback mechanisms.
    • The flexor pollicis longus muscle controls thumb flexion.
    • Understanding muscle servo properties is key to motor control research.

    Purpose of the Study:

    • To investigate the servo-like properties of human muscle.
    • To determine the latency and characteristics of muscle responses to perturbations.
    • To explore the relationship between muscle activation, load, and servo gain.

    Main Methods:

    • Subjects performed standardized thumb flexion tracking tasks against controlled torque loads.
    • Unexpected perturbations (halts, releases, stretches) were introduced during movements.

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  • Muscle activity was recorded via surface electromyography (EMG) and analyzed.
  • Voluntary reaction times were measured to differentiate reflex from voluntary responses.

    • Main Results:

    • Muscle activity adjusted approximately 50ms after perturbations to counteract them.
    • This rapid response, termed servo action, is attributed to the stretch reflex.
    • Servo gain increased proportionally with initial load and muscle activation level.
    • Fatigued muscle showed increased gain to maintain performance.

    Conclusions:

    • Human muscle exhibits rapid, automatic servo-like control based on the stretch reflex.
    • Servo gain is modulated by muscle activation and external load.
    • The findings challenge theories suggesting stretch reflex initiation of contractions in relaxed muscle.