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Motor-unit activity differs with load type during a fatiguing contraction.

Carol J Mottram1, Jennifer M Jakobi, John G Semmler

  • 1Deptartment of Integrative Physiology, University of Colorado, Boulder, CO 80309-0354, USA.

Journal of Neurophysiology
|October 16, 2004
PubMed
Summary
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During fatiguing contractions, the position task led to greater increases in spinal cord motor output compared to the force task, despite similar muscle activity. This suggests different neural control strategies are employed.

Area of Science:

  • Neuroscience
  • Human Physiology
  • Motor Control

Background:

  • Previous research indicates varying physiological responses during fatiguing contractions with different load types, despite similar electromyographic (EMG) activity.
  • Understanding motor unit discharge characteristics offers direct insight into spinal cord motor output during fatiguing tasks.

Purpose of the Study:

  • To compare the discharge characteristics of single motor units in the biceps brachii during two distinct fatiguing contraction types: a force task and a position task.
  • To investigate the differences in motor output from the spinal cord under isometric force versus position control during fatiguing contractions.

Main Methods:

  • Single motor unit discharge in the biceps brachii was recorded during fatiguing isometric force and position tasks.

Related Experiment Videos

  • Subjects maintained either constant upward force or constant elbow angle at a submaximal target force (22.2% MVC).
  • Dependent variables included motor unit discharge rate, motor output fluctuations, cardiovascular responses, and perceived exertion.

    • Main Results:

    • Despite similar increases in average EMG activity, the position task showed greater increases in mean arterial pressure, perceived exertion, and motor output fluctuations.
    • Motor unit discharge rate declined faster, and its variability increased significantly only during the position task.
    • A greater number of motor units were recruited during the position task compared to the force task.

    Conclusions:

    • The position task elicits a greater rate of increase in spinal cord motor output compared to the force task, even with similar net muscle torque.
    • These findings highlight distinct neural control strategies and greater central command involvement during position-controlled fatiguing contractions.
    • The results provide direct evidence of differences in motor unit recruitment and discharge patterns under varying task demands during fatigue.