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Related Experiment Videos

Motor unit activation patterns during isometric, concentric and eccentric actions at different force levels.

V Linnamo1, T Moritani, C Nicol

  • 1Neuromuscular Research Center, Department of Biology of Physical Activity, University of Jyväskylä, Jyväskylä, Finland. linnamo@sport.jyu.fi

Journal of Electromyography and Kinesiology : Official Journal of the International Society of Electrophysiological Kinesiology
|December 19, 2002
PubMed
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Motor unit activation differs between concentric and eccentric muscle actions. Eccentric actions may recruit fast motor units at lower forces, while concentric and isometric actions recruit them at higher forces.

Area of Science:

  • Exercise Physiology
  • Motor Control
  • Biomechanics

Background:

  • Understanding motor unit recruitment is crucial for optimizing training and rehabilitation strategies.
  • Distinct activation patterns exist for concentric and eccentric muscle contractions, influencing force production and muscle adaptation.

Purpose of the Study:

  • To investigate and compare motor unit activation patterns during concentric and eccentric forearm flexion at various force levels.
  • To analyze the influence of isometric preactivation on motor unit recruitment during dynamic contractions.

Main Methods:

  • Eight male subjects performed forearm flexions at 20%, 40%, 60%, and 80% of maximal voluntary contraction (MVC).
  • Electromyography (EMG) was recorded from the biceps brachii during isometric, concentric, and eccentric actions, with and without preactivation.

Related Experiment Videos

  • Intramuscular spike-amplitude frequency (ISAF) analysis was used to assess motoneuron pool activity.

    • Main Results:

    • Mean spike amplitude increased with force in isometric and concentric actions, but plateaued earlier in eccentric actions.
    • Mean spike frequency was generally higher in eccentric actions compared to concentric actions at lower force levels when no preactivation was used.
    • Eccentric actions showed higher mean spike amplitude at lower forces without preactivation, suggesting selective fast motor unit recruitment.

    Conclusions:

    • The recruitment threshold for motor units may be lower during dynamic (concentric and eccentric) contractions compared to isometric contractions.
    • Eccentric contractions appear to rely more on increasing the firing rate of active motor units to achieve higher forces.
    • Selective activation of fast motor units might occur at lower force levels during eccentric actions, particularly without prior isometric contraction.