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Transitory force decrease following a sudden reduction in stimulation frequency in motor units of rat medial

Joanna Grzesiak1, Katarzyna Kryściak1, Hanna Drzymała-Celichowska1

  • 1Department of Neurobiology, Poznan University of Physical Education, Poland.

Journal of Electromyography and Kinesiology : Official Journal of the International Society of Electrophysiological Kinesiology
|March 17, 2019
PubMed
Summary

Researchers discovered a new phenomenon in rat muscle contractions: a temporary force drop when stimulation frequency decreases. This effect, most pronounced in fast-resistant motor units, may stem from collagen disturbances around muscle fibers.

Keywords:
Force regulationMotor unitRate codingStimulation frequencyUnfused tetanus

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Area of Science:

  • Muscle Physiology
  • Motor Unit Behavior

Background:

  • Motor units generate force through tetanic contractions.
  • Stimulation frequency influences muscle force output.

Purpose of the Study:

  • To investigate the effects of decreased stimulation frequency on motor unit force.
  • To characterize a novel transitory force decrease phenomenon.

Main Methods:

  • Studied 161 single motor units (S, FR, FF) in rat medial gastrocnemius.
  • Evoked unfused tetanic contractions using three-phase stimuli (low-high-low frequency).
  • Analyzed force recordings during variable fusion degree tetani.

Main Results:

  • A transitory force decrease was observed in 78 units during the third phase of tetanus.
  • This phenomenon was most frequent in fast fatigue-resistant (FR) units (65.9%).
  • The force decrease was strongest in FR units (up to 36.5%) and prominent in middle-fused tetani.

Conclusions:

  • A new phenomenon of transitory force decrease in motor unit tetanic contractions was identified.
  • The effect is likely linked to impaired force transmission by perimysial collagen.
  • Fast fatigue-resistant motor units are particularly susceptible to this frequency-dependent force drop.