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Related Concept Videos

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When the neuron of a motor unit fires an action potential, it triggers a series of events, leading to a twitch contraction in the muscle fibers. The process of excitation-contraction coupling is crucial in relaying the action potential to the muscle fibers.
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The contraction strength of muscles is regulated by motor neurons, which modulate the frequency of action potentials dispatched to the motor units based on the body's requirements. This process of varying the muscle stimulation frequency allows muscles to contract with a force that is precisely tailored to the needs of the moment, whether lifting a feather or a heavy box.
Wave summation
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Excitation-contraction coupling is a series of events that occur between generating an action potential and initiating a muscle contraction. It occurs at the triad, a structure found in skeletal muscle fibers that comprise a T-tubule and terminal cisternae of the sarcoplasmic reticulum on each side. These triads are visible in longitudinally sectioned muscle fibers. They are typically located at the A-I junction — the junction between the A and I bands of the sarcomere.
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Related Experiment Video

Updated: Feb 20, 2026

Assessment of Neuromuscular Function Using Percutaneous Electrical Nerve Stimulation
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Voluntary activation and variability during maximal dynamic contractions with aging.

Vianney Rozand1, Jonathon W Senefeld1, Hamidollah Hassanlouei1

  • 1Exercise Science Program, Department of Physical Therapy, Marquette University, P.O. Box 1881, Milwaukee, WI, 53201, USA.

European Journal of Applied Physiology
|October 24, 2017
PubMed
Summary

Older adults show similar average voluntary activation but more variable motor cortex and muscle activation during maximal contractions. This increased variability in activation may signal subclinical central nervous system changes in aging.

Keywords:
ConcentricEccentricElbow flexor musclesElectrical stimulationSex differencesTranscranial magnetic stimulation

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Area of Science:

  • Neurology
  • Exercise Physiology
  • Gerontology

Background:

  • Age-related declines in maximal muscle torque are well-documented.
  • The contribution of reduced supraspinal activation to these declines, particularly during dynamic contractions, remains unclear.

Purpose of the Study:

  • To investigate age-related differences in voluntary activation and its variability.
  • To assess activation using transcranial magnetic stimulation (TMS) and electrical muscle stimulation.
  • To compare activation during maximal isometric, concentric, and eccentric contractions.

Main Methods:

  • Thirty young and 31 older adults performed maximal voluntary contractions (MVCs) of elbow flexors.
  • Voluntary activation was assessed via superimposed TMS and electrical stimulation.
  • Contractions included isometric, concentric, and eccentric actions at specific velocities and angles.

Main Results:

  • Older adults exhibited lower MVC torque across all contraction types compared to young adults.
  • No significant age-related differences were found in average voluntary activation assessed by TMS or electrical stimulation.
  • Older adults demonstrated greater variability in voluntary activation and increased antagonist coactivation during dynamic contractions.

Conclusions:

  • Average voluntary activation does not decline with age, but its predictability decreases.
  • Increased variability in supraspinal activation during maximal contractions in older adults may indicate subclinical central nervous system changes.
  • These findings suggest that motor control variability, not reduced activation capacity, may contribute to age-related strength loss.