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Related Concept Videos

Motor Unit Stimulation01:20

Motor Unit Stimulation

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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.
The latent period of contraction marks the onset of excitation-contraction coupling, when the action potential propagates across the sarcolemma, preparing the muscle fibers for contraction. As the fibers enter the contraction phase, the...
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Muscle Stimulation Frequency01:22

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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 in Skeletal Muscles01:20

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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.
When an action...
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Isotonic and Isometric Muscle Contractions01:22

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Two primary types of muscle contractions are isotonic and isometric, each serving unique functions and involving distinct mechanisms. Both isotonic and isometric contractions are integral to the body's complex system of movement and stability. Isotonic exercises contribute significantly to functional strength and movement, while isometric contractions are crucial for maintaining posture and joint stability.
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Relaxation of Skeletal Muscles01:29

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The period of muscle contraction primarily influences the duration of stimulation at the neuromuscular junction (NMJ), the presence of free calcium ions in the sarcoplasm, and the availability of energy or ATP to support contractions.
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Generation of Action Potential in Skeletal Muscles01:24

Generation of Action Potential in Skeletal Muscles

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Every cell in the body maintains a membrane potential due to an uneven distribution of positive and negative charges across its plasma membrane. The membrane potential is measured in millivolts and quantifies the difference in charge across the membrane.
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Related Experiment Video

Updated: Apr 16, 2026

In Vivo Measurement of Hindlimb Dorsiflexor Isometric Torque from Pig
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Torque decrease during submaximal evoked contractions of the quadriceps muscle is linked not only to muscle fatigue.

Boris Matkowski1, Romuald Lepers2, Alain Martin2

  • 1Laboratoire INSERM U1093, Cognition, Action et Plasticité Sensorimotrice, Université de Bourgogne, Faculté des Sciences du Sport, Dijon, France boris.matkowski@u-bourgogne.fr.

Journal of Applied Physiology (Bethesda, Md. : 1985)
|March 14, 2015
PubMed
Summary

Submaximal electrical muscle stimulation (EMS) causes significant quadriceps torque loss, not just from muscle fatigue, but also reduced motor unit recruitment due to altered nerve excitability.

Keywords:
electrical stimulationfemoral nerve stimulationisometric contractiontorque-frequency relation

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Area of Science:

  • Neuromuscular Physiology
  • Exercise Physiology
  • Motor Control

Background:

  • Electrical muscle stimulation (EMS) is used to induce muscle contractions.
  • Understanding the mechanisms behind torque decrease during submaximal EMS is crucial for optimizing training and rehabilitation.
  • Previous research suggests muscle fatigue is a primary factor, but neural contributions are less understood.

Purpose of the Study:

  • To investigate the neuromuscular mechanisms responsible for torque reduction during submaximal quadriceps EMS.
  • To test the hypothesis that reduced motor unit (MU) recruitment, due to altered axonal excitability, contributes to torque loss.
  • To differentiate between muscular and neural factors influencing torque output during EMS.

Main Methods:

  • Two experiments were conducted on 20 healthy men.
  • Experiment 1 assessed superimposed and resting twitches during EMS (n=9).
  • Experiment 2 analyzed twitch response and torque-frequency relationships of EMS-activated MUs (n=11), using femoral nerve stimulation and analyzing torque loss during repeated EMS contractions (50 Hz, 6s on/6s off) eliciting 20% maximal voluntary contraction (MVC) torque.

Main Results:

  • Torque decreased by approximately 60% during EMS-evoked contractions, compared to 18% during MVCs.
  • A rightward shift in the torque-frequency relationship of activated MUs was observed.
  • An increased ratio between superimposed and post-tetanic twitches during EMS indicated altered neural activation.

Conclusions:

  • Torque reduction during submaximal EMS involves both muscular mechanisms and a decrease in the number of recruited MUs.
  • Changes in axonal excitability threshold contribute to the reduced MU recruitment observed during EMS.
  • These findings highlight the importance of considering neural factors in the response to electrical muscle stimulation.