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Related Concept Videos

Motor Unit Stimulation01:20

Motor Unit Stimulation

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...
Muscle Stimulation Frequency01:22

Muscle Stimulation Frequency

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
At low firing rates, motor neurons induce individual twitch contractions in muscle fibers. These twitches...
Excitation-Contraction Coupling in Skeletal Muscles01:20

Excitation-Contraction Coupling in Skeletal Muscles

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 potential...
Tetanus01:29

Tetanus

Tetanus is a life-threatening neurological disorder characterized by persistent muscle contractions and spastic paralysis. It is caused by Clostridium tetani, a motile, Gram-positive, rod-shaped, obligate anaerobe. These bacteria produce terminal endospores, giving them a distinctive “lollipop” or “tennis-racket” appearance. They thrive in anaerobic environments, such as those found in deep puncture wounds.Once introduced into the body, the spores germinate into vegetative cells. These cells...
Isotonic and Isometric Muscle Contractions01:22

Isotonic and Isometric Muscle Contractions

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.
Isotonic contractions
Isotonic contractions occur when a muscle changes length while the...
Relaxation of Skeletal Muscles01:29

Relaxation of Skeletal Muscles

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.
When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated sodium channels. Sodium ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated calcium channels to open.

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Related Experiment Video

Updated: Jun 23, 2026

In Vivo Measurement of Hindlimb Dorsiflexor Isometric Torque from Pig
09:41

In Vivo Measurement of Hindlimb Dorsiflexor Isometric Torque from Pig

Published on: September 3, 2021

Evoked tetanic torque and activation level explain strength differences by side.

Chandramouli Krishnan1, Glenn N Williams

  • 1Graduate Program in Physical Therapy and Rehabilitation Science, University of Iowa, Iowa City, IA 52242-1190, USA.

European Journal of Applied Physiology
|April 28, 2009
PubMed
Summary
This summary is machine-generated.

Healthy young adults show about 10% knee strength differences between legs. Quadriceps activation and evoked torque explain most of this strength disparity, suggesting both central and peripheral factors contribute to variations in muscle strength.

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Last Updated: Jun 23, 2026

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Area of Science:

  • Biomechanics
  • Human Physiology
  • Neuromuscular Function

Background:

  • Healthy young individuals exhibit approximately 10% side-to-side differences in knee extensor strength.
  • The underlying physiological mechanisms driving these inter-limb strength disparities remain incompletely understood.

Purpose of the Study:

  • To investigate the contributions of voluntary quadriceps activation, concurrent antagonistic hamstrings activity, and electrically evoked torque to side-to-side knee extensor strength differences.
  • To determine if central and/or peripheral factors account for inter-limb strength variations.

Main Methods:

  • Twenty-two healthy volunteers participated in the study.
  • Knee extensor strength, voluntary activation (using interpolated twitch technique), and electrically evoked torque were measured for both legs.
  • Antagonistic hamstrings co-activation levels were also assessed.

Main Results:

  • Inter-limb differences in quadriceps voluntary activation and electrically evoked torque collectively explained 69% of the observed side-to-side strength variations.
  • Antagonistic hamstrings activity did not significantly contribute to the strength differences between limbs.

Conclusions:

  • Both central (voluntary activation) and peripheral (evoked torque) mechanisms play a role in inter-limb knee extensor strength differences.
  • These findings provide insight into the neuromuscular factors underlying asymmetries in human limb strength.