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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.
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In skeletal muscles, acetylcholine is released by nerve terminals at the motor endplate—the point of synaptic communication between motor neurons and muscle fibers. The binding of acetylcholine to its receptors on the sarcolemma allows entry of sodium ions into the cell and triggers an action potential in the muscle cell. Thus, electrical signals from the brain are transmitted to the muscle. Subsequently, the enzyme acetylcholinesterase breaks down acetylcholine to prevent excessive...
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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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Corticospinal Excitability During Explosive Voluntary Contractions and Its Association With Rapid Torque Production.

Federico Castelli1, Omar S Mian1, Adam Bruton1,2

  • 1School of Life and Health Sciences, Whitelands College, University of Roehampton, London, UK.

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Summary

Corticospinal excitability and inhibition increase during explosive contractions, influencing rapid torque production. These neural factors are key determinants of explosive voluntary muscle power.

Keywords:
corticospinal excitabilitycorticospinal inhibitionexplosive strengthmotor‐evoked potentialmuscle activationsilent period durationtranscranial magnetic stimulation

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Area of Science:

  • Neuroscience
  • Human Physiology
  • Motor Control

Background:

  • Understanding the neural mechanisms underlying explosive voluntary contractions is crucial for optimizing athletic performance and rehabilitation.
  • The relationship between corticospinal excitability and inhibition during rapid torque development remains incompletely understood.

Purpose of the Study:

  • To investigate the associations between rapid torque production and corticospinal excitability (motor-evoked potential; MEP) and inhibition (silent period duration; SPD) during explosive contractions.
  • To examine how MEP and SPD change across different phases of explosive contractions and at maximum voluntary contraction (MVC) plateau.

Main Methods:

  • 14 adult participants performed isometric knee-extensor explosive contractions.
  • Transcranial magnetic stimulation (TMS) was used to measure MEP and SPD in the quadriceps muscle during early, middle, and late phases, and at MVC plateau.
  • Torque was measured concurrently, and statistical analyses included repeated measures correlation, Spearman rho correlations, and linear mixed effects models.

Main Results:

  • MEP and torque were correlated within participants during the early phase of explosive contractions (r=0.43, p<0.001).
  • MEP and torque were significantly correlated across participants during the middle phase (rho=0.73, p=0.004).
  • Both absolute MEP, normalized MEP, and SPD increased significantly across contraction phases up to MVC plateau (p<0.025).

Conclusions:

  • Corticospinal excitability appears to be a significant factor in determining rapid torque generation during explosive contractions.
  • Both corticospinal inhibition and excitability progressively increase throughout the torque-time curve, reaching their peak at MVC plateau.