Assessing voluntary muscle activation with the twitch interpolation technique

Area of Science:

• * Human physiology
• * Neuromuscular function
• * Exercise science

Background:

• * Twitch interpolation is a common technique to assess skeletal muscle activation during voluntary contractions.
• * Previous studies suggested full muscle activation in healthy adults, but recent sensitive applications reveal incomplete activation even with maximal effort.
• * Discrepancies exist in quantifying voluntary activation using twitch interpolation data.

Purpose of the Study:

• * To critically evaluate methods for quantifying voluntary muscle activation using twitch interpolation.
• * To address the disagreement on whether linear scaling or extrapolation of evoked-voluntary force relationships is more accurate.
• * To highlight the importance of experimental technique in sensitive twitch interpolation.

Main Methods:

• * Application of sensitive twitch interpolation technique during voluntary contractions.
• * Measurement of evoked twitch force and voluntary force.
• * Analysis of linear and non-linear evoked-voluntary force relationships.
• * Consideration of experimental details: stimulation site, intensity, and number of stimuli.

Main Results:

• * Sensitive twitch interpolation reveals routine incomplete skeletal muscle activation in healthy adults.
• * Small to moderate voluntary activation deficits occur during brief maximal efforts.
• * Progressive activation deficits (central fatigue) are observed during exhausting exercise.
• * Evidence suggests resistance training may improve activation in some muscles but not others.
• * Voluntary activation declines with bed rest, joint injury, and degeneration.

Conclusions:

• * The twitch interpolation technique, when applied with high sensitivity and careful methodology, is essential for detecting subtle deficits in voluntary muscle activation.
• * Critiques of linear scaling methods often overlook the underlying reasons for non-linear force relationships, which can be minimized experimentally.
• * Extrapolation methods face greater challenges to validity than linear equations when considering the causes of non-linearity.
• * Twitch interpolation is a valuable tool for studying factors influencing voluntary activation, including fatigue, training, and clinical conditions.