Muscle inactivation: assessment of interpolated twitch technique

Area of Science:

• Exercise Physiology
• Neuromuscular Function Assessment

Background:

• The interpolated twitch technique (ITT) is used to estimate voluntary muscle activation.
• Accurate assessment of muscle inactivation is crucial for understanding neuromuscular performance.

Purpose of the Study:

• To investigate the validity, reliability, and optimal protocol for the interpolated twitch technique (ITT).
• To assess the accuracy of ITT in estimating maximal voluntary contraction (MVC) across different contraction intensities.

Main Methods:

• Isometric plantar flexor and leg extension contractions were performed.
• Muscle inactivation was estimated by comparing superimposed and potentiated evoked torques with voluntary contractions.
• Both nerve and surface stimulation were used to elicit the interpolated twitch.

Main Results:

• The ITT demonstrated reliability with both nerve and surface stimulation, though maximal stimulation was challenging in specific conditions.
• A shallow hyperbolic curve best described the relationship between interpolated twitch ratio and force, yielding low MVC prediction errors (1.1-6.9%) with second-order polynomials.
• Prediction error was higher at lower MVC levels (<40% MVC), impacting accuracy in patient populations like those with immobilized ankle fractures.

Conclusions:

• The interpolated twitch technique is valid and reliable for assessing muscle activation during high-intensity contractions when analyzed with a second-order polynomial.
• The ITT's accuracy is reduced at lower MVC intensities, limiting its utility in certain clinical populations.
• No significant difference in ITT sensitivity was observed when using single twitches, doublets, or quintuplets.