WITHDRAWN: Maximal motor unit response is modulated by contraction intensity, but not contraction type

Abstract

Determining a maximal motor response (Mmax) or superimposed Mmax (MSUP) are commonly used reference values in experiments eliciting raw electromyographic, motor evoked potentials, H-reflexes and V-waves. However, literature is scant on the most appropriate method to normalise these electrophysiological measures. The aims of the present study were to examine the differences and level of agreement in Mmax/MSUP under different muscle actions and contraction intensities. Following a familiarisation session, 22 males visited the laboratory on a single occasion. Mmax was recorded under 10% isometric and 25% and 100% shortening and lengthening MVC. MSUP was also recorded during 100% shortening and lengthening MVC. There were no differences in Mmax or MSUP between contraction types. All variables showed a strong positive correlation (P < 0.001, r2 ≥ 0.64). Mmax amplitude was larger (P < 0.001) at 100% shortening and lengthening intensity compared to Mmax amplitude at 10% isometric and 25% lengthening MVC. Bland-Altman plots revealed a bias towards higher Mmax at the higher contraction intensities. Despite MSUP being significantly smaller than Mmax (P < 0.001) at 100% MVC, MSUP showed a strong positive correlation (P < 0.001, r2 ≥ 0.64) with all variables. It is our recommendation that Mmax should be recorded at specific contraction intensity but not necessarily a specific contraction type.