Compound Maximal Motor Unit Response is modulated by contraction intensity, but not contraction type in Tibialis Anterior

Determining a single compound maximal motor response (MMAX) or an average superimposed MMAX response (MSUP) are commonly used reference values in experiments eliciting raw electromyographic, motor evoked potentials, H-reflexes and V-waves. However, existing literature is limited in detailing the most appropriate method to normalise these electrophysiological measures. Due to the accessibility of assessment from a cortical and spinal perspective, the tibialis anterior is increasingly used in literature and hence investigated in this study. 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 maximal voluntary contractions (MVC) at an angular velocity of 15°^-1. MSUP was also recorded during 100% shortening and lengthening with an average of five responses recorded. There were no differences in MMAX or MSUP between contraction types. All variables showed large, positive correlations (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 large 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.