Compound maximal motor unit response is modulated by contraction intensity, but not contraction type in tibialis...

Determining a single compound maximal motor response (M_MAX ) or an average superimposed M_MAX response (M_SUP ) 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 normalize 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 M_MAX /M_SUP under different muscle actions and contraction intensities. Following a familiarization session, 22 males visited the laboratory on a single occasion. M_MAX was recorded under 10% isometric and 25% and 100% shortening and lengthening maximal voluntary contractions (MVC) at an angular velocity of 15° sec^-1 . M_SUP was also recorded during 100% shortening and lengthening with an average of five responses recorded. There were no differences in M_MAX or M_SUP between contraction types. All variables showed large, positive correlations (P < 0.001, r²  ≥ 0.64). M_MAX amplitude was larger (P < 0.001) at 100% shortening and lengthening intensity compared to M_MAX amplitude at 10% isometric and 25% lengthening MVC. Bland-Altman plots revealed a bias toward higher M_MAX at the higher contraction intensities. Despite M_SUP being significantly smaller than M_MAX (P < 0.001) at 100% MVC, M_SUP showed a large positive correlation (P < 0.001, r²  ≥ 0.64) with all variables. It is our recommendation that M_MAX should be recorded at specific contraction intensity but not necessarily a specific contraction type.