Time to task failure in shoulder elevation is associated to increase in amplitude and to spatial heterogeneity of upper trapezius mechanomyographic signals.

This study investigates the changes in mechanomyographic (MMG) topographical maps of the upper trapezius muscle during short and sustained isometric contractions until task failure. MMG signals were detected over the dominant upper trapezius muscle of 12 volunteers using an unstructured grid of 12 accelerometers. The subjects performed isometric shoulder elevation at (1) maximal voluntary contraction (100% MVC), (2) 10-20-40-60-80-100% MVC for 10 s and (3) 20% MVC until task failure. Maps of absolute and normalised average rectified value (ARV) and mean power frequency (MNF) were obtained from the two-dimensional MMG recordings. Entropy (measure of heterogeneity of maps) of the MMG ARV and MNF distributions, changes over time of ARV and MNF, and the ratio between ARV at the task failure and at 100% MVC (activation ratio) were analysed in relation to the time to task failure. For the short duration and sustained contractions, MMG ARV and MNF depended on accelerometer location (P < 0.001) while normalised values did not. The activation ratio was positively correlated with the time to task failure (R (2) = 0.36, P < 0.05). Lower ARV entropy values (P < 0.05), greater MMG ARV increase during the endurance test (P < 0.001) and higher values of ARV activation ratio (P < 0.001) were observed in the subjects with longer time to task failure. The results demonstrate an association between time to task failure and MMG activation ratio and ARV entropy, which indicates that spatial variations in MMG activity and its changes over time underlie functional mechanisms for the maintenance of force during fatiguing contractions.