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

Abstract

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.

Longitudinal and transverse propagation of surface mechanomyographic waves generated by single motor unit activity

“…Moreover, the present results provide evidence of a major role of fiber displacement in the origin of the MMG. The dependence of MMG responses on accelerometer location is in agreement with the observations that muscle morphology and architecture affects the MMG signal features [8,19,20,21,29,30].…”

“…Recently, the MMG signal activity has been mapped in multiple locations in striated muscles [9,18,19,21]. Moreover, the distribution of the surface MMG over the skin plane generated by the activation of single motor units was assessed by means of twodimensional grids of accelerometers [9].…”

Motor unit acceleration maps and interference mechanomyographic distribution

“…These interpretations are complicated by the dependence of MMG responses on motor unit location observed in this study. The dependence of the MMG signal on motor unit location also confirms that the surface mechanical waves are affected by the muscle shape and physical properties of the surrounding tissues (Madeleine et al, , 2008Orizio et al, 2003;Yoshitake and Moritani, 1999), which explains the lack of consistency in trends of MMG global variables with increasing force and with fatigue in different muscles (Madeleine et al, , 2008Maton et al, 1990;Orizio et al, 1989;Orizio., 2004).. Homogenous and heterogenous interference MMG temporal and spectral maps (Madeleine et al, , 2008 emphasize major difficulties in relating interference MMG features to the contribution of neural, muscular and anatomical factors and underline that the interpretation of the interference MMG cannot be related to single mechanisms.…”

Novel insights into skeletal muscle function by mechanomyography: from the laboratory to the field

“…Eleven studies over twenty used an ACC to detect MMG signal [2,[16][17][18][89][90][91][92][93][94], seven used a PIZ [82,[95][96][97][98][99][100], and two utilized an MIC [101,102].…”