Mechanomyographic amplitude and mean power frequency responses during isometric ramp vs. step muscle actions

The purpose of the present study was to compare the mechanomyographic amplitude (MMG_RMS) and mean power frequency (MMG_MPF) vs. torque relationships during isometric ramp and step muscle actions for the vastus lateralis (VL) and rectus femoris (RF) muscles. Nineteen subjects (mean ± S.D. age = 24 ± 4 years) performed 2 isometric maximal voluntary contractions (MVCs) before and after 2 or 3 isometric ramp muscle actions from (5-95% MVC) to 9 submaximal step muscle actions (15, 25, 35, 45, 55, 65, 75, 85, and 95% MVC). MMG signals were recorded from the VL and RF muscles, and MMG_RMS and MMG_MPF values were computed for each corresponding percentage of the MVC. Absolute and normalized MMG_RMS and MMG_MPF vs. torque relationships were analyzed and interpreted on a subject-by-subject and composite pattern basis using polynomial regression and repeated measures ANOVAs. For MMG_RMS and MMG_MPF, only 16-53% and 11-26% of the individual responses were consistent with the composite polynomial models, respectively. In addition, the normalized composite MMG_RMS values were greater for the RF than the VL from 35 to 85% MVC. Only 47% of the MMG_RMS and 5% of the MMG_MPF individual patterns of responses were the same for the ramp and step muscle actions, and differences were also observed for the composite MMG_RMS and MMG_MPF patterns between the ramp and step muscle actions. Overall, these findings indicated that the torque-related patterns of responses for MMG_RMS and MMG_MPF were different among subjects (i.e., inter-individual variability) and were muscle- (VL vs. RF) and mode-specific (ramp vs. step).