Mechanomyographic amplitude and frequency vs. isometric torque relationships in women with higher vs. lower strength

PURPOSE: To examine the MMG amplitude and mean power frequency vs. torque relationships during isometric muscle actions in women with higher vs. lower muscular strength. METHODS: Eighteen women volunteered to perform isometric leg extensions at varying submaximal percentages and 100% of their maximal voluntary contraction (MVC) on a HUMAC NORM isokinetic dynamometer. The women were classified into lower strength (n = 10; 21.8 +/- 2.5 y, 164.3 +/- 6.7 cm, 58.2 +/- 7.9 kg) and higher strength (n = 8; 22.5 +/- 2.3 y, 165.4 +/- 4.4 cm, 71.7 +/- 10.6 kg) groups based on their isometric MVC values (lower = 98.4 +/- 18.3 Nm, higher = 162.8 +/- 26.1 Nm). An accelerometer (EGAS S704 10_Rev C) was placed over the vastus lateralis to detect the MMG amplitude (expressed as root mean square, RMS) and frequency (expressed as mean power frequency, MPF). Torque (Nm) was recorded by the dynamometer. RESULTS: Polynomial regression analyses indicated the relationship for normalized MMG amplitude vs. isometric MVC was quadratic for the lower strength group (R2 = 0.989) and linear for the higher strength group (R2 = 0.917). The MMG amplitude of lower strength women increased most between 60 and 100% MVC. For MMG MPF, the relationships were linear for both lower (R2 = 0.495) and higher strength women (R2 = 0.824). CONCLUSION: The different torque-related responses for MMG amplitude for lower vs. higher strength women likely reflected differences in absolute torque, and thus muscle stiffness, between groups.