An examination of the relationships among myosin heavy chain isoform content, isometric strength, and mechanomyographic median frequency.

The purpose of the present study was to determine if the combination of isometric knee extension strength and mechanomyographic (MMG) median frequency (MDF) could be used to estimate the percent (%) myosin heavy chain (MHC) Type II isoform content of the vastus lateralis. Five resistance-trained (mean +/- SD age = 23.2 +/- 3.7 years) and 5 aerobically trained (mean +/- SD age = 32.6 +/- 5.2 years) men volunteered to perform a 6-second isometric maximum voluntary contraction (MVC) of the dominant knee extensors at a joint angle of 90 degrees between the thigh and leg. During the isometric MVC, the surface MMG signal was detected from the vastus lateralis, and the MDF was calculated with the discrete Fourier transform. Following the isometric MVC and MMG measurements, muscle biopsies were taken from the vastus lateralis and analyzed for % MHC Type II isoform content. The results showed that neither isometric knee extension strength nor MMG MDF alone was significantly correlated with the % MHC Type II isoform content. The combination of isometric knee extension strength and MMG MDF, however, explained a significant proportion (i.e., 59.8%) of the variance in % MHC Type II isoform content, with a multiple correlation of R = 0.773 and a standard error of the estimate (SEE) of 15.4%. These findings indicated that a simple, time-efficient, and noninvasive test that simultaneously measures isometric strength and MMG MDF could be useful for estimating the % MHC Type II isoform content in well-trained men.