A Noninvasive Test For Estimating Type I Myosin Heavy Chain Expression In Women Using Mechanomyography

Abstract

PURPOSE: To determine if mechanomyographic amplitude (MMGRMS)-torque relationships could estimate type I percent myosin heavy chain expression (%MHC) of the vastus lateralis (VL) in sedentary women. METHODS: Fifteen healthy women (mean ± SD; age = 21.3 ± 5.3 yrs) volunteered for this study. Subjects performed 3 isometric maximal voluntary contractions (MVCs) of the knee extensors on an isokinetic dynamometer. The highest torque output determined the target torque levels for the subsequent randomly ordered isometric trapezoidal muscle actions at 30% and 70% MVC. An MMG sensor was placed on the VL. Simple linear regression models were fit to the log-transformed MMGRMS-torque relationships for the linear increasing and decreasing segments. MMGRMS was averaged for the steady torque segment. After testing, muscle biopsies were taken from the VL. %MHC was analyzed using SDS-PAGE. Pearson’s product moment correlation coefficients determined relationships among type I %MHC expression and each MMG variable (6 total). Sequential multiple-regression procedures determined if a predictive model for type I %MHC of the VL could be developed with the MMG variables significantly correlated with type I %MHC. Alpha was set at 0.05. RESULTS: Type I %MHC was correlated with the b terms from the MMGRMS-torque relationships for the linearly increasing segments at 70% MVC (p = 0.003; r = -0.72) and MMGRMS for the steady torque segments at 30% (p = 0.008; r = -0.65) and 70% MVC (p = 0.040; r = -0.54). No other relationships existed (p > 0.05). For the regression model, correlated variables were added in order of significance. The addition of each variable significantly added to the model (p < 0.05) and overall accounted for 81.2% of the variance in type I %MHC (Table 1). CONCLUSION: MMGRMS may provide a noninvasive method for estimating type I %MHC of the VL in untrained women. Future research should investigate the utility of this model in other populations.