The influence of myosin heavy chain isoform content on mechanical behavior of the vastus lateralis in vivo.

This study examined correlations between type I percent myosin heavy chain isoform content (%MHC) and mechanomyographic amplitude (MMGRMS) during isometric muscle actions. Fifteen (age=21.63±2.39) participants performed 40% and 70% maximal voluntary contractions (MVC) of the leg extensors that included increasing, steady force, and decreasing segments. Muscle biopsies were collected and MMG was recorded from the vastus lateralis. Linear regressions were fit to the natural-log transformed MMGRMS-force relationships (increasing and decreasing segments) and MMGRMS was selected at the targeted force level during the steady force segment. Correlations were calculated among type I%MHC and the b (slopes) terms from the MMGRMS-force relationships and MMGRMS at the targeted force. For the 40% MVC, correlations were significant (P<0.02) between type I%MHC and the b terms from the increasing (r=-0.804) and decreasing (r=-0.568) segments, and MMGRMS from the steady force segment (r=-0.606). Type I%MHC was only correlated with MMGRMS during the steady force segment (P=0.044, r=-0.525) during the 70% MVC. Higher type I%MHC reduced acceleration in MMGRMS (b terms) during the 40% MVC and the amplitude during the steady force segments. The surface MMG signal recorded during a moderate intensity contraction provided insight on the contractile properties of the VL in vivo.