The Influence of Myosin Heavy Chain Isoform Composition and Training Status on the Patterns of Responses for Mechanomyographic Amplitude versus Isometric Torque

The purpose of this study was to examine the influence of myosin heavy chain (MHC) isoform composition and training status on the mechanomyographic (MMG) amplitude versus isometric torque relationship for the vastus lateralis. Five resistance-trained (mean ± SD age = 23.2 ± 3.7 years), 5 aerobically trained (mean ± SD age = 32.6 ± 5.2 years), and 5 sedentary (mean ± SD age = 23.4 ± 4.1 years) men performed isometric muscle actions of the leg extensors in 20% increments from 20% to 100% of the maximum voluntary contraction. Biopsies from the vastus lateralis revealed that the MHC composition for the resistance-trained subjects was 59.0 ± 4.2% Type IIa, 0.1 ± 0.1% Type IIx, and 40.9 ± 4.3% Type I. The aerobically-trained subjects had 27.4 ± 7.8% Type IIa, 0.0 ± 0.0% Type IIx, and 72.6 ± 7.8% Type I MHC. The sedentary subjects had 42.1 ± 7.8% Type IIa, 17.8 ± 6.4% Type IIx, and 40.1 ± 10.9% Type I MHC. There were no consistent patterns of responses for the resistance-trained, aerobically trained, or sedentary subjects for MMG amplitude versus torque. Thus, differences in MHC isoform composition and training status did not explain the unique torque-related patterns for MMG amplitude.