Electrophoretic separation of myosin heavy chain isoforms in the human m. vastus lateralis: references to reproducibility and relationships with force, electromechanical delay, fibre conduction velocity, endurance and electromyography.

The aim of this investigation was to determine the relationship between muscle performance and the myosin heavy chain (MHC) composition and the reliability of electrophoretically determined MHC compositions. A total of thirty-one male subjects participated in the experiments. Maximal voluntary isometric contractions (MVC) of the knee extensors were performed at an arbitrary knee angle of 90 degrees and the following variables were recorded: maximal isometric force, muscle fibre conduction velocity (MFCV), electromechanical delay (EMD), maximal rate of force development (MRFD), median frequency of EMG (MF) and iEMG. Static isometric contractions of the knee extensors were held at an angle of 90 degrees using contractile forces of 10%, 50% and 100% MVC, respectively. These tests were conducted on separate days. Muscle biopsy samples were obtained from the left m. vastus lateralis before MVC and static endurance tests. MHC protein isoform differences were determined through sodium dodecyl-polyacrylamide gel electrophoresis (SDS-PAGE) followed by densitometric analysis. Type I-MHC compositions of the m. vastus lateralis ranged from 20-68% with a mean of 49 +/- 18%, mean type IIa-MHC and type IIb-MHC percentages were 35 +/- 16% and 16 +/- 10%, respectively. MHC compositions of duplicate biopsy samples were not significantly different from that of original samples. The coefficients of variation calculated for duplicate biopsy samples suggested reasonable reproducibility for MHC isoform differentiation for type I-MHC and type-II MHC composition (CV = 12.6%). Differentiation between type IIa-MHC and type IIb-MHC was not always clear using the densitometric traces. Subjects with higher percentages of type II-MHC displayed significantly faster MFCV (r = 0.67, P < 0.1), isometric force development (r = 0.68, P < 0.1) and shorter periods of EMD (r = -0.72, P < 0.05). There was also a tendency toward faster MRFD in these subjects although results did not reach significance. Endurance times for isometric contractions held at 10%, 50% and 100% MVC to exhaustion were not correlated with MHC composition. No relationships between II-MHC composition and MF or iEMG were observed. It was suggested that surface electromyographic recordings obtained during isometric MVC did not reflect underlying differences in muscle fibre composition.