Motor Unit Behavior in Young Healthy Subjects Differs by Sex and between the Vastus Medialis and Vastus Lateralis Muscles
Article ID: JJPTF_2024_01
Details
Article ID: JJPTF_2024_01
The purpose of this study was to examine whether motor unit behaviors would differ between in vastus medialis (VM) and vastus lateralis (VL), and between males and females. Seventy young healthy subjects (8females) performed a trapezoidal task with a target value of 60% of their maximum isometric knee extension strength. The EMG waveforms obtained from the VM and VL during the task were decomposed into individual motor unit action potential, and individual motor unit amplitude (MUAPAMP) and firing rate (MFR) were calculated. In addition, the recruitment threshold (RT) of each motor unit was calculated using the knee extension torque. MUAPAMP uses the average maximum amplitude of the rectified template waveform from each of the four electrodes. MFR is the inverse of the firing interval in the first quarter of the plateau period (5.0 s) during the trapezoid contraction task. The activity pattern of the motor unit is shown by the slope and intercept of the regression line obtained by scattering MUAPAMP or MFR on the vertical axis and RT on the horizontal axis for each participant. The value of intercept of MUAPAMP–RT were higher in males than in females and in the VL than in the VM, showing significant main effects of sex and muscle factors. The slope of MUAPAMP–RT were steep in males than females, showing significant main effects of sex factors. The intercept value of MFR–RT was higher in females than in males and in the VM than in the VL, showing significant main effects of sex and muscle factors. The slope of the MFR–RT tended to be gentler in the VM in males and in the VL in females, indicating different VM and VL motor unit activity patterns by sex and a significant interaction between sex and muscle factors. These results indicate motor unit activity patterns in young healthy subjects and suggest that differences in sex and individual muscles must be taken into account to understand pathology-specific differences in motor unit activity patterns.
