Muscle size-strength relationship including ultrasonographic echo intensity and voluntary activation level of a muscle group.

This study aimed to investigate the relationship between muscle volume (MV) and joint torque for the plantar flexors (PF) in 40 young (20 men and 20 women) and 33 elderly (19 men and 14 women) individuals in consideration of the voluntary activation (VA) of PF and ratio of intramuscular adipose tissue within PF assessed by ultrasonographic echo intensity (EI). MV was estimated from the thickness of PF on ultrasonography and the lower leg length using a prediction equation previously reported. The maximal voluntary contraction torque of isometric plantar flexion was measured as TQ. VA (%) was assessed using the twitch interpolation technique, and maximal joint torque calculated by TQ/VA × 100 was adopted as TQ. The correlation coefficients between MV and TQ (r = 0.518) and between MV and TQ (r = 0.602) were both significant, with the latter being significantly higher than the former. When a stepwise multiple regression analysis using MV and EI as independent variables and TQ as the dependent variable was performed, MV (β = 0.554) and EI (β = -0.203) were both selected as significant contributors for estimating TQ. Additionally, the residual errors of TQ using the multiple regression equation (independent variables: MV and EI; 18.6 ± 14.4 Nm) were significantly lower than those using the simple regression equation (independent variable: MV; 36.6 ± 28.0 Nm). These results suggest that the consideration of VA and EI with muscle size results in a closer muscle size-strength relationship than previously achieved.