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 TQMVC. VA (%) was assessed using the twitch interpolation technique, and maximal joint torque calculated by TQMVC/VA × 100 was adopted as TQ100%. The correlation coefficients between MV and TQMVC (r = 0.518) and between MV and TQ100% (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 TQ100% as the dependent variable was performed, MV (β = 0.554) and EI (β = -0.203) were both selected as significant contributors for estimating TQ100%. Additionally, the residual errors of TQ100% 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.