The relationship between passive stiffness and muscle power output: Influence of muscle cross-sectional area normalization

Introduction: We examined the relationship between passive stiffness of posterior hip and thigh muscles and muscle power output before and after normalization of passive stiffness to muscle cross-sectional area (CSA). Methods: Pearson correlation coefficients (r) were used to assess the relationships between the normalized and non-normalized slopes of the initial (phase 1) and final (phase 2) portions of the angle-torque curve and peak power output (Pmax). Results: A significant positive relationship was observed between the non-normalized slope of phase 1 and Pmax (r=0.723; P≤0.001); however, no correlations were observed between the normalized slope of phase 1 and Pmax (r=0.244; P=0.299) nor between Pmax and the normalized and non-normalized slopes of phase 2 (r=-0.159-0.418; P=0.067-0.504). Conclusions: The findings suggest that muscle size, rather than stiffness, accounted for a significant portion of the variance in muscle power output.