The maximum shortening velocity of muscle should be scaled with activation

The purpose of this study was to determine whether the maximum shortening v elocity (V-max) in Hill's mechanical model (A. V. Hill. Proc. R. Soc. Londo n Ser. B. 126: 136-195, 1938) should be scaled with activation, measured as a fraction of the maximum isometric force (F-max). By using the quick-rele ase method, force-velocity (F-V) relationships of the wrist flexors were ga thered at five different activation levels (20-100% of maximum at intervals of 20%) from four subjects. The F-V data at different activation levels ca n be fitted remarkably well with Hill's characteristic equation. In general , the shortening velocity decreases with activation. With the assumption of nonlinear relationships between Hill constants and activation level, a sca led V-max model was developed. When the F-V curves for submaximal activatio n were forced to converge at the V-max obtained with maximum activation (co nstant V-max model), there were drastic changes in the shape of the curves. The differences in V-max values generated by the scaled and constant V-max models were statistically significant. These results suggest that, when a Hill-type model is used in musculoskeletal modeling, the V-max should be sc aled with activation.