The purpose of this study was to determine whether the maximum shortening velocity (V_max) in Hill's mechanical model (A. V. Hill. Proc. R. Soc. London 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-release method, force-velocity (F-V) relationships of the wrist flexors were gathered at five different activation levels (20-100% of maximum at intervals of 20%) from four subjects. The F-V data at different activation levels can 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 scaled V_max model was developed. When the F-V curves for submaximal activation were forced to converge at the V_max obtained with maximum activation (constant 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 scaled with activation.

force-velocity relationship; Hill model; wrist flexors

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