FIGURE 6.

(A) Double-hyperbolic force-velocity relationship. Data were obtained from Figure 2 in Edman (1988a) using specialized software (ImageJ 1.51q8, NIH, United States). This modified version represents the force-velocity relationship of a single muscle fiber from the anterior tibialis muscle of a frog. Note the deviation of the experimental data from those predicted by the rectangular hyperbola in the high-force region (>0.78 maximal isometric force or P₀) despite the excellent fit ((F + 0.06) × (V + 0.59) = (0.21 + 0.06) × 0.59; R² = 0.994) (dashed line) (least squares method). In contrast, all measurement data are well-represented by a double-hyperbolic F-V equation ($V=\frac{(0.21-F)\times 0.59}{F+0.06}-(1-\frac{1}{1+{\mathrm{e}}^{-154.65\times (F-0.82\times 0.17)}})$; R² = 0.999) (solid line) (least squares method). (B) Sigmoidal transition of the force-velocity relationship from concentric (CON) to eccentric (ECC) dynamic muscle actions (open circles). Data were obtained from Figure 7 in Edman (1988a) using specialized software (ImageJ 1.51q8, NIH, United States). This modified version represents the eccentric and concentric force-velocity relationship of a single muscle fiber from the anterior tibialis muscle of a frog. A double-hyperbolic function was fitted to the concentric data (see above) and a hyperbolic function was fitted to the eccentric data ($V=\frac{0.028\times (1.000-F)}{2\times 1.000-F+(-0.384)}$; R² = 0.990) (least squares method). Note the drastic differences in force around the isometric force (open square) (0.90–1.20 P₀) with only minimal changes in contraction velocity (1.8% of maximal unloaded shortening velocity).