Comparison of force-velocity relationships from MUSICO (black line), Huxley 1957 (green line), and Huxley PL (red line). For reference, A.V. Hill’s force-velocity data are shown as cyan closed circles (Hill, 1938). Force per myosin filament (MUSICO) is normalized by the maximum isometric force, F_o, and tension in mass action kinetic models is normalized to maximum isometric tension, T_o. (A) Large differences between the predictions of isotonic shortening velocities are observed at the same force (e.g., 0.6 and 0.4 of maximum isometric force, F_o, or tension, T_o) or with forces at constant velocities (e.g., 0.04 and 0.133 of maximum shortening, v_max). (B) Isotonic lengthening velocities for the original Huxley 1957 model rate constants (solid lines and closed symbols) and with additional Zahalak factor f_Zah = 1.8 for x > h_Zah = 15.6 nm (Zahalak, 1986; dashed lines and open symbols) in order to increase cross-bridge detachment at higher cross-bridge strains at x > h_Zah during lengthening. To generate a curve in the observed range (dotted green line), the Huxley 1957 mass action kinetic model requires f_Zah > 4. The velocities in range from −60 to 20 nm/s are shown in the inset. Also, for reference, the force-velocity data for lengthening (Katz, 1939) are shown as cyan closed circles in addition to A.V. Hill’s data for shortening (Hill, 1938).