The hypothesis that cross-bridge (XB) transition rate (g) from the strong to the weak state (weakening) depends on the shortening velocity (V_SL), , provides analytical solution for the force ? velocity relationship and energy conversion by the sarcomere. The effect of V_SL on XB weakening was quantified in 6 trabeculae from the rat right ventricle, using K-H solution, [Ca]_o=1.5 mM, 25^o C. Two protocols were utilized: a) Identical V_SL were imposed at various instants during the twitch. b) Various amplitudes of V_SL were imposed at the same instant. Sarcomere length was measured with laser diffraction techniques. The response to shortening consisted of an initial exponential decline of force (F) (t = 2 ms) followed by a slow decrease of F. The instantaneous difference between the isometric F (F_M) and F during the second slow phase, depended on the shortening duration (Dt), the instantaneous F_M and V_SL: where g₁ was 1.189 ? 0.521 mm^-1 (mean ? s.d.). F/F_M was independent of the time onset of shortening but exhibits the effect of V_SL on the rate of XB weakening. The propertionalty coefficient, g₁, provides measurable time independent index of muscle contractilty based on XB dynamics.?