When skeletal muscles are activated and mechanically shortened, the force that is produced by the muscle fibers decreases in two phases, marked by two changes in slope (P1 and P2) that happen at specific lengths (L1 and L2). We tested the hypothesis that these force transients are determined by the amount of myosin cross-bridges attached to actin and by changes in cross-bridge strain due to a changing fraction of cross-bridges in the pre-power-stroke state. Three separate experiments were performed, using skinned muscle fibers that were isolated and subsequently (i) activated at different Ca2+ concentrations (pCa2+ 4.5, 5.0, 5.5, 6.0) (n = 13), (ii) activated in the presence of blebbistatin (n = 16), and (iii) activated in the presence of blebbistatin at varying velocities (n = 5). In all experiments, a ramp shortening was imposed (amplitude 10%Lo, velocity 1 Lo？sarcomere length (SL)？s？1), from an initial SL of 2.5 μm (except by the third group, in which velocities ranged from 0.125 to 2.0 Lo？s？1). The values of P1, P2, L1, and L2 did not change with Ca2+ concentrations. Blebbistatin decreased P1, and it did not alter P2, L1, and L2. We developed a mathematical cross-bridge model comprising a load-dependent power-stroke transition and a pre-power-stroke cross-bridge state. The P1 and P2 critical points as well as the critical lengths L1 and L2 were explained qualitatively by the model, and the effects of blebbistatin inhibition on P1 were also predicted. Furthermore, the results of the model suggest that the mechanism by which blebbistatin inhibits force is by interfering with the closing of the myosin upper binding cleft, biasing cross-bridges into a pre-power-stroke state.
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