FIGURE 7.
The force-displacement relations of the left and the right half of a myosin filament before (A) and after (B) a length step. The sum of their displacements must equal the total sarcomere stretch, Δx1 + Δx2 = 2Δx. This condition is imposed by plotting the force-displacement relation of one set of motors as a function of Δx1 (x axis) and that of the other set as a function of Δx2 = 2Δx − Δx1 (upper x axis). The forces on the myosin filament are in equilibrium if the forces produced on both sides (T2(Δx1) and T2(Δx2)) are equal, which is given by the intersection of both curves. The equilibrium is stable if the first curve crosses the second from below and unstable if it crosses it from above. If there are three stationary points (B), the central one is always unstable and those to either side stable. The force-displacement relation of the two-filament unit is given by the force at the stable intersection as a function of the mean displacement Δx.