Shortening amplitude affects the incomplete force recovery after active shortening in mouse soleus muscle

Abstract Compared to isometric contraction, the force producing capacity of muscle is reduced (force depression, FD) after a work producing shortening phase. It has been suggested that FD results from an inhibition of cross-bridge binding. Because the rate constants of the exponential force (re)development are thought to be primarily determined by cross-bridge attachment/detachment rate, we aimed to investigate the components of force redevelopment (REDEV) after 0.6, 1.2 and 2.4 mm shortening, resulting in varying amounts of FD (from about 5% to about 16%), in mouse soleus muscle ( n =11). Compared to isometric force development (DEV), the time to reach steady-state during REDEV was about 3 times longer (370 versus 1261 ms) increasing with increasing amplitude. Contrary to a single, a double exponential function with one component set equal to the rate constant of DEV (14.3 s −1 ), accurately described REDEV (RMS R 2 =0.75) and FD ( R 2 =0.77). We concluded that a work related slow exponential component is induced to the trajectory of incomplete force recovery after shortening, causing FD. These results suggest that after shortening, aside from cross-bridges with normal attachment/detachment rate, cross-bridges with reduced cycling rate are active.