Force depression and relaxation kinetics after active shortening and deactivation in mouse soleus muscle

After active shortening, isometric force production capacity of muscle is reduced (force depression, FD). The mechanism is incompletely understood but increasing cross-bridge detachment and/or decreasing attachment rate might be involved. Therefore we aimed to investigate the relation between work delivered during shortening (W), and change in half-relaxation time (Δ0.5RT) and change in the slow phase of muscle relaxation (Δkslow), considered as a marker for cross-bridge detachment rate, after shortening and after a short (0.7 s) interruption of activation (deactivation). We hypothesized that shortening induces an accelerated relaxation related to W which is, similar to FD, largely abolished by a short deactivation. In 10 incubated supra-maximally stimulated mouse soleus muscles, we varied the amount of FD at L0 by varying shortening amplitude (0.6, 1.2 and 2.4 mm). We found that W not only induces FD (R2=0.92) but also a dose dependent accelerated relaxation (R2=0.88 and R2=0.77 for respectively Δkslow and Δ0.5RT). In cyclic movements this is of functional significance, because the loss in force generating capacity might be (partially) compensated by faster relaxation. After a short deactivation, both FD and Δkslow were largely abolished but Δ0.5RT remained largely present. Under the assumption that Δkslow reflects a change in cross-bridge detachment rate, these results support the idea that FD is an intrinsic sarcomeric property originating from a work induced reduction of the number of force generating cross-bridges, however not via decreased attachment but via increased detachment rate.