Journal of biomechanics vol:46 issue:5 pages:1021-6
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 (Δ(0.5)RT) and change in the slow phase of muscle relaxation (Δk(slow)), considered as a marker for cross-bridge detachment rate, after shortening and after a short (0.7s) 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 L(0) by varying shortening amplitude (0.6, 1.2 and 2.4mm). We found that W not only induces FD (R(2)=0.92) but also a dose dependent accelerated relaxation (R(2)=0.88 and R(2)=0.77 for respectively Δk(slow) and Δ(0.5)RT). 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 Δk(slow) were largely abolished but Δ(0.5)RT remained largely present. Under the assumption that Δk(slow) 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.