The effect of muscle length on force depression after active shortening in soleus muscle of mice.

Isometric muscle force after active shortening is reduced [force depression (FD)]. The mechanism is incompletely understood but work delivered during shortening has been suggested to be the main determinant of FD. However, whether muscle length affects the sensitivity of FD to work is unknown, although this information might add to the understanding of the phenomenon. The aim of this study is to investigate the length dependence of the FD/work ratio (Q). Therefore, isometric force production (ISO) of 10 incubated mouse soleus muscles was compared to isometric force after 0.6, 1.2, and 2.4 mm shortening (IAS) at different end lengths ranging from L(0) - 3 to L(0) + 1.8 mm in steps of 0.6 mm. FD was calculated as the force difference between an ISO and IAS contraction at the same activation time (6 s) and end length. We confirm the strong relation between FD and work at L(0) (R² = 0.92) and found that FD is length dependent with a maximum of 8.8 ± 0.3% at L(0) + 1.2 mm for 0.6 mm shortening amplitude. Q was only constant for short muscle lengths (<85% L(0)) but increased exponentially with increasing muscle length. The observed length dependence of Q indicates that FD is not only determined by work produced during shortening but also by a length-dependent factor, possibly actin compliance, which should be incorporated in any mechanism explaining FD.