title: Force depression and relaxation kinetics after active shortening and deactivation in mouse soleus muscle.
abstract: 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 (0.7s) 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 L0 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 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.
authors: Research Centre for Exercise and Health, Biomedical Kinesiology, Faculty of Kinesiology and Rehabilitation, Department of Science, KU Leuven, Belgium. Pieter.van.noten@gmail.com
authors: P
authors: P
authors: Van Noten
authors: M
authors: M
authors: Van Leemputte
mesh_terms: D000818:Animals; D007537:Isometric Contraction / Q000502:physiology*; D007700:Kinetics; D008297:Male; D051379:Mice; D008954:Models, Biological; D009126:Muscle Relaxation / Q000502:physiology*; D053580:Muscle Strength / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*