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

• Published in: Journal of biomechanics Vol. 46; no. 5; pp. 1021 - 1026
• Main Authors: Van Noten, P, Van Leemputte, M
• Format: Journal Article
• Language: English
• Published: United States Elsevier Ltd 15.03.2013; Elsevier Limited
• Subjects: Animals; Conflicts of interest; Cross-bridge kinetics; Fatigue; Force deficit; Isometric Contraction - physiology; Kinetics; Male; Mice; Models, Biological; Muscle mechanics; Muscle Relaxation - physiology; Muscle Strength - physiology; Muscle, Skeletal - physiology; Physical Medicine and Rehabilitation; Production capacity; Relaxation; Force deficit; Cross-bridge kinetics; Relaxation; Muscle mechanics
• ISSN: 0021-9290; 1873-2380
• DOI: 10.1016/j.jbiomech.2012.07.011

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 (Δ0.5 RT) and change in the slow phase of muscle relaxation (Δ kslow ), 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 Δ kslow and Δ0.5 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 Δ kslow were largely abolished but Δ0.5 RT remained largely present. Under the assumption that Δ kslow 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.