Key result
Active shortening significantly decreased cross-bridge cycling kinetics compared with purely isometric contractions, whereas active stretching showed no change.
Why the study?
Experiments directly evaluating cross-bridge cycling kinetics in actively shortened and stretched muscle preparations compared with purely isometric contractions had not yet been performed.
Does active shortening and stretching alter cross-bridge cycling kinetics in rabbit psoas muscle fibres compared to purely isometric contractions?
Does active shortening and stretching alter cross-bridge cycling kinetics in rabbit psoas muscle fibres compared to purely isometric contractions?
Force depression following active shortening in skeletal muscle is associated with changes in cross-bridge kinetics, whereas force enhancement following active stretch is not.
Active shortening alters cross-bridge kinetics in skeletal muscle fibres; leaves open relevance to cardiac or in vivo force regulation.
The steady-state isometric force produced by skeletal muscle after active shortening and stretching is depressed and enhanced, respectively, compared with purely isometric force produced at corresponding final lengths and at the same level of activation. One hypothesis proposed to account for these force depression (FD) and force enhancement (FE) properties is a change in cross-bridge cycling kinetics. The rate of cross-bridge attachment (f) and/or cross-bridge detachment (g) may be altered following active shortening and active stretching, leading to FD and FE, respectively. Experiments elucidating cross-bridge kinetics in actively shortened and stretched muscle preparations and their corresponding purely isometric contractions have yet to be performed. The aim of this study was to investigate cross-bridge cycling kinetics of muscle fibres at steady-state following active shortening and stretching. This was done by determining muscle fibre stiffness and rate of active force redevelopment following a quick release-re-stretch protocol (kTR). Applying these measures to equations previously used in the literature for a two-state cross-bridge cycling model (attached/detached cross-bridges) allowed us to determine apparent f and g, the proportion of attached cross-bridges, and the force produced per cross-bridge. kTR, apparent f and g, the proportion of attached cross-bridges and the force produced per cross-bridge were significantly decreased following active shortening compared with corresponding purely isometric contractions, indicating a change in cross-bridge cycling kinetics. Additionally, we showed no change in cross-bridge cycling kinetics following active stretch compared with corresponding purely isometric contractions. These findings suggest that FD is associated with changes in cross-bridge kinetics, whereas FE is not.
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Ames et al. (2022) studied this question. Active shortening and stretching vs. Purely isometric contractions was evaluated on Cross-bridge cycling kinetics (kTR, apparent f and g, proportion of attached cross-bridges, force produced per cross-bridge). Active shortening significantly decreased cross-bridge cycling kinetics compared with purely isometric contractions, whereas active stretching showed no change.
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