Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Calgary, AB, Canada, T2N 1N4.
J Exp Biol. 2022 Nov 15;225(22). doi: 10.1242/jeb.244703. Epub 2022 Nov 18.
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.
与相应的最终长度和相同激活水平下产生的纯等长力相比，主动缩短和拉伸后的骨骼肌产生的稳定等长力分别受到抑制和增强。一种用于解释这些力抑制（FD）和力增强（FE）特性的假设是，横桥循环动力学发生了变化。主动缩短和主动拉伸后，横桥附着（f）和/或横桥脱离（g）的速率可能会发生改变，分别导致 FD 和 FE。尚未进行阐明主动缩短和拉伸肌肉标本中的横桥动力学及其相应的纯等长收缩的实验。本研究的目的是研究主动缩短和拉伸后稳定状态下肌肉纤维的横桥循环动力学。这是通过快速释放-再拉伸方案（kTR）确定肌肉纤维的刚性和主动力重新发展的速率来实现的。将这些措施应用于以前用于文献中的双态横桥循环模型（附着/脱离横桥）的方程中，使我们能够确定表观 f 和 g、附着横桥的比例以及每横桥产生的力。与相应的纯等长收缩相比，主动缩短后 kTR、表观 f 和 g、附着横桥的比例以及每横桥产生的力均显著降低，表明横桥循环动力学发生了变化。此外，与相应的纯等长收缩相比，主动拉伸后横桥循环动力学没有变化。这些发现表明 FD 与横桥动力学的变化有关，而 FE 则不然。
