Abstract
Force attained during concentric contraction (active shortening) is transiently enhanced following eccentric contraction (active stretch) in skeletal muscle. This phenomenon is called stretch-shortening cycle (SSC) effect. Since many human movements contain combinations of eccentric and concentric contractions, a better understanding of the mechanisms underlying the SSC effect would be useful for improving physical performance, optimizing human movement efficiency, and providing an understanding of fundamental mechanism of muscle force control. Currently, the most common mechanisms proposed for the SSC effect are (i) stretch-reflex activation and (ii) storage of energy in tendons. However, abundant SSC effects have been observed in single fiber preparations where stretch-reflex activation is eliminated and storage of energy in tendons is minimal at best. Therefore, it seems prudent to hypothesize that factor(s) other than stretch-reflex activation and energy storage in tendons contribute to the SSC effect. In this brief review, we focus on possible candidate mechanisms for the SSC effect, that is, pre-activation, cross-bridge kinetics, and residual force enhancement (RFE) obtained in experimental preparations that exclude/control the influence of stretch-reflex activation and energy storage in tendons. Recent evidence supports the contribution of these factors to the mechanism of SSCs, and suggests that the extent of their contribution varies depending on the contractile conditions. Evidence for and against alternative mechanisms are introduced and discussed, and unresolved problems are mentioned for inspiring future studies in this field of research.
Highlights
When we attempt to jump as high as possible, we naturally make a countermovement before the main movement
stretch-shortening cycle (SSC) in skeletal muscle are frequently observed in Enhanced Performance in Stretch-Shortening Cycle be interpreted that stretch-reflex activation and energy storage/ release do not play a role in the SSC effect in human/animal movement
We compared the SSC effect between fast and slow twitch fibers and found that the SSC effect was greater in the slow compared to the fast twitch fibers while residual force enhancement (RFE) was the same (Fukutani and Herzog, 2020). These results suggest that the increased SSC effect in the slow fibers is associated with the slower cross-bridge kinetics compared to the fast fibers and is not caused by differences in RFE
Summary
When we attempt to jump as high as possible, we naturally make a countermovement (i.e., eccentric contraction or active stretch) before the main movement (i.e., concentric contraction or active shortening; Figure 1). Findings in human muscle and single skinned fiber preparations showed that most of the SSC effect disappears in the initial shortening phase of SSCs (Fukutani et al, 2015a,b, 2016, 2017a,b), thereby supporting the idea that the cross-bridge kinetics might be a significant contributor to the SSC effect It has been argued, based on stiffness measurements that the number of attached cross-bridges is greater in an eccentric compared to an isometric or concentric contraction (Lombardi and Piazzesi, 1990; Brunello et al, 2007; Fusi et al, 2010). We assume that the influence of stretch activation on the SSC effect in mammalian skeletal muscles is small at best
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