The stretch-shortening cycle (SSC) revisited: residual force enhancement contributes to increased performance during fast SSCs of human m. adductor pollicis.

Wolfgang Seiberl,Walter Herzog,Geoffrey A Power,Daniel Hahn

doi:10.14814/phy2.12401

Abstract

The stretch-shortening cycle (SSC) occurs in most everyday movements, and is thought to provoke a performance enhancement of the musculoskeletal system. However, mechanisms of this performance enhancement remain a matter of debate. One proposed mechanism is associated with a stretch-induced increase in steady-state force, referred to as residual force enhancement (RFE). As yet, direct evidence relating RFE to increased force/work during SSCs is missing. Therefore, forces of electrically stimulated m. adductor pollicis (n = 14 subjects) were measured during and after pure stretch, pure shortening, and stretch-shortening contractions with varying shortening amplitudes. Active stretch (30°, ω = 161 ± 6°s−1) caused significant RFE (16%, P < 0.01), whereas active shortening (10°, 20°, and 30°; ω = 103 ± 3°s−1, 152 ± 5°s−1, and 170 ± 5°s−1) resulted in significant force depression (9–15%, P < 0.01). In contrast, after SSCs (that is when active stretch preceded active shortening) no force depression was found. Indeed for our specific case in which the shortening amplitude was only 1/3 of the lengthening amplitude, there was a remnant RFE (10%, P < 0.01) following the active shortening. This result indicates that the RFE generated during lengthening affected force depression when active lengthening was followed by active shortening. As conventional explanations, such as the storage and release of elastic energy, cannot explain the enhanced steady-state force after SSCs, it appears that the stretch-induced RFE is not immediately abolished during shortening and contributes to the increased force and work during SSCs.

Highlights

Stretch-shortening cycles (SSC) occur in most natural movements
Work during shortening was significantly (P < 0.001) lower during pure shortening compared to stretch-shortening, reaching about 60% of the work performed during SSCs for corresponding shortening amplitudes (Table 1)
Forces were significantly (P < 0.001) lower at the end of the shortening phase for the pure shortening compared to the corresponding stretch-shortening cycles (Figs. 2, 5)

Summary

Introduction

Stretch-shortening cycles (SSC) occur in most natural movements. During a SSC, a lengthening contraction is immediately followed by a shortening contraction that may be shortly delayed due to a brief transition phase (Komi and Gollhofer 1997; Komi 2000). In SSCs the force/work performed during the shortening phase is typically greater than that observed when shortening is not preceded by active stretching This increase in performance has been observed in isolated muscles, in situ animal preparations (Cavagna et al 1968; Gregor et al 1988), and during maximal voluntary contractions of human muscles (Cavagna et al 1968; Bosco et al 1987). Similar observations on enhanced forces can be made in steady-state isometric forces following an active muscle stretch Such forces are increased compared to those obtained for purely isometric contractions (Edman 2012). Isometric steady-state forces following active shortening are decreased compared to the corresponding purely isometric forces: a property referred to as force depression (FD) (Edman 1976; Lee and Herzog 2003). Observations of FD and RFE have been made across all structural levels of muscle, in a variety of muscles, and most importantly in the current context, have been made for in vivo human muscles activated voluntarily or artificially via electrical stimulation (Lee et al 1999, 2000; Lee and Herzog 2002; Oskouei and Herzog 2005; Hahn et al 2010, 2012; Seiberl et al 2010, 2012; Power et al 2012, 2013, 2014)

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