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

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          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.

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          Most cited references 57

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          Voluntary strength and fatigue.

           Daniel Merton (1954)
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            Stretch-shortening cycle: a powerful model to study normal and fatigued muscle.

             Paavo V. Komi (2000)
            Stretch-shortening cycle (SSC) in human skeletal muscle gives unique possibilities to study normal and fatigued muscle function. The in vivo force measurement systems, buckle transducer technique and optic fiber technique, have revealed that, as compared to a pure concentric action, a non-fatiguing SSC exercise demonstrates considerable performance enhancement with increased force at a given shortening velocity. Characteristic to this phenomenon is very low EMG-activity in the concentric phase of the cycle, but a very pronounced contribution of the short-latency stretch-reflex component. This reflex contributes significantly to force generation during the transition (stretch-shortening) phase in SSC action such as hopping and running. The amplitude of the stretch reflex component - and the subsequent force enhancement - may vary according to the increased stretch-load but also to the level of fatigue. While moderate SSC fatigue may result in slight potentiation, the exhaustive SSC fatigue can dramatically reduce the same reflex contribution. SSC fatigue is a useful model to study the processes of reversible muscle damage and how they interact with muscle mechanics, joint and muscle stiffness. All these parameters and their reduction during SSC fatigue changes stiffness regulation through direct influences on muscle spindle (disfacilitation), and by activating III and IV afferent nerve endings (proprioseptic inhibition). The resulting reduced stretch reflex sensitivity and muscle stiffness deteriorate the force potentiation mechanisms. Recovery of these processes is long lasting and follows the bimodal trend of recovery. Direct mechanical disturbances in the sarcomere structural proteins, such as titin, may also occur as a result of an exhaustive SSC exercise bout.
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              Why is countermovement jump height greater than squat jump height?

              In the literature, it is well established that subjects are able to jump higher in a countermovement jump (CMJ) than in a squat jump (SJ). The purpose of this study was to estimate the relative contribution of the time available for force development and the storage and reutilization of elastic energy to the enhancement of performance in CMJ compared with SJ. Six male volleyball players performed CMJ and SJ. Kinematics, kinetics, and muscle electrical activity (EMG) from six muscles of the lower extremity were monitored. It was found that even when the body position at the start of push-off was the same in SJ as in CMJ, jump height was on average 3.4 cm greater in CMJ. The possibility that nonoptimal coordination in SJ explained the difference in jump height was ruled out: there were no signs of movement disintegration in SJ, and toe-off position was the same in SJ as in CMJ. The greater jump height in CMJ was attributed to the fact that the countermovement allowed the subjects to attain greater joint moments at the start of push-off. As a consequence, joint moments were greater over the first part of the range of joint extension in CMJ, so that more work could be produced than in SJ. To explain this finding, measured and manipulated kinematics and electromyographic activity were used as input for a model of the musculoskeletal system. According to simulation results, storage and reutilization of elastic energy could be ruled out as explanation for the enhancement of performance in CMJ over that in SJ. The crucial contribution of the countermovement seemed to be that it allowed the muscles to build up a high level of active state (fraction of attached cross-bridges) and force before the start of shortening, so that they were able to produce more work over the first part of their shortening distance.

                Author and article information

                [1 ]Department of Biomechanics in Sports, Faculty of Sport and Health Sciences, Technische Universität München Munich, Germany
                [2 ]Human Performance Laboratory, Faculty of Kinesiology, University of Calgary Calgary, Canada
                [3 ]Human Movement Science, Faculty of Sports Science, Ruhr-Universität Bochum Bochum, Germany
                Author notes
                Correspondence Wolfgang Seiberl, Technische Universität München, Georg-Brauchle-Ring 62, 80992, Munich, Germany., Tel: +49 89 289 24585, Fax: +49 89 289 24582, E-mail: wolfgang.seiberl@
                Walter Herzog, University of Calgary, 2500 University Dr. N.W., Calgary, AB, Canada, T2N 1N4., Tel: 1-403-220-8525, Fax: 1-403-284-2070, E-mail: wherzog@

                Funding Information This study was supported by funding from the Canadian Institutes of Health Research, the Natural Sciences and Engineering Research Council of Canada, the Canada Research Chair Programme, and The Killam Foundation. This work was supported by the German Research Foundation (DFG) and the Technische Universität München within the funding program Open Access Publishing.

                Physiol Rep
                Physiol Rep
                Physiological Reports
                BlackWell Publishing Ltd (Oxford, UK )
                May 2015
                13 May 2015
                : 3
                : 5
                25975646 4463830 10.14814/phy2.12401
                © 2015 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society.

                This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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