Lead Summary 6This article reviews research relating to the stretch shortening cycle and plyometrics. 7The article introduces strength and conditioning practitioners to using ground contact 8 times and the reactive strength index in plyometric training. It documents how these 9 measurements can be used to optimize plyometrics and improve athletes' fast stretch 10 shortening cycle performance. Recommendations are made regarding the use of ground 11 contact times to improve training specificity and the use of the reactive strength index to 12 optimize plyometrics, to monitor training progress and as a motivational tool. A four step 13 progression of implementation, previously used with elite rugby players is detailed. 15The Stretch Shortening Cycle 16The stretch-shortening cycle (SSC) is a natural type of muscle function in which muscle 17 is stretched immediately prior to being contracted. This eccentric/concentric coupling of 18 muscular contraction produces a more powerful contraction than that which would result 19 from a purely concentric action alone (14). When the force velocity curve is measured 20 during a complex SSC movement involving a number of joints and muscle groups, such 21 as a vertical jump, the utilization of a preceding eccentric phase shifts the force-velocity 22 curve to the right. In comparison with purely concentric movements, the SSC allows 23 higher forces to be produced at any given velocity during the concentric phase (13). 25The SSC is observed in a wide range of activities. In real life situations, exercise seldom 31One view has been that the SSC causes an enhancement during the concentric phase 32 due to the storage and reutilization of elastic energy (7;16
The reliability of the reactive strength index (RSI) and time to stabilization (TTS) during maximal-effort plyometric depth jumps was examined. Twenty-two subjects performed three depth jumps from a height of 30 cm. Measures such as height of jump (JH), ground-contact time (CT), RSI, and TTS were obtained and analyzed for reliability using Cronbach alpha reliability coefficient and intraclass correlations. The JH, CT, and RSI were shown to be highly reliable from trial to trial (evidenced by high Cronbach reliability coefficients (alpha > 0.95) and high single- and average-measures intraclass correlations (>0.9). Time to stabilization was not reliable from trial to trial, as evidenced by a low Cronbach reliability coefficient (alpha < 0.7) and poor single- (<0.5) and average-measures (<0.7) intraclass correlations. The RSI was observed to be consistent for single measures, suggesting that coaches dealing with large numbers of athletes can conduct only a single trial from each depth jump height when attempting to optimize plyometric depth jump heights for their athletes. Time to stabilization could be a useful tool for strength and conditioning investigators to quantify the landing portion of plyometric exercises, but the protocol used in the current study to measure this variable did not prove to be reliable. Investigators wishing to use this measurement in such a context in future research studies may need to allow subjects appropriate habituation periods and control for arm movement during the landing phase of the exercise.
The study assessed the effect of current activation potentiation by evaluating jaw clenching and its effect on the rate of force development (RFD), time to peak force (TTPF), and peak force (PF) during the countermovement jump. Fourteen subjects performed the countermovement jump on a force platform while maximally clenching their jaw on a dental vinyl mouthguard (JAW) as well as without clenching their jaw by jumping with an open mouth (NON-JAW). Results reveal that the RFD was 19.5% greater in the JAW compared with the NON-JAW condition (p < 0.05). The TTPF was 20.15% less in the JAW compared with the NON-JAW condition (p < 0.05). There were no significant differences (p = 0.60) in PF between the JAW and NON-JAW conditions. These findings indicate that concurrent activation potentiation is manifested through jaw clenching during the countermovement jump. As a result, athletes may employ this strategy of maximally clenching their jaws to gain an ergogenic advantage during the countermovement jump.
The purpose of this article is to provide strength and conditioning practitioners with an understanding of the role of elastic energy in activities with high force and power requirements. Specifically, the article covers 1) the nature of elasticity and its application to human participants, 2) the role of elastic energy in activities requiring a stretch-shorten cycle such as the vertical jump, 3) the role of muscular stiffness in athletic performance, 4) the control of muscular stiffness through feedforward and feedback mechanisms, and 5) factors affecting muscular stiffness. Finally, practical applications are provided. In this section, it is suggested that the storage and reuse of elastic energy is optimized at relatively higher levels of stiffness. Because stiffness decreases as fatigue ensues as well as with stretching before an event, the article emphasizes the need for proper preparation phases in a periodized cycle and the avoidance of long static stretches before high-force activities. The importance of teaching athletes to transition from eccentric to concentric movements with minimal time delays is also proposed due to the finding that time delays appear to decrease the reuse of elastic energy. In addition to teaching within the criterion tasks, evidence is provided that minimizing transitions in plyometric training, a technique demonstrated to increase musculotendinous stiffness, can optimize power output in explosive movements. Finally, evidence is provided that training and teaching programs designed to optimize muscular stiffness may protect athletes against sports-related injuries.
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