Reactive strength has been defined as an athlete`s capacity to bear a stretch load and change movement from a rapid eccentric to rapid concentric contraction (36) and provides an indication of fast stretch-shortening cycle (SSC) function (43). SSC actions produce more powerful contractions than concentric alone due to the storage and release of elastic energy and can be classified as either slow (over 250 ms) or fast (under 250 ms) depending on the duration of the ground contact time (GCT) (21,43). The fast SSC is utilized in many sporting movements such as running, sprinting, jumping, and hopping and can be quantified using the reactive strength index (RSI) (8). Practitioners can use this metric to monitor the effectiveness of training interventions and to determine neuromuscular readiness to perform (9). For example, RSI calculated from a drop jump was found to be more sensitive to training-induced fatigue than other commonly used tests, such as the countermovement jump (CMJ) and squat jump (7). A number of previous experimental studies that have measured RSI have used the drop jump test from various box heights (typically 20-40 cm) (8,39,44). However, drop jump testing has some limitations, particularly in children and adolescents who exhibit greater variability in technique (30). Practitioners should also consider what drop height is appropriate when their athletes span a wide range of maturational stages. For example, a standardized height of 30 cm may provide an insufficient or excessive stimulus given the disparity in neuromuscular qualities that will exist in young athletes across varying levels of maturity (41). Low and moderate levels of eccentric loading are proposed as being appropriate for athletes that are pre- and circa Peak Height Velocity (PHV), respectively (27), further questioning the suitability of drop jump assessments from set box heights, particularly if these exceed the athlete`s own maximum CMJ height. While it is possible to individualize the box height to each athlete, this is logistically difficult, time inefficient, and makes it more difficult to draw comparisons across athlete groups. More recently, repeated jump tests such as the 5 maximum hop test (29) and 10 to 5 repeated jump test (10-5 RJT) (4,9,13,14,44) have been used to assess RSI capabilities in athletes. In these tests, the intensity of eccentric loading is constrained by the athletes own jumping capabilities, potentially offering a safer and more appropriate method to measure the longitudinal development of RSI in athletes of different ages and stages of maturation. In a cohort of older adolescent males (aged 17-19 y), the 10-5 RJT performed on a contact mat demonstrated validity against a force plate, and acceptable reliability when assessing RSI variables (14). Two separate studies with adult athletes, also demonstrated excellent reliability and acceptable variability (ie, intraclass correlation coefficient [ICC] >.8 and coefficient of variation [CV] <10%) in the 10-5 RJT (4,44). Comyns et al (4) also investigated the usefulness, determined by comparing the smallest worthwhile change (SWC) to the typical error (TE), of the 10-5 RJT with adult male and female team sport athletes. The SWC helps practitioners determine if a meaningful change in performance has occurred in a set of test results (47) by examining if the SWC is greater than the TE. Coaches and athletes can then be confident in the test`s ability to detect small changes. The findings from Comyns et al (4) raise questions about the test`s ability to detect the SWC. The 10-5 RJT was rated "good" at detecting the SWC in RSI only for the female athletes, and for the males, it was rated as "marginal." However, the 10-5 RJT was rated "good" at detecting moderate worthwhile changes (MWCs) in RSI for both male and female athletes (4). This is the only study to examine the 10-5 RJT`s ability to detect the SWC in performance, and none of the studies have examined the validity or reliability of the 10-5 RJT with a younger adolescent population. It remains unknown if the 10-5 RJT is valid, reliable, and able to detect the SWC with younger athletes in the earlier stages of maturity. Considering the greater variability observed in a similar maximal hopping task with younger adolescent athletes (29), it cannot be assumed that the level of reliability and validity demonstrated with older adolescents and adults exists and with a younger, less mature cohort. Further research is required to establish if it is possible to detect the SWC with the 10-5 RJT. Evidence suggests that younger participants display greater variance during jumping tasks, which reduces in adulthood (10,15,30) as a result of enhanced motor development with increasing age, and the acquaintance of specialized movement skills with age, experience, and practice (10). SSC function and RSI also improves with age and maturation, resulting in an increased ability to attenuate and produce ground reaction force (40). Higher RSI values are the result of older, more mature subjects producing significantly greater jump heights, while maintaining similar GCT, suggesting as children mature they can apply greater force in the same amount of time (28). This can be attributed to changes in muscle size (5,20,26); fiber type (11,37); activation (6,12); architecture (2,20); and mechanical tendon properties (24,25), in addition to improvements in neuromuscular capacity, which demonstrates transitioning from a reactive to more preactive movement control strategy (28). Combined, these adaptations improve SSC function through mechanisms such as elastic energy utilization, neural potentiation, and an enhanced stretch-reflex contribution as a result of greater force producing capabilities and reduced electromechanical delay (40). Despite the potential importance of these adaptations, currently there is a lack of evidence available examining the effect of growth and maturation on RSI in field-based tasks using a repeated jump protocol. This warrants further investigation to enhance the practical utility in settings where frequent monitoring is required. Therefore, the aims of this study were to: (1) establish the intraday and interday reliability and validity of the 10-5 RJT in male adolescent athletes and (2) identify if differences exist in 10-5 RJT performance variables across elite youths who are at various stages of growth and maturation. We hypothesize that the 10-5 RJT test will be reliable with an adolescent population, and RSI will increase across maturational groups, primarily as a result of greater flight time (FTs) and jump heights with GCTs remaining similar.
© Copyright 2022 Pediatric Exercise Science. Human Kinetics. Alle Rechte vorbehalten.