Original article written by John McMahon, Paul Jones and Paul Comfort
Background
Reactive strength is a physical quality that has received much attention in the literature over the years[9], and can be measured across a range of different tests such as: drop jumps[1,11], countermovement rebound jumps[7], the 10-5 repeated jump test[3] or even repeated hopping [4]. Because it is measured during a variety of repeated jump tests, the measurement is a ratio of jump height/distance (or flight time) divided by ground contact time [6,9]. However, Ebben et al.[5] suggested dividing jump height by time to take-off during a countermovement jump, giving the metric of reactive strength index modified (RSI-Mod), which has received growing attention in recent years[2,10,13,14]. Interestingly, Ebben et al.[5] showed that RSI-Mod was both highly reliable and sensitive enough to elicit meaningfully different values across different exercises in both men and women; thus, providing practical utility for coaches who wish to use this metric. This was further supported in the present study being discussed, when McMahon and colleagues[12] stated that RSI-Mod is likely to be easier for coaches to comprehend, compared to some force and velocity variables. Additionally, time to take-off (which is one of the component parts of RSI-Mod and represents an assessment of time-constrained force production during the CMJ)[12] is believed to be an important attribute for team-sport athletes[17].
What did the authors do?
In the present study, 60 Rugby League players (30 from the English Super League, 30 from the English Championship – which represents the top and second tier professional leagues in the UK, respectively) performed 3 countermovement jumps during a single test session, on a force platform, during the start of pre-season. A variety of force-time variables were monitored, but given the title of this study, the key variables of interest for this review are: 1) jump height, 2) time to take-off and, 3) RSI-Mod (remembering that jump height and time to take-off are needed to calculate RSI-Mod). Although null-hypothesis statistical tests were run, for the purpose of this review, effect size data will be referred to, to determine differences between groups and were interpreted as: < 0.19 = trivial, 0.20-0.49 = small, 0.50-0.79 = moderate, and > 0.80 = large.
What did the authors find?
When looking at differences in our 3 key variables between playing levels, jump height showed only small differences between groups (Super League = 36.56 ± 3.99 cm; Championship = 35.57 ± 3.99 cm), resulting in an effect size of 0.25. However, time to take-off showed much bigger differences between groups, (Super League = 0.712 ± 0.074 s; Championship = 0.822 ± 0.099 s), resulting in a large effect size of 1.26. Finally, RSI-Mod also showed large differences between groups (Super League = 0.52 ± 0.05; Championship = 0.44 ± 0.08), resulting in an effect size of 1.11.
When interpreting these data, it’s clear to see the value of monitoring strategy metrics (i.e., time to take-off and RSI-Mod), in addition to an outcome measure such as jump height. Specifically, the results show that whilst there were minimal differences in jump height, Super League players were able to achieve their outcome in a noticeably shorter time period (as represented by the shorter time to take-off). Practically speaking, coaches would be able to infer that higher standard of players have better ballistic qualities during the countermovement jump, because they are able to achieve the same (or slightly better) outcome in a shorter period of time. Given many sporting actions are underpinned by having only a finite period of time to perform them[15.16.17], a similar outcome being achieved in a shorter time period can be viewed as a favourable adaptation. This also had a knock-on effect for RSI-Mod, making these 2 variables arguably more useable than jump height.
Limitations
Although one limitation was outlined by the authors, this related specifically to additional force-time variables not discussed in this review, and therefore, will not be focused on. Although a relatively simple study design, it appears to have been conducted well. That said, it should be acknowledged that this study represents data for a single time point only. Although reporting such data during pre-season holds value (i.e., providing baseline data to refer back, which is not impacted by cumulative fatigue throughout the season), it would be interesting to determine if such between-group differences remained the same over repeated time points (i.e., throughout the course of a competitive season), or whether fluctuations impacted one group more than the other. This may potentially help to inform coaches as to the sensitivity of these metrics, relative to training and competition loads that the athletes experience.
Practical applications
The authors indicated that coaches should consider using both time to take-off and RSI-Mod in their routine monitoring processes, when using force platforms for countermovement jump testing. Given previous research has highlighted the importance of monitoring strategy metrics (i.e., how the jump is performed) during jump testing [2,8,13], this represents a fair suggestion for coaches. However, for coaches who do not have the money to utilise force platforms, 2 potential options exist:
- Work alongside others who do have access to force platforms.
- The newly developed My Jump Lab smartphone app, has just added the metrics of time to take-off and RSI-Mod, in addition to jump height. Thus, coaches need only download the app in order to be able to implement these suggestions now.
Reviewer Comments
This is a nicely written up study and the author team have a history of undertaking and publishing excellent work, related to jump testing with force platforms. The lead author in particular is a real expert in this area and for anyone who is unsure how to conduct reliable and accurate jump testing on force platforms, I would encourage them to reach out to him.
Recommended resources
- Article – Using the reactive strength index modified to evaluate plyometric performance – William Ebben and Erich Petushek
- Article – Using reactive strength index-modified as an explosive performance measurement tool in Division I athletes – Tim Suchomel and colleagues
- Podcast – Reactive strength index protocols and threasholds – Eamonn Flanagan
- Article – A guide to using force plates in sports performance – John McMahon