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  • John R. Harry, PhD, CSCS

CMJ "Explosiveness" and RSImod: A magical or misunderstood performance metric?

Hello there. Yes, that was an homage to this week's holy day of May the 4th (be with you).

This week I wanted to share some thoughts on the magical, crime-stopping, Grammy-winning, all-around perfect CMJ performance metric: reactive strength index modified (RSImod), which is the ratio of jump height and time to takeoff. All you have to do is search for studies on the CMJ in the last 5 years, and I'd bet it all on the fact that >80% of the studies returned in the search will have included RSImod as a key performance metric. There's good reason for its near exponential rise of inclusion in both the literature and practitioner settings. But, I feel compelled to also discuss it's limitations to help us all loosen our collective death grip we have on this metric, because it's not always the magical, crime-stopping, Grammy-winning, all-around perfect metric that we sometimes make it out to be. Before you start requesting that this blog be canceled because what I'm about to say doesn't align with your RSImod opinion, keep in mind that I, too, am a proponent and frequent user of it. Don't believe me? Go check my CMJ publications. I just want you all to understand it as best as possible so we can keep moving the field forward.

First, let's start with why RSImod can be a useful metric of performance. Most of us who conduct jump tests do so with athlete populations. I think it's fair to say that most athletes must perform physical movements within a time-constrained environment. For instance, a basketball player might be able to jump 70 cm. But if the task (e.g., rebounding a basketball; Figure 1) requires that athlete to jump 55 cm, they will need to achieve that jump height faster than another athlete, even if 55 cm is the opposing athlete's maximum jump height. So, height alone is not going to dictate success in this situation. Because of that gravity thing, when we jump to a specific height, we will spend the same amount of time in the air no matter what. In order to get to that specific height faster, we will need to spend less time on the ground before takeoff.

Figure 1. The jumper who gets to the desire height sooner often succeeds in competition.

RSImod is so frequently studied because it represents the time-constrained quality of ground-based sports by accounting for the height achieved and the time required to get off the ground. RSImod was shown to reside in the quadrant of CMJ profiles characterized by "high forces and fast jumps", so it can be also be thought of as a representation of "speed-strength" (Kipp, et al 2016). We generally state that RSImod is therefore a reflection of an athlete's explosiveness, or the ability to suddenly increase speed or power while playing a sport (or performing a sport-specific action). Please, please, please don't jump on my butt about athletes not actually "exploding" (i.e., they don't go boom) and then claim it's not the right term to use. After all, what I stated above is a formal definition of explosiveness. Don't believe me? Take a look for yourself."

Now let's talk about the limitations of RSImod. A prominent issue I see is that users are not well-versed with what RSImod "means," which I infered from the way they display it with units, notably meters (jump height units) per second (time units). I think most of us know that m/s is velocity, or an object's rate of change of position. We cannot label RSImod as a "velocity" with units of m/s because the change of position (jump height) occurs after takeoff while the time component (time to takeoff) occurs prior to takeoff (the change of position doesn't occur during the time-to-takeoff duration). So it's not a velocity. In fact, this is why it‘s an index. It should be presented without units or with "arbitrary units" so it's not confused with velocity.

Another potential limitation of RSImod is the fact that it is a ratio. Fabricated relationships can occur among variables when using ratio-based indices, which can make it difficult to generate meaningful interpretations (Allison et al 1995). Another related limitation is that some researchers and practitioners fail to concurrently study the constituent parts of RSImod (jump height and time to takeoff). This makes useful interpretation impossible. For example, recreationally active samples demonstrate greater RSImod scores (i.e., greater explosiveness) by increasing jump heights (Harry et al 2018) but high-level basketball players demonstrate greater RSImod scores by increasing jump heights and decreasing times to takeoff (Krzyszkowski et al 2020). Without assessing the constituent parts alongside RSImod, you can't determine why or how they improved their explosiveness. The same issue reveals itself when comparing groups or individuals. For example, two athletes can demonstrate equal explosiveness via RSImod but very different constituent parts, so there's unique jump abilities masked by the RSImod score. An example of this is shown in Table 1. So, RSImod is probably a population- or comparison-specific metric and not the end-all-be-all metric that some of us hold on a pedestal so high you'd think it was sent to us from the maker of all things CMJ.

Table 1. Jump performance results from two athletes. Each demonstrated different jump height and time-to-takeoff performances but equal RSImod scores (i.e., "equal" explosiveness).

Something else to consider is what the constituent parts represent from a philosophical perspective. I am of the opinion that jump height and time to takeoff are both "performance" metrics, though others might consider time to takeoff a "strategy" metric. My opinion is based on the fact that the temporal strategy is how you manipulate the phase durations that make up the total time to takeoff. Thus, phase-specific temporal strategy leads to a change in temporal performance. But, not everyone thinks like me, and that's okay. You probably should not think completely like me anyway. I mean, I've been jazzed about some pretty ridiculous stuff I look back on with a disappointing shake of the head (like when I thought rate of power development was a cool and informative jumping variable - but then I went to graduate school and really began to understand mechanics [thanks Max Paquette]). If you fall into the group that considers time to takeoff a strategy metric, do you think it is realistic to have a primary performance metric that is defined as the ratio of performance and strategy? I know that's not really a data issue, but the philosophy is important. For instance, someone who is against eating meat wouldn't use a meat-based ingredient in their dinner just because it was part of the recipe found on Google. Right? So, if you're against time to takeoff as a performance metric, why would you use it as a performance metric ingredient? There's surely another metric that can provide similar information. Hell, there might even be a better one.

To wrap this up, RSImod is a useful metric of CMJ performance. But, we absolutely must know what it "means" and how to explore changes in performance represented by RSImod. We also need to know what population we are studying, as some populations seem to only manipulate one component of RSImod, while other population sub-groups don't demonstrate varied RSImod scores. All this means is that there might not be a reason to bother calculating RSImod in some circumstances, not matter how badly we need (want?) to.

Okay, people, it's time to sign off and get back to "work." Keep on keepin' on, and I hope to see you for next week's edition of the Fun Friday Blog. Peace!

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