Improving Modified Reactive Strength Index Scores

This article will refer to the Reactive Strength Index – Modified (RSImod) rather than the original Reactive Strength Index (RSI). The calculation for RSImod is jump height divided by time to takeoff in a countermovement jump. RSI refers to jump height divided by contact time in a drop jump.

RSI and RSImod do not always align within athletes (1), likely due to skill and familiarization issues in drop jumps, so we will use the simpler version here.

This “problem” is a typical one because as Kipp showed, RSImod essentially represents an athlete’s explosiveness (2), a training objective in most sports. We will approach the problem with three strategies, based on the likelihood that they need to be addressed.

The first approach and the “simplest” is to increase the relative strength of the athlete. As Cormie showed, in weak athletes, increasing strength will also increase explosiveness (3). You can pick your measure of choice, whether it be 1RM in squat, trap bar deadlift, split squat, or peak relative force in the isometric midthigh pull. Increasing strength per unit of body mass will make athletes more explosive.

The second approach to improving RSImod comes via plyometric training. If the athlete has a good amount of relative strength, then a more specific stimulus is needed to “tune the system”. In a CMJ, the underlying aspects of RSI are different than those for jump height (4), so it’s important to isolate these. This is about the ability to relax and unload, then stiffen quickly to change the body’s momentum. The best examples of this behavior come from plyometrics and sprinting (the ultimate plyometric!). When choosing your drills, choose exercises where ground contact time can be kept to a minimum, preferably 200ms and less. The objective is rapid spikes in eccentric force, not high external loads.

The third approach to improving RSImod will come from a load-management strategy. We know that RSImod is one of the more sensitive metrics to increased loading in an athlete, being reduced up to 72hrs after exhaustive exercise (5, 6) and during periods of overreaching (7). The lowest hanging fruit as a strength coach and sport scientist is to look at the accumulated load/stress of the athlete. Ask yourself if it’s time for that athlete to take a break. A 3-4 day unloading period (maybe longer if overreaching has been extensive) can make for very sharp increases in RSImod. This represents a peaking or tapering strategy, so it’s important to remember, you can’t taper what you don’t have.

Using regular jump monitoring, whether with force plates, jump mats, lasers, or cameras, can provide the resolution required to make important decisions around workout programming strategies and overall load management of athletes. RSImod is a good metric to evaluate explosiveness. When looking for a boost: get STRONG, learn how to be REACTIVE, and be fresh if you want to SHOW IT OFF!