How to Test Reactive Strength Index (RSI): A Precision IMU Protocol

The Reactive Strength Index (RSI) is one of the most reliable indicators of stretch-shortening cycle (SSC) performance. Defined as jump height divided by ground contact time (jump height / contact time, expressed in m/s), this deceptively simple ratio captures something profound: not just how high an athlete can jump, but how quickly they produce that height. Flanagan & Comyns (2008) called RSI an "integrated measure of explosiveness," and decades of follow-up work have linked RSI to sprint speed, jumping ability, and change-of-direction performance.

RSI is, however, exquisitely sensitive to methodology. Box height, landing instructions ("as fast as possible" vs. "as high as possible"), arm position, and the sampling frequency of your measurement device all materially shift the result. This guide presents a fully standardized RSI protocol built around an 800Hz IMU, drawing on the reliability work of Healy et al. (2018) and the longitudinal data of Beattie et al. (2017) to ensure repeatable, defensible measurements.

For coaches, trainers, and clinicians, RSI is most useful as a tracked variable, not a single snapshot. The protocol below is suitable for in-season monitoring, return-to-play assessment, and verifying training adaptation. See also our RSI research deep-dive for the underlying physiology.

RSI quantifies the "efficiency of explosiveness." Jump height alone tells you raw output. Pair it with the time spent on the ground producing that output, and you get a window into how rapidly the neuromuscular system reacts. Two athletes with identical jump heights are functionally very different if one has a 0.18 s contact time and the other 0.30 s.

Two main variants exist: classic RSI from a drop jump (DJ), and RSI modified (RSImod) from a countermovement jump (CMJ). They reflect different neuromuscular mechanisms.

Drop jump RSI evaluates fast SSC under 0.5 s, making it directly relevant for sprinters and jumpers. RSImod uses a slower, more voluntary action and is better for general explosiveness assessment and return-to-play tracking. Pair this with our countermovement jump guide to use both variants as complementary metrics.

The reliability of RSI hinges on protocol standardization. The procedure below follows recommendations from Beattie et al. (2017) and applies cleanly in an 800Hz IMU environment.

Step 1: Warm-up. 5 min of easy cycling, dynamic mobility, and 3-5 sub-maximal jumps. See our how to warm up before heavy lifting guide for a complete neuromuscular activation template.

Step 2: Set the box height. Start at 30, 45, or 60 cm. Recreational adult males typically begin at 30 cm; trained athletes start at 45 cm. The height that yields the highest RSI is the "optimal drop height," which is athlete-specific.

Step 3: Cue the movement. "Step softly off the box and, as soon as you land, jump as fast and as high as you can. Spend as little time on the ground as possible." Hands stay on the hips to remove arm-swing variability.

Step 4: Capture. Take the best of three trials or the average—best-of-three usually reflects the athlete's ceiling more cleanly. Reference our drop jump technique guide for movement quality cues.

RSI has historically been measured with contact mats or force plates. Mats lack mobility; force plates are expensive and stationary. An 800Hz IMU mounted at the ankle or pelvis provides equivalent information in a fully field-portable form factor.

The IMU detects touchdown and takeoff from acceleration and angular velocity signals. Algorithms typically use a vertical acceleration threshold (e.g., >5 g) to mark touchdown and a stable -1 g signature to mark takeoff. At 800 Hz sampling, this transition is resolved to 1.25 ms—roughly 8x finer than the typical 100 Hz contact mat.

Jump height is computed from flight time using h = g·t²/8, or by direct integration from the IMU. The two methods typically agree within ±1 cm. For a deeper accuracy analysis, see our jump mat vs IMU accuracy research piece.

To interpret RSI you need population norms. Healy et al. (2018) reported the following ranges across athletic populations.

More important than any single value is tracking change over time. A drop of ≥10% from an athlete's baseline is a classic signal of neuromuscular fatigue or elevated injury risk. In-season testing once or twice per week is typical. Cross-reference with diagnostic guides like why my squat isn't getting stronger to triangulate plateau causes.

The most common error is jumping off the box rather than stepping. The protocol calls for a soft step-off; jumping increases drop velocity and inflates RSI. The IMU's descent-phase acceleration signal flags this automatically.

Second, heel-first landings. RSI assumes a fast forefoot reaction; landing on the heels artificially extends contact time. Examining the first-peak shape of the acceleration trace reliably distinguishes heel from forefoot landings.

Third, excessive knee flexion at landing. Once contact time exceeds ~0.25 s, you have left the fast SSC and entered the slow SSC, and RSImod becomes the more appropriate metric. For data integrity, only accept trials with contact time between 0.10 and 0.25 s.

Finally, fatigue destroys reliability. Always test fresh after warm-up, and avoid testing within 24-48 h of a high-intensity session.