RSI-Modified Explained: The Countermovement Jump Reactive Metric

Two athletes post an identical 34 cm countermovement jump on the same force plate. Same jump, same output — until the trace prints: one left the ground in 0.48 seconds, the other took 0.71 to build the exact same height. Same result, different quality of movement, and jump height alone would never have shown you that.

That's the gap RSI-modified was built to close. It reuses the countermovement jump you're already running and adds one number describing how efficiently an athlete converts time into height — no drop box, no timing mat, no extra test. This guide covers what RSImod calculates, how it differs from the classic drop-jump RSI most coaches already know, a clean measurement protocol, what the published research does and doesn't support, and where testing goes wrong on a real team.

The Formula

RSI-modified (RSImod) is calculated as: RSImod = Jump Height (m) ÷ Movement Time (s), where movement time runs from the first sign of the countermovement — the instant vertical ground reaction force drops below bodyweight as the athlete begins to dip — through to takeoff.

Example: an athlete jumps 0.35 m with a movement time of 0.62 seconds. RSImod = 0.35 ÷ 0.62 ≈ 0.56.

Why It's Not on the Same Scale as RSI

If you've used classic RSI from drop jumps, that 0.56 will look alarmingly low — a decent athlete's drop-jump RSI usually sits between 1.5 and 2.5. RSImod isn't broken; it's measuring across a much longer window. Ground contact time on a drop jump runs 150–250 milliseconds. Movement time on a CMJ, from first dip to takeoff, runs 450–750 milliseconds — three to four times longer, because the athlete is generating their own eccentric loading instead of absorbing a fall. Divide roughly similar jump heights by a denominator three times larger and the ratio lands in a completely different range by design. Compare RSImod values only against other RSImod values, never against drop-jump RSI norms.

What the Number Is Actually Telling You

A high RSImod means the athlete produced good jump height without spending excessive time getting there — fast eccentric braking, a clean transition at the bottom of the dip, and quick concentric drive. A low RSImod next to a perfectly normal jump height usually means one of two things: the countermovement was unusually deep and slow to build force, or the athlete paused at the bottom before driving up. Both patterns can still produce a respectable jump, and both cost time — and on the field, time under tension at the bottom of a jump is time an opponent isn't going to wait around for.

Two Metrics From the Same Family, Different Jobs

Both metrics divide a height by a time to describe stretch-shortening cycle efficiency, but they load the tissue differently and answer different questions in a training program.

The distinction that actually matters day to day: a CMJ needs nothing beyond a force plate, jump mat, or wearable the athlete already stands on — no box, no landing mechanics risk, no cumulative joint stress from repeated drops. That's why RSImod has become the default reactive metric for morning readiness checks, while drop-jump RSI stays reserved for periodic testing blocks a few times per training cycle.

Equipment Options

Force plates give the cleanest movement-time data because they capture the exact instant force drops below bodyweight. Validated jump mats and wearables (see our jump mat vs. force plate comparison) estimate the same event from acceleration or displacement thresholds — accurate enough for daily field monitoring, though they carry slightly more trial-to-trial noise than a lab force plate.

Step-by-Step Protocol

1. Athlete stands still for 1–2 seconds to establish a stable bodyweight baseline before jumping.
2. Athlete performs a maximal CMJ at a self-selected countermovement depth, hands on hips to remove arm swing as a variable (unless arm-swing contribution is specifically what you're testing).
3. Movement time begins when vertical force first drops a set threshold below bodyweight — commonly 5 standard deviations of the quiet-standing baseline, or a fixed 10 N threshold.
4. Movement time ends at takeoff, when vertical force falls to near zero, typically below 10–20 N.
5. Jump height is derived either from flight time or from the impulse-momentum method using takeoff velocity. The two can differ by 1–3 cm on the same jump, so pick one method and stay consistent with it across sessions.
6. Run 3 trials with 45–60 seconds of rest between each. Use the trial with the highest RSImod, not the trial with the highest jump height alone — they aren't always the same rep.

One detail that trips up a lot of programs: the threshold used to mark the start of the countermovement changes the resulting movement time by 30–60 milliseconds depending on the software default. Switch testing systems mid-season and your baseline will shift even though performance hasn't. Re-baseline every time you change hardware or software versions.

Ebben and Petushek (2010): Where RSImod Came From

RSImod was first proposed by Ebben and Petushek in a 2010 study published in the Journal of Strength and Conditioning Research. Testing roughly 50 NCAA Division I female athletes across multiple sports on a force plate, they compared RSImod against other force-time variables already used to describe CMJ performance — eccentric and concentric rate of force development, peak power, and time to peak force. RSImod correlated moderately to strongly with several of these variables, loosely in the r = 0.5–0.8 range depending on which one, suggesting a single, easy-to-calculate ratio captures much of what practitioners were otherwise pulling from more complex force-time curve analysis. That's still the core case for using RSImod today: most of the diagnostic value of a full curve breakdown, from one division, without curve-analysis software.

McMahon et al. (2018): The Same RSImod Can Hide Different Strategies

Worth taking seriously before you trust RSImod as a standalone number: McMahon and colleagues (2018, International Journal of Sports Physiology and Performance) grouped college-age team-sport athletes by RSImod and matched them for jump height, then compared the underlying force-time curves. Athletes with the same RSImod and the same jump height still showed meaningfully different countermovement depths and peak force profiles — some got there with a shallow, fast dip and high relative force, others with a deeper dip and higher peak power expressed over a longer window. Two athletes can post an identical RSImod for different mechanical reasons, and the ratio alone won't tell you which strategy is in play. If a training decision hinges on the result, glance at countermovement depth or the force-time trace before acting on the number in isolation.

What This Means for Field Use

Both studies used lab-grade force plates in modest college samples, and neither tracked RSImod across a full competitive season under real training loads — closer to how most coaches actually use it. Treat RSImod as a strong trend-monitoring tool, not a number to base a roster or return-to-play decision on by itself.

Typical RSImod Ranges by Training Status

Published normative tables for RSImod are sparser than for drop-jump RSI, and values shift with whatever movement-time threshold a given lab uses. Treat the ranges below as a starting reference, not a hard cutoff — build each athlete's own baseline over 3–4 sessions before trusting any change against it.

A 70 kg athlete who moves from a 0.70-second to a 0.58-second movement time at the same 34 cm jump height — a change most coaches would completely miss on jump height alone — pushes RSImod from roughly 0.49 to 0.59. That's a meaningful efficiency gain, and it usually shows up weeks before the athlete's actual jump height improves. See also: Countermovement Jump Test: Complete Protocol & Norms.

A Simple Weekly Monitoring Routine

RSImod earns its keep as a trend, not a one-off snapshot. A basic in-season cadence:

This is a monitoring cadence, not a diagnostic one. A single low session after a poor night's sleep isn't a training problem — the same drop repeating for two straight weeks usually is.

Errors Practitioners Actually Make

• Comparing across different onset thresholds. If a software update changes how the system defines the start of the movement, last month's RSImod numbers and this month's aren't the same measurement anymore. Re-baseline after any software or hardware change.
• Reading RSImod on the drop-jump RSI scale. A 0.55 isn't a poor score — it's a normal CMJ score. Coaches new to the metric routinely panic over numbers that are completely healthy.
• Letting countermovement depth drift between sessions. A deeper dip this week versus last week changes movement time independent of anything neuromuscular. Cue a consistent depth — a visual line or a verbal cue works — so the same movement pattern is being compared over time.
• Using flight time on a compliant surface. Landing mats and soft turf inflate flight time and, with it, jump height, which drags RSImod up artificially. Test on the same hard, flat surface every session.
• Chasing a single session's number. One low RSImod reading tells you almost nothing on its own. A rough night of sleep, a heavy lower-body session the day before, or ordinary warm-up variability can all move the number 10–15% with no real change in readiness.

A Quick Self-Check Before You Trust a Change

Before flagging a drop as fatigue or a rise as progress, run through this: same testing surface, same movement-time threshold and software version, same hand position, at least 3 trials rather than 1, and compared against the athlete's own 3–4 session baseline rather than a published norm. If all four hold and the change persists across two sessions, it's real enough to act on.