How to Use RSI for Plyometric Readiness: Drop Jump Assessment Guide

A 2019 meta-analysis by Beattie et al. (Sports Medicine) found that Reactive Strength Index (RSI) measured via drop jump was the single best predictor of sprint acceleration among all jump-test metrics, explaining 62% of variance in 10-metre sprint time across team sport athletes — outperforming countermovement jump height, broad jump, and squat strength. RSI's predictive power stems from what it uniquely measures: the athlete's ability to produce force rapidly in the amortisation phase of the stretch-shortening cycle. That same stretch-shortening cycle capacity is what determines whether an athlete can safely tolerate high-intensity plyometric training on a given day, making RSI the most logical readiness gate for plyometric session intensity.

This guide explains how to measure RSI via drop jump, how to find each athlete's optimal drop height, what RSI scores classify as training-ready versus at-risk, and how to use real-time RSI data to autoregulate plyometric volume and intensity.

What Is Reactive Strength Index?

RSI was formalised by McClymont (2003) and subsequently validated by Young (1995) and numerous subsequent researchers. The standard formula is:

RSI = Jump Height (m) ÷ Ground Contact Time (s)

For example, an athlete who achieves a 0.38 m jump height with a 0.185-second ground contact time produces an RSI of 2.05. The ratio captures the quality of the stretch-shortening cycle (SSC): a high RSI requires both sufficient reactive force (numerator) and efficient energy transfer within a short time window (denominator). An athlete who jumps high but contacts the ground for a long time has poor SSC efficiency — a relevant distinction that jump height alone cannot detect.

RSI vs. CMJ Height: Why the Distinction Matters

CMJ height reflects the athlete's capacity to utilise elastic energy stored in the SSC over a self-selected countermovement duration — it primarily captures power output. Drop jump RSI, by contrast, constrains the time available for force production to the actual ground contact time, making it a direct measure of SSC stiffness and reactive force production. These are related but distinct neuromuscular qualities, and each responds differently to training and fatigue. An athlete may have high CMJ height but low RSI if they rely on a slow, deep countermovement rather than elastic rebound — a pattern that predicts poor acceleration mechanics and elevated ankle and Achilles tendon injury risk (Dessing et al., 2019).

How RSI Reflects Plyometric Readiness

RSI is acutely sensitive to neuromuscular fatigue in ways that CMJ height alone is not. When the stretch-shortening cycle is compromised by accumulated fatigue — particularly from heavy eccentric loading, high-volume sprint work, or intense plyometric sessions — ground contact time increases more than jump height decreases. This means RSI drops more sharply than jump height in response to SSC-specific fatigue.

Flanagan & Comyns (2008, Journal of Strength and Conditioning Research) showed that RSI declined significantly 24 and 48 hours after a high-volume plyometric session even when CMJ height had nearly fully recovered. This temporal dissociation makes RSI a uniquely sensitive marker for SSC-specific readiness: an athlete whose CMJ looks fine but whose RSI is suppressed still carries meaningful plyometric injury risk, particularly in activities demanding rapid ground contact (depth jumps, bounding, sprint acceleration from blocks).

The practical implication: for sessions involving true reactive plyometrics (drop jumps, hurdle hops, bounding), RSI should gate the session rather than CMJ. For sessions involving slower-SSC movements (box jumps, jump squats), CMJ height is the more appropriate readiness marker.

Drop Jump RSI Testing Protocol

Standardisation is critical for RSI reliability — small changes in drop technique dramatically affect both jump height and ground contact time. Follow this protocol precisely:

Setup

1. Use a box with a measured, precise height (not a guess). Standard testing heights: 20 cm (low), 30 cm (standard), 40 cm (moderate), 50 cm (high).
2. Athlete stands on box with toes at the front edge, arms by sides or on hips (fix arm position and keep consistent across all tests).
3. Athlete steps off (does not jump off) the box with one foot leading, landing on both feet simultaneously.
4. Upon landing, immediately rebound maximally — the goal is minimum ground contact time with maximum jump height.
5. Arms may swing freely on rebound if consistent across tests; the arm-fixed protocol is preferred for research comparisons.

Repetitions and Rest

Perform 3 trials per drop height with 45 seconds rest between trials. Use the best RSI value (not mean) as the data point. A coefficient of variation above 10% across 3 trials indicates technique inconsistency; retest after additional practice trials.

Cuing

Instruct athletes to: "Land and leave the floor as fast as possible — imagine the floor is hot." This cue elicits reactive (not absorb-then-push) mechanics. Avoid "jump as high as you can" without the ground contact time emphasis — this encourages a longer, more forceful push rather than true reactive rebound.

Finding Optimal Drop Height

Each athlete has an optimal drop height — the height that produces peak RSI for that individual. Testing above optimal drop height increases landing force beyond what the SSC can efficiently utilise, increasing ground contact time (the denominator) more than jump height (the numerator), causing RSI to decline. This is the basis of the optimal drop height assessment.

To find an athlete's optimal drop height: test RSI at 20, 30, 40, and 50 cm in a single session (allow 3 minutes rest between heights). Plot RSI vs. drop height. The height producing peak RSI is the optimal. Use this height for all subsequent readiness testing to ensure comparable values across sessions.

Importantly, optimal drop height should be re-evaluated every 8–12 weeks because it increases as athletes develop SSC stiffness and reactive strength through training (Byrne et al., 2017).

RSI Norms and Classification

Population-specific RSI norms from a 30 cm drop height provide context for classifying athletes and identifying training targets. Values below reflect best-of-3 trials, arms on hips protocol:

These norms are drawn from Flanagan et al. (2008) and updated by Byrne et al. (2017). Note that norms differ between sports: rugby players average RSI 1.4–1.9; sprinters and jumpers typically score 2.4–3.5; basketball players average 1.8–2.4 depending on position.

Using RSI to Gate Plyometric Session Intensity

The gating framework works by comparing today's pre-session RSI to the athlete's personal 7-day rolling RSI baseline. This removes absolute RSI score as the gate (which disadvantages lower-baseline athletes) and instead flags relative readiness — the only meaningful question before a training session.

Gate Rules

• RSI within 5% of baseline: Green gate — proceed with planned plyometric session, including highest-intensity exercises.
• RSI 5–10% below baseline: Yellow gate — reduce session foot-contacts by 20–30%; eliminate true depth jumps; maintain box jump and CMJ-based exercises.
• RSI >10% below baseline: Red gate — replace reactive plyometrics with slow SSC alternatives (box step-downs, controlled squat jumps); no bounding or multi-directional reactive work.

This system allows athletes to train productively even when RSI is suppressed — the training stimulus shifts from SSC-intensive reactive training to controlled jump training that does not demand rapid amortisation. Recovery of SSC capacity is accelerated by avoiding taxing the already-fatigued mechanism further.

Tracking RSI Across a Training Cycle

Longitudinal RSI data reveals adaptation patterns that are directly actionable for programming. In a well-designed plyometric training block, RSI shows a characteristic trajectory:

1. Weeks 1–2 (loading): RSI may decline slightly as accumulated SSC fatigue exceeds current adaptation.
2. Weeks 3–4 (adaptation): RSI stabilises or shows small improvement as the athlete adapts to the plyometric stimulus.
3. Post-deload (Week 5+): RSI typically peaks, reflecting the supercompensation of SSC stiffness and reactive force production.

Byrne et al. (2017, Journal of Strength and Conditioning Research) documented mean RSI improvements of 12–18% following a 6-week progressive drop jump programme in collegiate sprinters, with the optimal drop height increasing from 30 cm to 40 cm across the block — confirming that the neuromuscular system was genuinely adapting to higher eccentric loads rather than simply tolerating them. Regular RSI testing (2–3 times per week) allows coaches to identify when athletes are in the supercompensation window and schedule the highest-demand plyometric sessions accordingly.