Reactive Strength Index (RSI): What It Is & How to Measure

The Reactive Strength Index (RSI) is a measure of how quickly and effectively you can switch from an eccentric (lengthening) muscle action to a concentric (shortening) action — essentially, how well you use the stretch-shortening cycle. You calculate it by dividing jump height (in meters) by ground contact time (in seconds) during a rebound jump, such as a drop jump or a repeated hop test. The result is a single number that coaches and athletes use to assess plyometric ability, explosive power, and readiness to train.

Formula

The standard RSI formula is:

RSI = Jump Height (in meters) ÷ Ground Contact Time (in seconds)

Where:

• Jump height – the vertical displacement of the body’s center of mass during the jump, typically measured in meters (convert from cm by dividing by 100).
• Ground contact time – the duration between landing and takeoff during a rebound jump, measured in seconds.

The result is expressed in meters per second (m/s). A higher value indicates a better ability to produce force quickly after landing.

Worked Example

Using the default inputs: jump height = 30 cm, contact time = 0.25 s.

1. Convert jump height to meters: 30 cm ÷ 100 = 0.30 m
2. Divide by contact time: 0.30 m ÷ 0.25 s = 1.20 m/s

So the RSI is 1.20 m/s. This is a typical value for a moderately trained athlete.

Why RSI Matters

RSI is used in sports science to evaluate an athlete’s reactive strength — the ability to absorb and return force quickly. This quality is critical in sports that involve sprinting, jumping, cutting, and change-of-direction. A high RSI suggests efficient use of the stretch-shortening cycle, which can improve performance in acceleration and vertical jump. It also helps identify asymmetries between legs and monitor fatigue or readiness. Because it requires minimal equipment (a contact mat or force plate), it’s a practical field test.

How to Measure RSI

The most common protocols are:

• Drop Jump (DJ): Step off a box (typically 20–40 cm), land on both feet, then immediately jump as high as possible. Contact time is measured from landing to takeoff.
• Rebound Jump / Repeated Hops: Perform a series of maximal hops with minimal ground contact, and RSI is calculated for each hop.
• Countermovement Jump (CMJ) with contact time: Some protocols use a CMJ performed on a force plate, but the classic RSI uses a drop jump.

Equipment options:

• Force plate – most accurate, measures flight time and contact time directly.
• Contact mat / jump mat – uses flight time to estimate jump height and contact time.
• Video analysis – using high-speed cameras (e.g., 240 fps) to measure contact time and jump height via displacement.

Whichever method you use, ensure consistent instructions: arms on hips or allowed to swing? Standardize the box height and the instruction to “jump as high and as fast as possible.�

How to Interpret Your RSI Score

RSI values vary by sport, age, and training level. General benchmarks (from drop jumps) are:

• Elite athletes: > 2.0 m/s
• Good / well-trained: 1.5 – 2.0 m/s
• Average / recreational: 1.0 – 1.5 m/s
• Below average / beginner: < 1.0 m/s

However, these are rough guides. Always compare an individual’s score to their own baseline and to norms for their specific sport and sex. A low RSI might indicate poor strength, poor technique, or fatigue. A high RSI suggests good reactive strength, but it doesn’t necessarily mean good absolute strength — it’s about speed of force production.

Common Mistakes and Misconceptions

• Using the wrong height unit: RSI is typically in m/s, but many people accidentally plug in cm and get a number 100x too large. Always convert to meters.
• Inconsistent box height: A higher drop increases contact time and may lower RSI. Use a standard box height (e.g., 30 cm) for all tests.
• Not controlling for arm swing: Arm swing can artificially increase jump height. Decide on a rule (e.g., hands on hips) and stick to it.
• Ignoring fatigue: RSI drops with fatigue, so testing when tired gives misleading results.
• Comparing across different protocols: RSI from a drop jump is not directly comparable to RSI from a repeated hop test. Always note the test type.

Limitations of RSI

RSI is a useful metric, but it has limitations:

• It doesn’t measure absolute strength – a strong but slow athlete may have a low RSI.
• It’s sensitive to technique – poor landing mechanics can inflate contact time.
• It’s only one piece of the puzzle – combine with other tests (CMJ, isometric mid-thigh pull, sprint times) for a full profile.
• It doesn’t account for body mass – two athletes with the same RSI may have very different power outputs.

Frequently Asked Questions

What is a good RSI score?

For drop jumps, a score above 2.0 m/s is generally considered excellent (elite level), 1.5–2.0 is good, 1.0–1.5 is average, and below 1.0 is below average. However, these numbers vary by sport and testing protocol, so always compare to relevant norms.

How do I improve my RSI?

Improving RSI involves increasing jump height, decreasing contact time, or both. Plyometric training (e.g., depth jumps, pogo hops), strength training (squats, deadlifts), and sprint work can all help. Focus on technique: land stiffly and rebound quickly, not softly.

What is the difference between RSI and RSI-modified?

RSI-modified (RSImod) is calculated from a countermovement jump (CMJ) by dividing jump height by time to takeoff (from start of movement to takeoff). It reflects slower, more deliberate force production, whereas classic RSI from a drop jump reflects fast reactive strength. Both are useful but measure different qualities.

Can I measure RSI without a force plate?

Yes. A contact mat or a mobile app with high-speed video can measure flight time and contact time. The accuracy is lower than a force plate, but for most training purposes, a contact mat is sufficient. Ensure you use the same device for all tests to maintain consistency.

Is RSI only for athletes?

RSI is most relevant for athletes in jumping and sprinting sports, but it can also be used for general fitness assessment, injury risk screening, and monitoring rehabilitation progress. However, for non-athletes, other tests may be more appropriate.

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in h | Jump height | cm | 30 | 0
in t | Contact time | s | 0.25 | 0
out rsi | Reactive Strength Index | m/s | (h / 100) / t | 2