Reactive Strength Index (RSI): What It Is and How to Calculate It

If you could only keep one number to summarize "how explosive is someone's lower body in short, reactive efforts," many strength and conditioning coaches would pick the Reactive Strength Index (RSI). It's not a test in itself, but a calculation derived from other jump tests, condensing a lot of useful information into a single value.

What does this test measure?

RSI quantifies the efficiency with which the neuromuscular system converts a force-absorption phase (eccentric) into a propulsive phase (concentric) in the shortest possible time. In other words: how much jump height you get for each unit of time spent in contact with the ground.

It can be calculated in two common ways:

• From the Drop Jump: jump height (m) ÷ ground contact time (s)
• Modified RSI (from the CMJ): jump height (m) ÷ total movement time (s), useful when a contact mat isn't available

Equipment needed

• Contact mat or force plate, to obtain flight time and contact time accurately
• A box of known height (if calculating from the Drop Jump)
• Alternatively, a vertical jump app that records phase times

Test protocol

1. Perform a Drop Jump from a known drop height (see the drop jump test for the full protocol)
2. The mat or software automatically records:

- Flight time → converted into jump height - Contact time → time the feet spend on the ground between landing and takeoff

1. Calculate the RSI by dividing jump height (in meters) by contact time (in seconds)
2. Repeat the calculation at different drop heights (20, 30, 40 cm) to identify at which height the athlete achieves their best RSI
3. Record the maximum RSI and the drop height at which it occurred

Common mistakes

• Calculating RSI from jumps without a stretch-shortening cycle (such as the Squat Jump), which doesn't make conceptual sense
• Not testing several drop heights, missing information about the individual's "optimal height"
• Confusing a low contact time with a "good RSI" without accounting for jump height (a very low contact time with a very low jump also gives a low RSI)
• Using RSI as the sole decision criterion without contextualizing it with maximal strength and other data

How to interpret the result

RSI	Interpretation
< 1.0	Low reactive capacity; prioritize maximal strength and basic explosive strength before intense plyometrics
1.0 - 1.5	Moderate reactive capacity
1.5 - 2.5	Good reactive capacity, typical of trained athletes
> 2.5	Very high reactive capacity, typical of elite athletes in explosive disciplines

RSI is especially useful for monitoring fatigue: a drop in RSI relative to the athlete's usual value (even without major changes in jump height) can be an early sign of accumulated neuromuscular fatigue.

Relationship with other tests

RSI doesn't exist without a jump test to feed it, so it makes sense to understand it alongside:

• Drop Jump test — the base test from which classic RSI is obtained
• CMJ test (Countermovement Jump) — to calculate modified RSI when a contact mat isn't available

Track progress over time

RSI is one of those indicators that only becomes valuable when tracked over time and across different drop heights. With Movalytics you can log the jump height and contact time for each Drop Jump, let the app calculate RSI automatically, and compare the evolution of your athlete's maximum RSI and "optimal height" mesocycle by mesocycle — very valuable information for adjusting plyometric volume.

Frequently asked questions

Do I need an expensive force plate to calculate RSI? It's ideal for maximum accuracy, but there are high-speed camera jump apps that provide reasonable estimates of contact and flight time, sufficient for practical tracking. What does it mean if RSI drops from one week to the next without much change in jump height? It's usually a sign that the athlete is taking longer to "leave" the ground, meaning their reactive capacity is reduced — common during periods of accumulated fatigue. Is RSI useful for all sports? It's especially relevant in sports with repeated, short explosive actions (jumps, sprints, changes of direction). In pure endurance sports it has less direct relevance, although it remains a good general indicator of lower-body neuromuscular health.