Reactive strength index explained: RSI, RSI-modified and what each jump test actually tells you

Reactive strength index is one of the most reported numbers in jump testing and one of the least consistently calculated. The same athlete can own three different 'RSI' values on the same morning, each from a different test, and two of them can come from the same jump with different formulas. This guide separates them, shows the arithmetic, and explains what each one is good for.

One name, three tests

Two formulas for the drop jump

Drop jump RSI is reported in two ways, and force plate software usually offers both.

• Jump height ÷ contact time, in metres per second.
• Flight time ÷ contact time, a plain ratio with no unit.

They rank athletes very similarly and produce very different values. A drop jump with 0.50 s of flight and 0.25 s of contact scores 2.0 on the flight-time formula. The same jump is about 31 cm high, which gives roughly 1.2 on the height formula. Neither is wrong. A target agreed on one formula is meaningless on the other, so the formula belongs on the report next to the number.

Read contact time and height separately

RSI is a ratio, and a ratio can improve in two ways. An athlete can jump higher from the same contact, or leave the ground sooner at the same height. Those are different adaptations and they call for different training, so the two parts should always sit next to the ratio.

• Contact time under about a quarter of a second is the conventional mark of a fast stretch-shortening cycle. Longer than that, the athlete has turned the drop jump into a small countermovement jump and the test is measuring something else.
• A falling RSI with unchanged height means contacts are getting longer: often fatigue, sometimes a protective strategy after injury.
• A rising RSI from shorter contacts alone, with height falling, is not automatically progress. Check the cue the athlete was given.

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RSI-modified answers a different question

RSI-modified divides countermovement jump height by the time from the start of the movement to take-off. There is no box and no imposed landing, so it says little about tolerance to fast impacts. What it shows is strategy: an athlete who is tired or guarding something reaches the same height by taking longer, and RSI-modified falls while jump height holds. That is why it is a better weekly monitoring number than jump height, and a poor substitute for a drop jump when the question is reactive ability.

The hop test and single-leg hops

Repeated hops load the ankle and lower leg with short, stiff contacts. Reported as a mean RSI and a mean contact time across the best hops, they describe how well the athlete sustains that stiffness. Done on one leg at a time they give something the bilateral tests cannot: a clean left–right comparison of reactive strength, which matters most in the late stages of a lower-limb rehabilitation.

EUR: what the countermovement adds

The eccentric utilisation ratio compares a countermovement jump with a squat jump from a held position, usually as height ÷ height. A value close to 1.0 says the countermovement added little: the athlete jumps almost as well from a dead stop. A clearly higher value says they are using the stretch well. It is a useful companion to RSI because it separates 'not strong' from 'strong but not using it'.

Protocol details that change the number

• Box height. RSI depends on drop height. Fix it per athlete group and record it.
• The cue. 'Jump as high as you can' and 'get off the ground as fast as you can' produce different jumps. Use one sentence, every time.
• Hands on hips, or not. Arm swing adds height and changes timing.
• Best trial or mean. Decide once and do not mix.
• Surface and footwear. Both change contact time.

A checklist for reading reactive strength

• Name the test and the formula with every RSI value.
• Read contact time and height beside the ratio.
• Use RSI-modified for week-to-week monitoring and the drop jump for reactive ability.
• Compare single-leg hops left against right, and name the weaker leg.
• Look at EUR when RSI is low to see whether the problem is strength or its use.

How Kinexis handles this

Kinexis reads the drop jump, hop test, countermovement jump and single-leg hop from ForceDecks and puts them in one reactive-strength section per athlete, with every ratio beside its parts and every number against its target. Across the squad the same metrics feed the elasticity score on the readiness view and the rankings, so the athlete whose contacts are getting longer shows up without anyone opening an export.

• Drop jump RSI with contact time and jump height on their own bars.
• Hop test RSI and contact time, RSI-modified and EUR in the same section.
• Single-leg hop left against right, with the gap and the weaker leg named.
• An elasticity score per athlete built from these metrics, and a squad ranking for each.