Reactive Strength Index (RSI) in sports

Learn everything you need to know about the Reactive Strength Index (RSI), including the test protocol, RSI formula, RSI normative chart and exercises to improve RSI.

By Loek Vossen

Reactive Strength Index (RSI) test

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What is the Reactive Strength Index in sports

The Reactive Strength Index (RSI) is a measure that demonstrates an athlete’s ability to rapidly change from an eccentric motion into a concentric muscular contraction. “Reactive” refers to this transition from eccentric to concentric, also known as the stretch-shortening cycle. RSI is an important strength test to track fitness progress.

It can be used to measure an athlete’s:

  • Ability to cope with plyometric exercises
  • Acceleration speed
  • Change of direction speed
  • Reactive strength
  • Stretch-shortening cycle

It is also used to measure central nervous system fatigue.

Reactive Strength Index vs Dynamic Strength Index

The Reactive Strength Index (RSI) is not the same as the Dynamic Strength Index (DSI). The dynamic strength index is also known as the dynamic strength deficit or explosive strength deficit.

The dynamic strength index is a ratio between your dynamic peak force (e.g. measured using a 1-repetition maximum (1RM) back squat) and a ballistic peak force (e.g. measured using a countermovement jump (CMJ) or squat jump).

This article will focus only on the RSI.

How to test Reactive Strength Index

RSI measures the reactive jump capacity of an athlete in a drop jump (also: depth jump).

The traditional test is an incremental drop jump RSI test. The athlete starts on a block or box. Strength coach Travis Mash recommends a box height between 30-45cm. This is high enough to get a good measure, but not too high to risk unnecessary damage to the body.

Once the athlete is on top of the box, he/she steps off, lands on the floor and immediately rebounds back in a vertical jump. The intent is to jump as high and as quickly as possible.

During the test, you measure the ground contact time and the jump height.

GymAware RS is a good example of equipment that measures the Reactive Strength Index. The GymAware Cloud helps you track (daily) RSI. Here’s the RSI protocol with GymAware equipment. Note that RSI is just one metric you can measure with this velocity based training device.

How to calculate Reactive Strength Index

The GymAware equipment and GymAware cloud automatically calculate RSI in the protocol. It uses the following formula:

Reactive Strength Index (RSI) = Jump Height / Ground Contact Time

This RSI equation makes clear that improving RSI comes down to two things:

  1. Increasing vertical jump height
  2. Decreasing ground contact time
Eamonn Flanagan RSI quadrant
RSI depends on jump height and contact time. These 2 factors together create a quadrant in which you can place the result of your RSI. Source.

An alternative formula to measure RSI is:

Reactive Strength Index (RSI) = Flight time / Ground Contact Time

There is a third RSI formula that is called the Modified Reactive Strength Index (RSImod):

Reactive Strength Index Modified (RSImod) = Jump Height / Time to Takeoff

Although all three formulas are very straightforward, having accurate data is key. GymAware equipment is highly accurate and has been independently validated. It is the system of choice for hundreds of elite coaches world-wide.

What is a good Reactive Strength Index score

Once you have an RSI score, you probably wonder what a good Reactive Strength Index score is.

Reactive Strength Index chart

At GymAware, we did our own research with a small population, to create an RSI chart with normative data:

Reactive Strength Index (RSI) chart
Reactive Strength Index (RSI) chart with normative data.

Although this RSI chart is a super helpful starting position, you can also use your own group of athletes to learn what a good RSI norm is.

For more example RSI scores, I recommend checking out:

  • This study when interested in RSI norms for female soccer and rugby players
  • This study for RSI reference values for male and female NCAA division I athletes
  • This study when interested in the influence of sex and maximum strength on RSI

How to improve Reactive Strength Index

Improving RSI is about improving the stretch-shortening cycle (eccentric > concentric) of a muscle. It is important to first know the RSI score of an athlete before prescribing a training program. Untrained athletes could easily injure themselves with an advanced RSI training program.

Eamonn Flanagan (Lead Strength and Conditioning Consultant at Sport Ireland Institute) suggests the following plyometric progression plan to improve reactive strength:

Eamonn Flanagan RSI training program
Plyometric progression plan for reactive strength development. Source. 

Typical exercises to improve RSI are: jumps and bounces, plyometrics, countermovement jumps, speed ladders, mini hurdle drills, reverse box jumps, rebound box jumps etc.

RSI in a nutshell

RSI is a simple and easy to implement test that looks at the ability of an athlete to change from an eccentric to a concentric movement. It can also help to assess daily readiness.

Although the calculations are pretty straightforward, you need accurate measurements to track RSI progress over time. GymAware is gold standard velocity based training equipment that tracks RSI, among others.

Before you create a training program to improve RSI, make sure you know your athlete’s RSI score. Otherwise you could easily injure an athlete with exercises that are too demanding.

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Loek Vossen

Loek Vossen

Human Movement Scientist | Content Marketing and Education