# Reactive Strength Index (RSI)

The Reactive Strength Index (RSI) measures muscle­tendon stress and their reactive jump capacity, important for athletes in sports requiring dynamic jumping activities. RSI is calculated by dividing the jump height by the ground contact time.

## Testing Reactive Strength

The most commonly used test to calculate reactive strength test is the incremental drop jump test, which involves drop jumps from a series of heights, starting from a 30cm box and working upwards to a 75cm box. The RSI can also be measured using repeat hopping and jumping tasks. A quick and easy test of RSI is the "rebound jump test" involving a single double jump. There is also the 10/5 RSI test. In this test, like the rebound jump test, the athlete performs a single countermovement jump but upon landing immediately transitions into a series of 10 repeated, bilateral hops. the triple jump requires good reactive strength

## Calculating RSI

Reactive strength index (RSI) is the ratio between the height jumped and the ground contact time, calculated by dividing the jump height by the ground contact time. The jump height time is the time between the subject's feet leaving the timing mat or force platform and when they contacted it again. The vertical jump height in meters can be calculated from the jump time (in seconds) using this formula: jump height = 4.9 x (0.5 x Time)2. Ground contact time is the time between the first foot contact with the force platform and when the subject's feet left the mat.

For example, if an athlete jumped 36cm with a contact time of 0.18secs then they would have an RSI of 200. The athlete can therefore improve their RSI by increasing the height of their jump, by decreasing their ground contact time, or both of these.

You could also calculate RSI using the flight time as opposed to jump height. The RSI calculation would still be valid as it is a simple formula to convert jump height from jump time, and jump time is often easier to obtain.

references:

• Flanagan, E.P., Ebben, W.P., and Jensen, R.L. (2008). Reliability of the reactive strength index and time to stabilization during depth jumps. Journal of Strength and Conditioning Research. 22(5), pp.1677–1682.

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