Flight time or jump height for calculation of Reactive Strength Index (RSI)?

  • July 2013
  • Posted By admin

Q: Why do some people calculate Reactive Strength Index (RSI) using flight time / contact time and others use jump height / contact time?

A:  I prefer to use flight time / contact time. Both FT and CT can be measured directly and are very accurate and it is clear what these physical quantities are. RSI can also be expressed as  jump height / contact time. In most instances flight time is used to estimate jump height based on the formula:

Jump Height = 9.81 * (flight time) ^ 2 / 8

It is important to note that this is only an estimate of jump height because take-off and landing position affect flight time and so jump height varies accordingly. In terms of RSI it makes little difference which calculation is used because the correlation between the two methods is perfect because it is a straight mathematical derivation. When testing using a force plate it is better to use actual jump height based on ground reaction force calculations and then divide by contact time as this provides an even more valid RSI measure. Having said this, a simple contact mat system and using RSI = flight /contact works well and has been used extensively for diagnosis and programming as well as research.