How to calculate the Reactive Strength Index
There are three common methods to calculate the performance of the RSI test. These are:
- Method 1: RSI = Jump Height / Ground Contact Time
- Method 2: RSI = Flight Time / Ground Contact Time
- Method 3: RSI = Jump Height / Time to Take-off
Jump height is an estimate of the height change in the athlete’s centre of mass. Jump height is best measured using the velocity data from a force platform. This can be calculated using the following formula:
Jump Height = 9.81 * (flight time)2 / 8
Flight time is quite simply the total time the athlete is in the air during a jump – from when they break contact with the floor, to when they first touchdown upon landing. This is often measured using a jump/ contact mat, however, results can be easily influenced by body position during take-off and landing. For example, if an athlete bends their legs during flight, this can alter the results and affect the accuracy of the test.
Time to take-off includes the eccentric and concentric phases of the stretch-shortening cycle (2).
Though both jump height and flight time can be measured directly and accurately, numerous professionals prefer to use flight time as opposed to jump height because it is easier to obtain and less time-consuming. It makes little difference which calculation is used as jump height and flight time are strongly correlated as both are a straight mathematical derivation (14). If using a force plate, it is better to use jump height based on ground reaction forces as this has been suggested to provide a more valid RSI measure. If no force plate is available, then using flight time calculated from a contact mat also works well and is often used in research and practise settings.