Reactive Strength Index Calculation and Meaning

Reactive strength index (RSI) is calculated by dividing jump height by ground-contact time during a jump or rebound task. Enter jump height in centimetres and contact time in seconds to get RSI in metres per second; the result describes how much height is produced relative to the time spent on the ground.

Formula

RSI = jump height ÷ ground-contact time, with height expressed in metres and contact time in seconds. In symbols, RSI = h ÷ t, where h is jump height in metres and t is ground-contact time in seconds. The calculator accepts height in centimetres, so it converts height to metres by dividing by 100: RSI = (height in centimetres ÷ 100) ÷ contact time in seconds. The unit is metres per second (m/s).

For the standard drop-jump calculation, jump height is commonly estimated from flight time using the ballistic relationship h = g × flight time² ÷ 8, where g is gravitational acceleration, approximately 9.81 m/s². If a device already reports jump height, use that value rather than converting flight time again. The height method and measurement setup should be kept consistent when comparing results.

Worked Example

Using the calculator defaults, jump height is 30 cm and ground-contact time is 0.20 s.

1. Convert height to metres: 30 ÷ 100 = 0.30 m.
2. Divide height by contact time: 0.30 ÷ 0.20 = 1.5 m/s.
3. Rounded to two decimal places, RSI = 1.50 m/s.

This example illustrates the ratio, not a target score. The result depends on the task, equipment, instructions and measurement method, so it should not be treated as a universal performance grade.

What Reactive Strength Index Measures

RSI is used in sport and exercise testing to describe an athlete’s ability to produce a jump quickly after a landing or other brief ground contact. It is a ratio: greater jump height raises the score, while longer contact time lowers it. The idea is to consider rebound performance alongside the time taken to leave the ground, rather than judging height alone.

In a drop jump, a person steps from a platform, lands, and immediately jumps vertically. The test aims to capture rapid stretch-shortening-cycle performance: muscles and tendons are loaded during landing and then contribute to the subsequent take-off. RSI is a summary of the outcome; it does not isolate the contribution of any one muscle, tendon, technique or physiological mechanism.

RSI is also used for other protocols, including repeated rebound or hopping tasks. Those measures may be calculated from flight time and contact time, or from jump height and contact time, depending on the protocol. The protocol name matters: scores from different tests should not automatically be treated as interchangeable.

How to Measure the Inputs

Both measurements need a consistent definition. Contact time is the period from initial landing to take-off. Jump height may come from flight time, a force platform, a jump mat or another measurement system. Devices can use different detection thresholds and assumptions, which may shift the reported values.

• Use the same jump task: Record the drop height, instructions, footwear, surface and arm-use rules. A change in setup can change the result.
• Measure contact time and height in the same repetition: Combining values from separate attempts does not describe one jump.
• Check the device’s units and definitions: Some systems display centimetres, others metres; some report flight time or contact time in milliseconds.
• Use clean trials: A pause on landing, an extra hop, or a deep squat may alter the movement being tested.
• Repeat consistently: Follow the same warm-up, rest intervals and number of trials when tracking change over time.

If contact time is displayed in milliseconds, convert it to seconds before calculation: 200 ms = 0.200 s. If height is in inches, convert to metres using 1 inch = 0.0254 m. The calculator above expects centimetres and seconds, so entering 200 as seconds or entering 0.30 as centimetres would create a large unit error.

How to Interpret a Score

RSI has units of m/s because it divides a distance by a time. It is not a percentage, and it is not the same as jump height or speed measured during the entire jump. A larger result means a greater height-to-contact-time ratio under the particular test conditions.

For example, a 0.30 m jump with 0.20 s contact time gives 1.50 m/s. A 0.30 m jump with 0.30 s contact time gives 1.00 m/s. The jump heights match, but the longer contact lowers RSI. Conversely, a higher jump does not necessarily produce a higher RSI if contact time also increases substantially.

There is no single cut-off that reliably classifies every person or sport. Comparisons are more meaningful within the same athlete over time, or between people tested with the same protocol and equipment. Age, training background, body size, sport demands and testing method can all affect interpretation. A change in score should also be considered alongside the underlying height and contact-time values: the same RSI can result from different combinations of those inputs.

Common Errors and Limits

The most frequent calculation error is mixing units. Convert height to metres and contact time to seconds, or use the equivalent centimetre-and-second expression in the calculator. Another common mistake is confusing flight time with contact time. They are different intervals: flight time is the time airborne, while contact time covers landing through take-off.

• Do not divide height by flight time when the intended formula calls for ground-contact time.
• Do not compare scores across different jump protocols without checking how each score was defined.
• Do not assume more decimal places mean greater measurement accuracy; rounding cannot remove sensor error.
• Do not interpret one trial as a definitive measure. Technique, timing thresholds and ordinary trial-to-trial variation affect results.

RSI compresses two measurements into one number, so it can hide useful detail. Report height and contact time with the score when possible. The index also does not directly measure landing forces, movement quality, injury risk or overall athletic ability. It is a test metric, not a complete assessment.

Frequently Asked Questions

What is the formula for reactive strength index?
For the height-based version, RSI = jump height in metres ÷ ground-contact time in seconds. The answer is expressed in m/s.

Can I calculate RSI from flight time?
Flight time can be used to estimate jump height under a ballistic-jump assumption: height = 9.81 × flight time² ÷ 8, with flight time in seconds. Divide that estimated height by ground-contact time to obtain height-based RSI. Follow the test or device’s stated method.

What is a good RSI score?
There is no universal “good” score that applies across all ages, sports, protocols and devices. Compare results using the same test conditions and interpret them in context.

Is RSI the same as RSI-modified?
No. RSI-modified commonly refers to a countermovement-jump measure calculated as jump height divided by time to take-off. Standard drop-jump RSI uses ground-contact time after landing. The names and protocols should not be confused.

Does a higher RSI always mean a better jump?
A higher ratio indicates more height relative to contact time in that test, but it does not explain the whole performance. Check the separate height and contact-time values and make comparisons only under consistent conditions.

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in h | Jump height | cm | 30 | 0
in t | Ground-contact time | s | 0.20 | 0
out rsi | Reactive strength index | m/s | (h / 100) / t | 2