Reactive Strength Index Formula & Calculator

This calculator works out the Reactive Strength Index (RSI) from a jump height and ground contact time. You enter the jump height in centimeters and the contact time in seconds, and the calculator returns the RSI value (height ÷ time) plus the flight time derived from that height. The RSI is a key metric in sports science for measuring how quickly an athlete can switch from an eccentric (absorbing) to a concentric (explosive) movement.

Formula

The standard formula for the Reactive Strength Index is:

RSI = Jump height (in meters) ÷ Ground contact time (in seconds)

In symbols: RSI = H ÷ t

• H = jump height in meters (if you measure in centimeters, divide by 100 first)
• t = ground contact time in seconds (the time your foot is on the ground during a rebound jump)

The result is expressed in meters per second (m/s). A higher value means you are able to produce more force in a shorter time – i.e., you are more “reactive� or “explosive.�

Worked Example

Using the default values from the calculator:

1. Jump height = 30 cm = 0.30 m
2. Contact time = 0.25 s
3. RSI = 0.30 ÷ 0.25 = 1.20 m/s

The calculator also shows the flight time (the time you are in the air) derived from the jump height: flight time = √(2 × 0.30 / 9.81) ≈ 0.247 s. This is a useful check – the contact time should be shorter than the flight time for a typical plyometric movement.

Why RSI Matters in Training

RSI is widely used by strength and conditioning coaches to assess an athlete’s “reactive strength� – the ability to quickly transition from a yielding (eccentric) phase to a forceful (concentric) phase. This quality is critical in sports like sprinting, jumping, and change-of-direction activities. A low RSI may indicate that an athlete is spending too long on the ground, absorbing force rather than reusing it. A high RSI suggests good stiffness and neuromuscular efficiency.

RSI is also used to monitor fatigue. After a heavy training block, RSI often drops before other performance markers change, making it a sensitive indicator of recovery status.

How to Measure Jump Height and Contact Time

In practice, RSI is usually measured with a contact mat or a force plate during a rebound jump (e.g., a drop jump or a countermovement jump with a short ground phase). The mat records the time your feet leave the ground and the time they are in contact. Jump height is typically calculated from flight time using the formula:

Jump height = (flight time² × g) / 8 (where g = 9.81 m/s²)

If you don’t have a mat, you can estimate jump height using a wall jump test (mark the highest point you can touch standing, then the highest point during a jump) and time the contact phase with a video at high frame rate. However, the most accurate and reliable method is a force plate or contact mat.

What Is a “Good� RSI? (Normative Values)

RSI values vary widely by sport, training background, and the type of jump used. For a drop jump from a 30 cm box, typical ranges are:

• Untrained individuals: 0.5 – 1.0 m/s
• Recreational athletes: 1.0 – 1.5 m/s
• Elite jumpers/sprinters: 1.5 – 2.5 m/s or higher

These are general guidelines, not strict cut-offs. It is more meaningful to track your own RSI over time than to compare with a fixed table, because testing protocols differ (box height, instruction, etc.).

Common Mistakes and Limitations

• Unit mix-ups: The most common error is entering jump height in centimeters but forgetting to convert to meters. If you enter 30 cm as “30� and divide by 0.25, you get 120 m/s – a huge, unrealistic number. Always convert cm to m (divide by 100) before dividing.
• Contact time measurement: Some people time the entire ground contact including the initial landing and the subsequent takeoff. Ensure you are measuring the time from first foot contact to toe-off.
• Jump height from flight time: The formula assumes the body’s center of mass is at the same height at takeoff and landing. In a drop jump, that is not exactly true – you land from a box, so the landing height is lower than the takeoff height. This can overestimate jump height. For a simple countermovement jump, the assumption is more valid.
• RSI is not a measure of maximal strength: It is a measure of power and reactivity, not pure force. A strong athlete may have a low RSI if they are slow off the ground.

Alternatives to RSI

If you don’t have a contact mat, you can use simpler tests like the standing long jump (distance) or the vertical jump (height) to gauge explosiveness. Another alternative is the “RSI-modified� (RSImod), which uses jump height divided by time to takeoff (the time from the start of the countermovement to the point of takeoff) instead of ground contact time. RSImod is often used with a force plate and is more practical for slower movements. However, the classic RSI with ground contact time remains the most common field test.

Frequently Asked Questions

What is a good reactive strength index for a 15-year-old?

There is no universal “good� value for a specific age because it depends on training history and sport. For a 15-year-old who is not a trained jumper, a value around 0.8–1.2 m/s is typical. For a trained young athlete, 1.2–1.8 m/s is common. The best approach is to test regularly and aim for gradual improvement.

Can I calculate RSI without a contact mat?

Yes, you can estimate it with a video analysis. Record a drop jump at 240 fps or higher, then count the frames from first contact to takeoff to get contact time. Jump height can be estimated from a wall jump or from flight time if you can measure that accurately. However, this is less precise than a contact mat.

Does RSI change with fatigue?

Yes, RSI is sensitive to fatigue. After intense training or a match, RSI typically decreases because contact time lengthens and jump height drops. Monitoring RSI can help coaches detect when an athlete needs more recovery.

Is a higher RSI always better?

Generally, a higher RSI indicates better reactive strength, but it is not the only factor in athletic performance. Extremely high RSI might come with a higher risk of injury if the athlete lacks proper landing mechanics. Also, some sports require longer ground contact (e.g., distance running) where a very high RSI is not necessary.

What is the difference between RSI and RSImod?

RSI uses ground contact time (from landing to takeoff), while RSImod uses time to takeoff (from the start of the movement to takeoff). RSImod is more suitable for countermovement jumps and is often measured on a force plate. Both are measures of explosiveness, but they are not directly comparable.

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in h | Jump height | cm | 30 | 0
in t | Contact time | s | 0.25 | 0
out rsi | Reactive strength index | m/s | (h / 100) / t | 2
out ft | Flight time | s | sqrt(2 * (h / 100) / 9.81) | 3