How to Calculate the Reactive Strength Index (RSI) Score for Health and Fitness

October 2, 2025 •

4 min read

The Reactive Strength Index (RSI) is a key metric in sports science, used to measure an athlete's explosive lower body power and ability to transition from an eccentric to a concentric muscle contraction. This guide will demystify how to calculate RSI score for athletic and fitness applications, outlining the methods and necessary measurements to help you assess an athlete's reactive strength.

Quick Summary

This guide explains the calculation for the Reactive Strength Index (RSI), a critical measure of athletic power derived from jump height or flight time and ground contact time. It distinguishes the fitness metric from the financial indicator of the same name.

Key Points

Reactive vs. Relative Strength Index: 'RSI' in health and fitness refers to the Reactive Strength Index, which measures explosive power, distinct from the financial Relative Strength Index.
Calculation Formula: Reactive Strength Index is calculated as either Flight Time divided by Ground Contact Time, or Jump Height divided by Ground Contact Time.
Measurement: A plyometric test, typically a drop jump using a jump mat or force plate, is used to obtain the necessary flight time and contact time data.
Interpretation: A higher Reactive Strength Index score indicates better explosive power and efficiency in the stretch-shortening cycle. Scores often improve with targeted plyometric training.
Performance & Health Implications: Tracking RSI helps monitor an athlete's progress, identify areas for improvement, and manage training intensity to prevent fatigue and injury risk.
Test Method: For a Reactive Strength Index test, athletes step off a box and immediately rebound jump. For best results, consistency in box height and jump execution is important.

Understanding the Reactive Strength Index (RSI)

It's important to clarify that the term 'RSI' has at least two common definitions. In the financial world, it refers to the Relative Strength Index, a technical indicator used to analyze stock momentum. For health and fitness, especially in athletic training, RSI stands for Reactive Strength Index, a measure of an individual's explosive strength. This reactive strength is crucial for sports that involve repetitive hopping, jumping, and quick changes of direction, as it quantifies how efficiently an athlete can use the stretch-shortening cycle (SSC).

Reactive strength is the ability to quickly and effectively transition from a muscle lengthening (eccentric) movement to a muscle shortening (concentric) movement. A high RSI suggests an athlete can absorb and produce force quickly, which is beneficial for powerful, explosive movements.

Equipment needed for RSI calculation

To accurately calculate RSI score, you will need a reliable way to measure two key variables from a plyometric test, typically a drop jump. Here is the standard equipment required:

Jump Mat or Force Plate: This device is essential for capturing precise measurements of flight time and ground contact time. Force plates are more advanced and can also measure force production.
Drop Box: A plyometric box of a specific height (e.g., 30cm, 45cm, or 60cm) is used for drop jumps to standardize the test.
Analysis Software: The software associated with your jump mat or force plate will process the raw data and provide the necessary metrics for calculation.

Step-by-step process for calculating Reactive Strength Index

The calculation for the Reactive Strength Index is straightforward once you have your two primary measurements. The most common method involves a drop jump.

Step 1: Perform the drop jump test

Stand on a drop box of a predetermined height (e.g., 30cm).
Step off the box and land with both feet on the force plate or jump mat.
Upon landing, immediately jump vertically as high as possible without pausing.
Land back on the mat with both feet.

Step 2: Record the required data

Your measurement device (jump mat or force plate) will automatically record two key data points from the jump:

Flight Time (FT): The duration the athlete is in the air, from takeoff to landing.
Ground Contact Time (CT): The duration the athlete's feet are in contact with the ground after stepping off the box and before the rebound jump.

Step 3: Apply the RSI formula

There are two widely accepted formulas for calculating the Reactive Strength Index:

$RSI = Flight~Time~ (seconds) / Ground~Contact~Time~ (seconds)$
$RSI = Jump~Height~ (meters) / Ground~Contact~Time~ (seconds)$

The second formula is often used when the measurement device provides jump height directly. If you only have flight time, you can convert it to jump height using the following formula, where g is the acceleration due to gravity (9.81 m/s²):

$Jump~Height~= (Flight~Time/2)^2 g 0.5$

Step 4: Interpret the RSI score

A higher RSI score generally indicates better reactive strength. For example, an athlete with a flight time of 0.40s and a ground contact time of 0.15s would have an RSI of 2.67 (0.40/0.15), which is a high score. A typical standard for many athletes is an RSI of 2.0 or higher. Lower scores may indicate a need to focus on improving reactive strength through specific plyometric training.

Comparing reactive strength index and relative strength index

To avoid confusion, here is a comparison of the two concepts that share the 'RSI' acronym.


Feature	Reactive Strength Index (Sports)	Relative Strength Index (Finance)
Purpose	Measures athletic explosive power.	Measures the speed and magnitude of recent price changes.
Calculation	Ratio of jump height or flight time to ground contact time.	Based on the ratio of average gains to average losses over a specified period.
Key Metric	Jump height (or flight time) and ground contact time.	Average upward and downward price changes.
Scale	Varies, with higher values indicating better performance.	An oscillator ranging from 0 to 100.
Application	Sports performance, fitness assessment, injury prevention.	Technical analysis for stock and market trading.
Interpretation	Higher score = more explosive power.	Above 70 (overbought), Below 30 (oversold).

The importance of reactive strength in athletic health

While the calculation is a straightforward formula, the implications for an athlete's health and performance are significant. A low RSI can indicate inefficient use of the SSC, potentially increasing the risk of injury, especially in sports that demand repeated jumping and landing. By monitoring RSI over time, coaches and athletes can track progress, identify areas for improvement, and manage training load to prevent central nervous system fatigue. Progressive plyometric training can help improve an athlete's RSI and, in turn, enhance their overall athletic health and performance.

Conclusion

Knowing how to calculate RSI score is a powerful tool for athletes and coaches focused on improving explosive power and managing athletic load. The Reactive Strength Index, derived from jump tests, provides objective data on an athlete's ability to transition force quickly. While the same acronym is used in financial analysis for a different purpose, understanding the context is key. By accurately measuring and tracking the Reactive Strength Index, trainers can develop targeted strategies to enhance athletic performance and mitigate injury risk. For further information on sports science, researchers and athletes can consult reputable sources such as the National Center for Biotechnology Information and other dedicated sports science resources.

Frequently Asked Questions

A good Reactive Strength Index score is highly dependent on the sport and the athlete's level. However, some sources suggest that a score of 2.0 and above is a strong benchmark for many athletes. Elite sprinters may aim for a score closer to 3.0 or higher.

Jump height can be converted from flight time using the formula: Jump Height = (Flight Time / 2)² 9.81 0.5, where 9.81 is the acceleration due to gravity in m/s². Some jump mats provide this conversion automatically.

Manual calculation without a force plate or jump mat is difficult and prone to error. The timing required for ground contact and flight time is too precise for manual stopwatches. Specialized equipment is recommended for accurate RSI calculation.

You can improve your Reactive Strength Index score through specific plyometric training, such as drop jumps and depth jumps. Focusing on minimizing ground contact time while maximizing jump height will train your body's ability to use the stretch-shortening cycle more efficiently.

The Reactive Strength Index (RSI) is a sports science metric for explosive power. The Relative Strength Index (RSI) is a financial technical indicator for analyzing stock momentum. They use the same acronym but have completely different purposes and calculations.

A low Reactive Strength Index score can indicate a weakness in reactive strength, suggesting inefficient use of the stretch-shortening cycle. This may increase injury risk and can be improved with appropriate training.

Yes, monitoring Reactive Strength Index over time can help assess central nervous system fatigue. A drop in an athlete's RSI score without a change in training can indicate fatigue and a need for a deload period.