Calculate reactive strength index from jump height or flight time and ground-contact time, or solve backward for a target contact time or jump height.
Reactive Strength Index
Reactive Strength Index (RSI) describes how much jump height an athlete produces relative to ground-contact time during a rebound or drop jump. Higher values can reflect a greater ability to use the fast stretch-shortening cycle, but test setup strongly influences the number.
Formula and Method
Height-based RSI = jump height in meters / contact time in seconds, reported in m/s. When flight time is entered, jump height = gravity x flight time squared / 8, using 9.80665 m/s² here. Rearranged modes solve contact time = jump height / target RSI or jump height = target RSI x contact time. Flight/contact-time ratio is a separate dimensionless metric; modified RSI uses time to takeoff rather than rebound contact time.
Inputs that must stay consistent across RSI tests
| Reference | Range or value | Use or interpretation |
|---|---|---|
| Jump height method | Flight time or device-reported height | Do not mix methods across sessions |
| Contact time | Time on the ground | Use the same device threshold |
| Drop height | Box or platform height | Keep constant for repeat tests |
| Technique | Hands, knee bend, landing instructions | Standardize before comparison |
How to Use the Result
Use the same device, surface, drop height, warm-up, arm-action rule, and jump instructions. Review jump height and contact time separately as well as the ratio because the same RSI can arise from different strategies.
Assumptions and Limitations
RSI does not by itself diagnose readiness, fatigue, injury risk, or sport performance. Flight-time-derived height assumes ballistic motion, equal center-of-mass height at takeoff and landing, and accurate event detection. Changes in landing posture can bias the estimate. Device algorithms can produce different values.
Sources
- Flanagan EP, Comyns TM. The use of contact time and the reactive strength index to optimize fast stretch-shortening-cycle training, 2008.
