03/09/2026
COACHS EYE - What is Reactive Strength Index? ⚡️
When I use a drop jump, I'm not just looking at how high an athlete jumps.
I'm also interested in how quickly they can produce that jump after hitting the ground.
That's where Reactive Strength Index (RSI) comes in.
RSI is calculated using:
Jump Height ÷ Ground-Contact Time
So, in simple terms, it gives us an indication of an athlete's ability to rapidly absorb force and transition into producing force again.
Think about two athletes who both jump 35 cm.
One spends 0.25 seconds on the ground.
The other spends 0.18 seconds on the ground.
They've achieved the same jump height — but the second athlete has produced that outcome in considerably less time and would therefore record a higher RSI.
Why does that matter?
In sport, athletes rarely have unlimited time to produce force.
Whether sprinting, jumping or rapidly changing direction, force often needs to be produced within very short timeframes.
But a higher RSI doesn't automatically mean a "better" performance.
I still want to understand how the athlete achieved it.
That's why alongside RSI I'll consider:
⚡ Jump height
⏱️ Ground-contact time
📈 Power output
🦘 The athlete's overall jump strategy
An athlete might improve RSI by jumping higher, reducing contact time, or a combination of both — and those adaptations don't necessarily tell us the same thing.
RSI gives us the score. The components help explain the adaptation.
