Reactive Strength Index (RSI)

Why this matters

RSI measures reactive strength — how fast an athlete turns a landing into a rebound (the stretch-shortening cycle). It's the headline plyometric metric: a high RSI means a short ground contact AND a good rebound height.

How it's computed here

From three marked frames: ground contact → takeoff → landing.

• contact time = takeoff − contact
• flight time = landing − takeoff
• jump height = 9.81 × flight_time² / 8 (flight-time method, no calibration)
• RSI = jump height ÷ ground-contact time

Capture requirement (read this)

Drop-jump contact time is only ~130–300 ms. At 30 fps the ±1-frame error is ~16% — unusable. Use ≥120 fps (ideally 240 fps) slow-motion capture. The tool reads the clip's capture fps and warns when resolution is too coarse.

What good looks like

Short, stiff ground contact with a strong rebound — "quick feet, stiff ankles." Minimal knee/hip collapse on contact.

Coaching cues

• "Off the ground fast" — minimize contact time.
• Progress drop height only as RSI holds up.

Related

• Modified RSI (mRSI) — the countermovement-jump analogue.