Reactive Strength
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The ability to switch quickly from absorbing force (eccentric) to producing force (concentric) — how well an athlete uses the stretch-shortening cycle.
Detail
Reactive strength is the capacity to rapidly change from an eccentric (yielding) to a concentric (overcoming) muscle action and to generate a large force in a very short ground-contact time. It is the practical, trainable expression of stretch-shortening-cycle (SSC) efficiency and is what distinguishes a "springy" athlete from a strong-but-slow one.
It is commonly quantified by the Reactive Strength Index (RSI), calculated from a drop/depth jump as:
RSI = jump height ÷ ground-contact time (or flight time ÷ contact time).
A higher RSI means more output per unit of contact time. Reactive strength is trained primarily with fast-SSC plyometrics (depth jumps, bounding, repeated hops, low-amplitude hops) that emphasize minimal, stiff ground contacts ("touch the ground like it's hot"). Prerequisite levels of maximal strength and tendon stiffness are needed before high-intensity reactive work is appropriate.
Key facts
- Reactive strength = fast eccentric-to-concentric transition with short contact time.
- Quantified by Reactive Strength Index: RSI = jump height ÷ contact time (or flight ÷ contact).
- Trained with fast-SSC plyometrics emphasizing brief, stiff ground contacts.
- Requires a base of maximal strength before high-intensity reactive drills.
Connections
- stretch-shortening-cycle — reactive strength is applied SSC ability.
- depth-jump — the classic reactive-strength training and testing drill.
- rate-of-force-development — short contacts demand high RFD.
- plyometrics-definition — the training method that builds it.
Educational content only — not medical advice. Always consult a qualified professional for individualized guidance, especially around injury, pregnancy, or medical conditions.