Free coach's guide · 17 pages

DRI and RSI

Two reactive numbers, one athlete. RSI is jump height over contact time and has no drop-height term. The Dynamic Rebound Index, developed by Lance Brooks, Ph.D., adds the drop to the displacement and squares the contact time. This guide covers what each measures, where each applies, and the single line of arithmetic that separates them.

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What is inside

  • Two formulas, side by side, drawn on the jump itself: which term is the drop, which is the rebound, which is squared.
  • How the math behaves. Halve the contact and the score quadruples. What each input moves, one at a time.
  • The Bounce Factor protocol. Two jumps, no box, DRI on every rep, with the demo video.
  • The standards. The five DRI tiers in the app's colours, and the RSI levels for reference.
  • Which drop height to start on, and the criteria for moving up.
  • Reading both on the quadrant, reporting without confusion, five common mistakes, and the references.

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03 Two formulas, side by side
04 Why they are not the same number
05 How the math behaves
06 The box problem
07 The Bounce Factor protocol
08 DRI standards
09 RSI standards, for reference
10 Which box first
11 Where RSI wins
12 Where DRI wins
13 Reading both on the quadrant
14 Which to use, when
15 Reporting both without confusing anyone
16 Common mistakes
17 Summary and references
The difference in one line

Same rebound, same contact, different drop

Take one athlete, a 30 cm rebound and a 0.20 second contact. Off a 12 inch box RSI reads 1.5 and DRI reads 1.54. Off a 24 inch box RSI still reads 1.5. DRI reads 2.32, about 50% higher, because twice the drop was reversed in the same contact. RSI has no term for the box.

RSI
RSI = Jump Height ÷ Contact Time

Thirty years of coaching familiarity and published benchmarks. Built for flat-ground rebound tests: 10/5, 4/2, Max RSI. Does not compare across boxes.

DRI
DRI = (DH + JH) ÷ (g × CT²)

Developed by Lance Brooks, Ph.D., published 2026. Built for Drop Jump and Bounce Factor modes. Calculates the drop into the score, so it compares across drop heights.

See the one-page DRI vs RSI chartTry the DRI and RSI calculator
Questions

About the guide

Who is this guide for?
Strength and conditioning coaches, athletic trainers and clinicians who test reactive jumps and want to compare results across drop heights, athletes and a season. It assumes you know what RSI is and explains everything else.
Do I need a Plyomat to use it?
No. The formulas work with any timing mat or force plate that reports contact time and jump height. Plyomat computes both scores live on every rep, which is the convenient version, but the guide stands on its own.
Where does DRI come from?
The Dynamic Rebound Index was introduced by Lance Brooks, Ph.D., in A Unified Mechanical Framework for Evaluating Stretch-Shortening Cycle Function (2026). The guide states the formula as published, with worked examples computed at the stated values. The five scoring tiers are Plyomat's field standard.
What happens with my email?
We send you the PDF and add you to the Plyomat coaches newsletter, which covers testing standards, new guides and app updates. Unsubscribe at any time from any email. We do not sell or share your address.

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