Dynamic Rebound Index, the drop-aware reactive score

The Dynamic Rebound Index (DRI) is a reactive strength score developed by Lance Brooks, Ph.D. It measures the same elastic, stretch-shortening quality that Reactive Strength Index (RSI) does, but it fixes RSI's single biggest blind spot: drop height.

Classic RSI is jump height divided by ground contact time. It is a clean number, but it is blind to how far the athlete dropped before they rebounded. Step off a taller box and you arrive on the floor with more downward energy to reverse, which tends to inflate the rebound and the RSI that comes with it. Two athletes can post the same RSI off completely different boxes, and RSI has no way to tell you which one is actually more reactive.

That is the gap Lance Brooks built DRI to close. By folding the drop height into the equation, DRI prices in the help an athlete got from the box, so the score reflects reactive ability rather than how high a platform they stepped off. Plyomat worked with Lance Brooks to bring DRI out of the research and onto the mat, where the Controller 3.0 computes it on every rep.

Drop height plus jump height is the total vertical displacement the athlete has to handle: how far they fell before the contact, plus how high they rebounded out of it. Contact time is how long they spend on the ground reversing it, the variable that separates a quick, stiff athlete from one who sinks and grinds; because it is squared in the denominator, a long, soft contact is penalized hard. Dividing the displacement by g × contact time² measures it against how far gravity alone would carry the body in that same ground time, which is what makes DRI a clean dimensionless number. A rep scores high only when a genuinely short contact is paired with real displacement, and that is what makes it honest for return-to-play screening, for progressing athletes onto taller boxes, and for longitudinal tracking where last month's test used a different box than today's.

Here is the difference in practice. Picture two athletes who post an identical RSI. Athlete A drops from a 30 cm box; Athlete B drops from 50 cm. Both rebound 38 cm with a 0.18 second ground contact, so RSI scores them exactly the same at 2.11. But B fell further and reversed far more downward energy in the very same ground time. DRI prices that in: add each drop to its rebound and divide by g and the square of the contact time, and Athlete A scores 2.14 while Athlete B scores 2.77. Same RSI, two different athletes, and DRI is what tells them apart, crediting the one who handled more total displacement without spending any longer on the floor.

The classic home for DRI is the drop jump (also called the depth jump): the athlete steps off a box, lands, and rebounds as high as possible while keeping ground contact as short as they can. Here the drop height is simply the box height, so DRI is straightforward to compute and immediately useful.

Most programs do not test off a single box, and that is where DRI earns its keep. A return-to-play athlete might start at 20 cm and work up to 40 over a rehab block. A developing jumper might progress from 30 to 50 cm across a year. With raw RSI, each of those boxes is its own island, and you are left guessing whether a number went up because the athlete improved or because the box changed. DRI lines those efforts up on one scale, so the trend you read is the athlete's reactivity, not the equipment.

It also reveals the drop height where an athlete is most reactive. Test across a range of boxes, compare the DRI at each, and the height where reactive output peaks is the height worth programming. That is a depth-jump prescription built from data instead of habit.

DRI needs a drop height, but you do not always need a box to give it one. That is the idea behind Bounce Factor, Plyomat's repeat-hop reactive protocol. The instruction is simple: stand on the mat, jump, and do not just pop off, jump and then jump again, cycle after cycle. Pogos, five-hop sets, the sticky-stick patterns many coaches already run.

The clever part is where the drop height comes from. In Bounce Factor there is no platform, so the athlete's own first jump becomes the drop height. Their initial pop off the floor sets the height they fall back from, the ground contact between the two jumps is the contact time, and the second jump is the rebound. Plyomat pairs each rep with the next rebound, computes a per-cycle DRI, and scores the whole set. It is reactive testing without a box, a wall, or a single piece of extra equipment.

That changes what you can see. A single drop jump shows an athlete's best contact on their freshest rep. Bounce Factor shows whether they hold that quality across a set, cycle after cycle, which is the reactive stamina that actually shows up late in a game. It is the workhorse mode for repeat-hop work, and DRI is the score underneath it.

DRI lives or dies on the contact time in its denominator. Ground contact in a sharp reactive jump lasts roughly a fifth of a second, so a timing error of a few hundredths swings the score, which is why you measure it on a contact mat rather than estimating it from video. Plyomat's pressure-sensitive switch mat and Controller 3.0 resolve ground contact to 0.001 seconds, derive jump height from flight time, and return DRI the moment the athlete steps off the mat.

It runs in both Drop Jump and Bounce Factor modes inside the free Plyomat 3.0 app, alongside RSI, the Reactive Strength Quadrant, Plyomat Power Score, and left-right asymmetry across eight assessment modes. No hand timing, no spreadsheets, no subscription. It is the same hardware behind the Plyomat vertical jump mat and a modern upgrade for anyone moving on from an older mat (see the Just Jump alternative).