Most people pick a drop-jump box the same way: whatever is in the corner of the gym. Then they do every drop jump from it for months.
The box height matters more than that. Drop from too low and there is not much to react to. Drop from too high and the athlete sinks into the ground, sits there, and comes off slowly. Somewhere in between is the height where they hit the floor and leave it fast and high. That is the height worth training from, and it is different for different athletes.
You can find it in one session with a phone and a few boxes.
What RSI tells you
RSI, reactive strength index, compares how high the athlete jumps with how long they spend on the ground. High and quick gives a big number. Low or slow gives a small one.
So when you raise the box, you are asking one question: does the athlete still get off the ground quickly, or does the extra height start to cost them?
The setup
You need three or four box heights. For example 20, 30, 40 and 50 cm. Measure them, do not guess.
Prop the phone low and side-on, still, with the box, the landing spot and the athlete's whole body in frame. Do not hold it. Film each drop, or film with the normal camera and import the clip afterwards.
Normal 60 fps video works. Slow motion is the most precise option.
Same instruction every rep: step off, do not jump off, land and go up as fast and as high as you can.
The ladder
Start at the lowest box. Three drops, full rest between them.
Move up one box. Three more drops.
Keep going until the last box, or stop earlier if the landings get heavy and the heels crash down. A drop the athlete cannot control is not a test result.
In the SpeedTrackr Android app, you pick the box height before you mark the jump. Then you mark take-off and landing frame by frame, and the app gives RSI. Because every result is saved with its box height, RSI is compared box by box. A drop from 40 cm is never judged against one from 30 cm.
Reading it
Take the best RSI at each box and put them side by side.
Usually RSI holds or climbs for the first box or two, then starts to fall as the ground contact gets longer. The box just before it falls is a good height to train from for now.
If RSI is already falling from the lowest box, start lower. If it is still climbing at the top, the athlete can handle more, but there is no need to chase it in the same session.
Two athletes in the same squad can land on different boxes. That is the point of running the ladder for each of them instead of setting one box for the whole group.
Keep it comparable
Use the same boxes, the same surface and the same camera spot when you repeat it. Run it fresh, near the start of a session, not after sprints or heavy lifting.
Repeat the ladder every few weeks. If the best box moves up, or RSI at the same box goes up, the athlete is handling more.
If you test a squad, turn on Coach Mode and pick the athlete before each jump, so every drop lands on the right athlete's page. Coach Mode works in the Android app and on the web app. Each athlete's body weight is saved once, so jump results are right for every athlete, not just the coach.
One last thing. This is a phone and a set of boxes, marked by hand from video. It is a practical way to compare an athlete with themselves on the same setup, which is what you need to pick a box.
The Android app is on Google Play: https://play.google.com/store/apps/details?id=com.speedtrackr.reactionpro
On an iPhone or a laptop, use the web app: https://www.speedtrackr.com/auth/login
