A drop jump without RSI is just a landing. You see the athlete leave the box, hit the ground, and leave again. What you do not see is how long they spent on the ground compared with how high they went.
That ratio is reactive strength index, or RSI. It is the height of the rebound divided by the time the feet spent on the ground. Short ground contact and a solid rebound means spring. Long contact and a soft rebound means the athlete is absorbing more than they are returning.
You do not need a force plate to start tracking it. You need a clear video and a consistent setup.
Here is how that works in the SpeedTrackr app.
You film the drop jump or hop on your phone, or import a clip you already have. Then you mark the contacts on the video and get jump height and RSI. It is not live analysis on the camera screen. You record or import first, and the results come after.
Normal 60 fps video works. Slow motion is the most precise option when you have it. The app refuses clips whose frame timing changes partway through, because RSI depends on knowing how long each frame lasts.
A few habits keep the numbers comparable week to week.
Use the same box height each time you care about the trend. A drop from 30 cm and a drop from 45 cm are different tests. If you change the box, treat it as a new baseline, not a new personal best against the old one.
Film from the side so both feet and the ground line are easy to see. If the landing disappears behind a rack or another athlete, you will fight the markers.
Mark the same way every time. Touchdown is when the feet hit. Take-off is when they leave. If you mark soft landings differently from stiff ones, the RSI will move even when the athlete did not.
Read each athlete against themselves. A heavy jumper and a light jumper will not share the same RSI. The useful question is whether this athlete is spending less time on the ground for a similar rebound than they were a few weeks ago.
If you coach a group, turn on Coach Mode. Coach Mode is on the web app and on Android. Save each jump to the right athlete from the Squad tab, or by typing their name when you save the result. Each athlete then has one page with their records, progress and every result, so you can see RSI trending without digging through a camera roll.
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
There is no native iPhone app. iPhone users use the web app.
Try it on your next jump day. One box height, one camera spot, three drops each. Save the RSI. That is the number to beat next week.
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