What the Numbers Mean
Jump height, take-off velocity, peak force and mRSI: where each number in a Honest Jump report comes from and what it tells a coach.
A countermovement jump report shows four numbers and three curves. This page says where each one comes from and what it is good for.
The four numbers
Jump height
centimetres
How far your centre of mass rose after you left the ground. It is worked out from the time you spent in the air, which the phone finds in its own motion record.
Once you are in the air, gravity alone decides how you rise and fall, so a flight of t seconds means a rise of g·t²/8. Nothing about your weight, your height or your technique enters that.
Take-off velocity
metres per second
How fast you were travelling upwards at the instant your feet left the floor. It comes from the same flight time: g·t/2.
Height is what take-off velocity turns into, so two athletes with the same height left the floor at the same speed.
Peak force
body weights (BW)
The hardest push against the floor during the jump, measured in multiples of your own standing weight. Standing still is 1 BW.
This one does not come from the flight time. A neural network reads the phone's motion and predicts the whole force curve, the same curve a force plate would draw, and peak force is its highest point 1. An erratum to this study has also been published 2. How Honest Jump measures a jump has the model and its error.
mRSI
metres per second
Modified reactive strength index: jump height divided by the time it took you to get off the floor, from the first movement down to the take-off.
Two athletes can reach the same height, one in 0.7 seconds and one in 1.1. Only mRSI tells them apart, which makes it the number to watch when the aim is a faster jump rather than a bigger one.
Most of what separates two scores is the height, not the time: across 151 college athletes, men and women took almost the same time to leave the floor while their heights were far apart 3. Read it as jump height, discounted by how long it took you.
In that same group the average was about 0.42 for men and 0.31 for women. The published range ran from 0.21 to 0.70 among men and from 0.14 to 0.55 among women 3.
It moves early under fatigue. Ten professional players in women's 3×3 basketball, tested the morning after a team practice, were down about 6% on jump height and about 12% on mRSI, the largest drop of the 22 things measured. By that afternoon everything was back to baseline 4.
Because it divides one measurement by another, the errors of both come along. A change of less than roughly eight per cent in a single session is within the noise 5.
Mean errors in the web app
In internal comparisons of this web app with force-plate measurements, mean errors were about 6% for the force waveform and discrete variables. These web app results are separate from the published findings cited on this page. How Honest Jump measures a jump describes how the app works.
The three curves
Under the numbers the report draws force, velocity and power against time, from the first movement to the take-off.
- Force is the predicted ground reaction force in body weights. It starts at 1, dips below as you drop into the countermovement, rises through the push, and ends as you leave the floor.
- Velocity is negative while you go down and positive while you go up. It crosses zero at the bottom of the countermovement and ends at your take-off velocity.
- Power is force times velocity. It is negative on the way down, where you are soaking up the drop, and positive on the way up.
The phases
The report splits the jump into three parts, and the curves are shaded to match.
- Unweighting. You start to drop and the force falls below your body weight. Nothing is being pushed yet.
- Braking. You stop the drop. Force rises sharply while you are still moving down.
- Propulsive. From the lowest point to the take-off. Velocity is positive and rising.
The split matters because two jumps of the same height can be built differently. A long braking phase and a short one are different problems to train.
Comparing one jump to another
These numbers are most useful against your own earlier ones: one athlete, the same phone, the same way of holding it, measured on different days. That is what the dashboard chart is for.
Read next
- How Honest Jump measures a jump is where each number comes from, and how far off it is.
- The countermovement jump is the test itself, and what it is good for.
References
- Kim H, Kong T, Han H, Ha D, Kipp K (2026). Smartphone IMU-Based Prediction of Vertical Ground Reaction Force During Countermovement Jumps: A Deep Learning Approach. Journal of Strength and Conditioning Research. Advance online publication. doi:10.1519/JSC.0000000000005597
- Kim H, Kong T, Han H, Ha D, Kipp K (2026). Erratum: Smartphone IMU-Based Prediction of Vertical Ground Reaction Force During Countermovement Jumps: A Deep Learning Approach. Journal of Strength and Conditioning Research. Advance online publication. doi:10.1519/JSC.0000000000005699
- Sole CJ, Suchomel TJ, Stone MH (2018). Preliminary Scale of Reference Values for Evaluating Reactive Strength Index-Modified in Male and Female NCAA Division I Athletes. Sports 6(4):133. doi:10.3390/sports6040133
- Cabarkapa D, Cabarkapa DV, Batra A, Fry AC (2025). Postpractice Neuromuscular Performance Changes in 3 × 3 Professional Female Basketball Players. Journal of Strength and Conditioning Research 39(5):e706–e710. doi:10.1519/JSC.0000000000005092
- Suchomel TJ, Bailey CA, Sole CJ, Grazer JL, Beckham GK (2015). Using Reactive Strength Index-Modified as an Explosive Performance Measurement Tool in Division I Athletes. Journal of Strength and Conditioning Research 29(4):899–904. doi:10.1519/JSC.0000000000000743