What RSI Testing Tells Us About an Athlete’s Jump
Two athletes can jump the same height and still perform very differently when they need to rebound quickly off the ground.
That’s why I look at more than jump height.
Reactive Strength Index, or RSI, gives us another piece of information to help understand an athlete’s performance and guide their training.
What is RSI?
RSI compares how high an athlete jumps with how long they spend on the ground during a rebound jump.
RSI = Jump Height ÷ Ground Contact Time
For example, an athlete who jumps 0.30 meters with a ground contact time of 0.20 seconds has an RSI of 1.50 meters per second.
The idea is to produce height while getting off the ground quickly. A higher score can come from jumping higher, shortening ground contact time or improving both.
Why I look beyond the score
The final number doesn’t tell the whole story.
One athlete might produce a higher jump but spend longer on the ground. Another might rebound quickly but produce less height.
Those performances can result in similar RSI scores, even though the athletes use different strategies.
I want to see the jump height, contact time and movement itself. Together, they give me more useful information than the score alone.
How does that help with training?
Consider two hypothetical athletes.
Athlete A jumps relatively high but takes longer to rebound. Depending on their technique and other testing results, I might include low-level rebound drills that emphasize quick, controlled contacts.
Athlete B gets off the ground quickly but produces little height. I might place more emphasis on strength and producing force through appropriate lifting and jumping exercises.
These are possible coaching decisions—not automatic prescriptions. RSI doesn’t tell me by itself why an athlete performed that way or which exercise will fix it.
I also consider their age, training experience, movement control, goals and the demands of their sport.
Why consistent testing matters
Changing the test can change the result.
A drop jump and a repeated-jump test are different tasks. Even within a drop jump, the box height and instructions can influence performance.
To track progress, we need a consistent setup: the same test, equipment, instructions, surface and approach to selecting attempts.
Athletes also need practice with the task. Learning how to perform the test can affect their score, especially when they’re new to it.
A small change on one day doesn’t automatically mean their ability improved or declined. I look for meaningful patterns across testing sessions.
What RSI doesn’t tell us
RSI isn’t a complete measure of athletic ability.
It doesn’t replace sprint testing, strength assessment, movement observation or watching an athlete play their sport. It also isn’t an injury diagnosis or a guarantee of future performance.
For younger athletes, I’m particularly interested in their progress over time rather than ranking them against teammates who may have different experience or be at a different stage of development.
How we use testing at JJR
Testing should help answer practical questions: Where is the athlete starting? What deserves attention? Is the training producing meaningful progress?
At JJR, RSI is one piece of that process. We combine testing with coaching observations to help choose appropriate challenges and guide training.
The goal is to understand the athlete—and use that understanding to build a purposeful plan.
Want to learn more about your athlete’s starting point? Message JJR about a free athlete evaluation at the LaSalle Dome.
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