Reactive strength index (RSI) is calculated by dividing jump height by ground contact time during a drop jump. The formula is simple: RSI = jump height ÷ ground contact time. A higher number means you’re producing more force in less time, which is the hallmark of explosive, elastic athletic movement. There’s also a modified version (RSImod) used with countermovement jumps, which swaps ground contact time for total time to takeoff.
The Standard RSI Formula
The classic RSI calculation comes from the drop jump, where you step off a box, land, and immediately rebound as high as possible. The formula uses two variables:
- Jump height (in meters): how high you jump after landing
- Ground contact time (in seconds): how long your feet are on the ground between landing and takeoff
RSI = jump height ÷ ground contact time. If you jump 0.35 meters with a ground contact time of 0.200 seconds, your RSI is 1.75. The score has no formal unit, though it’s sometimes expressed in meters per second.
What makes RSI useful is that it captures both outputs at once. Jumping high is good, but not if it takes you forever to get off the ground. A short contact time is good, but not if you barely leave the floor. RSI rewards the combination: maximum height with minimum ground time.
The Modified Version: RSImod
RSImod uses the countermovement jump (CMJ) instead of the drop jump. You start standing, dip down, and jump as high as you can. The formula changes slightly:
RSImod = jump height ÷ time to takeoff. Time to takeoff (TTT) includes the entire movement from the moment you start dipping downward through the braking and pushing phases until your feet leave the ground. This replaces the ground contact time used in standard RSI because there’s no landing phase before the jump.
RSImod is popular in team sport settings because the countermovement jump is easier to standardize and less physically taxing than repeated drop jumps. It captures the same underlying quality: how efficiently you convert a stretch into an explosive upward push.
Why RSI Matters: The Stretch-Shortening Cycle
RSI measures how well your muscles use what’s called the stretch-shortening cycle. This is the natural sequence where a muscle lengthens under load (the eccentric or “braking” phase) and then immediately shortens to produce force (the concentric or “pushing” phase). The preceding stretch stores elastic energy and activates reflexes that amplify the push. Adults who use this stretch before jumping can increase their jump height by 18 to 30% compared to jumping from a dead stop.
This cycle powers nearly all athletic movement: sprinting, cutting, bounding, and any action that requires you to absorb force and redirect it quickly. RSI quantifies how well that system is working. A declining RSI score over a training week can signal that your neuromuscular system is fatigued before you feel it subjectively.
How to Run the Drop Jump Test
The standard protocol uses a box (commonly 30 to 40 cm) and a device that records flight time and ground contact time. Step off the box with one foot, don’t jump up or step down aggressively. On landing, think “bounce.” The goal is to reverse your downward velocity as quickly as possible with minimal knee bend, then jump as high as you can.
This is technically called a “bounce drop jump,” and it’s distinct from a depth jump, where you allow deeper knee flexion and prioritize height over speed. For RSI testing, you want ground contact times under 250 milliseconds. If contact times are much longer than that, you’re likely bending too deep and turning it into a depth jump, which measures a different quality.
Most protocols call for three to five jumps after a few warm-up attempts, with the best score or the average used for analysis. Consistency between attempts matters. If your scores vary widely, your technique may need tightening before the numbers mean anything useful.
What You Need to Measure It
You need a device that captures flight time (time in the air) and contact time (time on the ground). Three common tools work well:
- Force plates: considered the gold standard, measuring ground reaction forces directly
- Contact mats: pressure-sensitive mats that detect when your feet leave and return to the surface
- Video-based apps: smartphone applications like My Jump that use slow-motion video to identify takeoff and landing frames
All three have been shown to produce highly reliable and interchangeable results. In a comparison study, force plates, contact mats, and the My Jump app all showed excellent test-retest reliability (ICC values above 0.98) and correlated strongly with each other (r = 0.987 or higher). For practical purposes, a $50 contact mat or a smartphone app can give you scores that closely match a laboratory-grade force plate.
One thing to note: devices that estimate jump height from flight time can introduce small errors if you tuck your knees in the air or land in a different position than you took off. Keeping your body position consistent between takeoff and landing improves accuracy.
Calculating Jump Height From Flight Time
If your device gives you flight time but not jump height directly, you can calculate it. The standard physics formula is:
Jump height = (gravity × flight time²) ÷ 8, where gravity is 9.81 m/s². So if your flight time is 0.54 seconds: jump height = (9.81 × 0.2916) ÷ 8 = 0.358 meters.
Some systems calculate jump height from takeoff velocity instead, using the formula: jump height = velocity² ÷ (2 × 9.81). Both methods are valid. The flight-time method is more accessible since most contact mats and apps give you flight time directly. The velocity method requires a force plate that integrates ground reaction forces to determine how fast you left the ground.
RSImod Score Benchmarks
Reference values from NCAA Division I athletes provide useful context for interpreting RSImod scores from countermovement jumps. These are measured in meters per second:
For male collegiate athletes, scoring below 0.352 falls in the bottom 25th percentile. The middle range sits between 0.352 and 0.492, while scores above 0.492 place you in the top 25%. Elite male performers averaged 0.549. For female collegiate athletes, the bottom quartile falls below 0.248, the middle range spans 0.248 to 0.379, and the top quartile starts above 0.379. Elite female performers averaged 0.429.
These benchmarks come from athletes across multiple sports, so they represent general athletic ability rather than sport-specific standards. A volleyball player or sprinter would typically score higher than a distance runner or a golfer. Use these numbers as a starting reference, but track your own scores over time. Your personal trend matters more than any single comparison to a normative table.
Using RSI to Guide Training
RSI is most powerful as a monitoring tool when tracked consistently. In semi-professional soccer, changes in RSI have been linked to training load variables like total distance covered, sprint volume, and acceleration demands. Because RSI integrates both jump height and contact time, it captures a broader picture of neuromuscular status than either variable alone.
The practical application is straightforward. Test RSI at regular intervals, ideally under the same conditions each time (same warm-up, same box height, same time of day). If your RSI drops meaningfully from your baseline, your stretch-shortening cycle efficiency is compromised, which typically reflects accumulated fatigue. This can signal that you need to reduce training intensity or volume before performance declines further or injury risk increases.
Individual responses vary considerably. Some athletes show RSI changes primarily driven by contact time getting longer (they’re spending more time on the ground), while others show it through reduced jump height. Tracking which component is shifting tells you whether the issue is more about stiffness and reactivity or raw force production, which can guide what you adjust in your program.