Updated on 26 de June de 2026 by Adrián Escobar Morales
He Reactive Strength Index (RSI), or reactive strength index, is one of the most valuable metrics for evaluating an athlete’s explosive performance. In short: it measures how much the athlete jumps in relation to the time it takes to do so. A high RSI means that the athlete produces a lot of force in a very short time, which directly translates into a better vertical jump, greater sprint speed and more efficiency in any explosive action.
Unlike simply measuring jump height, RSI captures the stretch-shortening cycle (SSC) quality: that physiological mechanism that allows elastic energy to be stored when landing and released explosively to jump again. To measure RSI you need to know two pieces of information: jump height and ground contact time. Devices like the ADR Jumping They calculate both variables automatically and show you the RSI in real time.
Índice
- 1 What is the Reactive Strength Index (RSI)?
- 2 The stretch-shortening cycle: the physiological basis of RSI
- 3 How is RSI measured? Protocols and tools
- 4 What is a good RSI? Reference values
- 5 Why does RSI matter in sports performance?
- 6 RSI and vertical jump: the direct connection
- 7 How to improve RSI: what the scientific evidence says
- 8 Common mistakes when training and measuring RSI
- 9 Conclusion
- 10 RSI FAQ
- 11 Literature
What is the Reactive Strength Index (RSI)?
The RSI was developed in 1995 by Young as part of the Strength Qualities Assessment Test (SQAT) from the Australian Institute of Sport. Its formula is simple:
RSI = Jump height (cm) ÷ Contact time (s)
An athlete who jumps 40 cm with 0.20 s of contact has an RSI of 2.0. Another that jumps the same height but with 0.30 s has an RSI of 1.33. The first is more reactive, although it jumps just as high.
According to Brooks (2026), the RSI provides a unified mechanical framework to evaluate the function of the SSC objectively, being especially useful to identify neuromuscular deficits and design individualized plyometric training programs.
| Variable | Definition | How to obtain |
|---|---|---|
| Jump height (cm) | Vertical distance traveled by the center of mass from takeoff to the highest point | Calculated from flight time |
| Contact time (s) | Time the foot remains in contact with the ground between consecutive jumps | Measured directly with contact platform or infrared system |
| RSI | Ratio between jump height and contact time | Automatically calculated by the device |
The stretch-shortening cycle: the physiological basis of RSI
To understand the RSI you have to understand the SSC. It is the mechanism by which muscles and tendons act like a spring: they are loaded during the braking phase (eccentric) and release that energy explosively in the propulsion phase (concentric), generating more force than in a pure concentric contraction.
There are two types of SSC with different practical implications:
| Feature | SSC slow | fast SSC |
|---|---|---|
| Contact time | > 250ms | < 250ms |
| Typical exercises | CMJ, squat jump, long jump | Drop jump, rebounds, sprint, hurdle jumps |
| Dominant mechanism | Active muscle contribution | Muscle-tendon stiffness + elastic energy |
| Does the RSI measure it? | Not directly (→ RSImod) | Yes (→ Classic RSI with drop jump) |
The RSI measures the fast SSC. An athlete may have an excellent CMJ and a mediocre RSI. They are different qualities that require specific training.
How is RSI measured? Protocols and tools
To calculate RSI you need to accurately measure jump height and contact time. These are the main options:
| Tool | Measure height | Measure contact time | Calculate RSI | Portability | Cost |
|---|---|---|---|---|---|
| force platform | ✓ | ✓ | ✓ | Low | very high |
| Mobile app (video) | ✓ | ✗ | ✗ | High | Low |
| ADR Jumping | ✓ | ✓ | ✓ (automatic) | High | Economic |
Most used RSI tests
- Incremental Drop Jump (DJ-RSI): The athlete falls from different heights (15, 30, 45, 60 cm) and the RSI is calculated for each one. The height that produces the best RSI is the optimal training height for that athlete.
- Repeated Jump Test (10-5 RSI): 10 consecutive jumps are made and the 5 best RSI are averaged. Fast, reproducible and widely used in field contexts.
- Modified RSI (RSImod): It is calculated by dividing the CMJ height by the time to takeoff. Useful when only CMJ data is available.
Measure RSI with ADR Jumping
He ADR Jumping is a vertical jumping platform that automatically records the jump height, flight time, contact time and RSI in each repetition using an infrared beam. The data appears in real time both on the device’s integrated display and in the ADR System app (iOS and Android, free), from where it can be exported to Excel or saved in the cloud.
Its operation is simple: the athlete places himself between the two units of the device, jumps, and the system automatically detects when the infrared beam is interrupted when taking off and landing, accurately calculating the flight time and contact time. ADR Jumping is scientifically validated by independent universities, which guarantees that its RSI values are reliable and comparable with those obtained in studies published in high-impact journals.
What is a good RSI? Reference values
RSI values vary depending on the test used, the sport and the level of the athlete. As a general reference for the drop jump:
| Level | RSI | Typical profile |
|---|---|---|
| Low | < 1.0 | Athletes without previous plyometric work. Needs base strength and slow SSC before progressing. |
| Intermediate | 1.0 – 2.0 | Recreational athletes or in initial stages of explosive development. |
| Well | 2.0 – 3.0 | Athletes trained with systematic plyometric work. |
| Elite | > 3.0 | Sprinters, jumpers or athletes with years of specific reactive training. |
The RSI is relative to the athlete and the sport. The most important thing is not the absolute value, but the trend over time and response to training.
Why does RSI matter in sports performance?
The RSI is not just a jump test: it is a window into the athlete’s neuromuscular status and their ability to produce explosive force in high-demand situations. Scientific research has established strong relationships between RSI and:
- Sprint speed and acceleration: Athletes with higher RSI accelerate more efficiently by taking better advantage of elastic stiffness in each stride.
- Change of address: The ability to absorb and redirect forces in a short time is decisive in team sports.
- Vertical jump in court sports: a study with volleyball players (Knezevic et al., 2022) found correlations of r = 0.68–0.73 between bilateral CMJ RSImod and attack jump in competition.
- Neuromuscular fatigue monitoring: A drop in RSI with respect to the athlete’s baseline value indicates accumulation of fatigue, even before the athlete himself perceives it subjectively.
This last use is especially valuable in daily practice: performing 3–5 drop jumps at the beginning of the session with the ADR Jumping It gives you in less than 2 minutes an objective measure of the athlete’s neuromuscular status. If the RSI drops more than 10–15% from your baseline value, it may be a signal to reduce the intensity for that day.
RSI and vertical jump: the direct connection
Improving RSI is one of the most direct ways to improve the vertical jump, because it specifically works on the ability to produce force in the shortest time possible. The physiological mechanism works like this:
- A higher RSI indicates greater stiffness of the muscle-tendon unit (especially the Achilles tendon and quadriceps).
- This rigidity allows more elastic energy to be stored during the eccentric phase of the jump.
- The release of that energy in the concentric phase produces greater vertical impulse in less time.
- Result: higher jump height with shorter contact time = higher RSI.
The key is that this process is trainable. Research shows that fast SSC responds very well to specific plyometric training, especially when systematically progressing in intensity and volume.
How to improve RSI: what the scientific evidence says
The meta-analysis of Ramirez-Campillo et al. (2023), which analyzed 61 studies with 2,576 participants, showed that plyometric training significantly improves RSI (ES = 0.54; 95% CI: 0.46–0.62; p < 0.001). This effect is superior to that of traditional strength training with high loads and slow speeds.
| Variable | Optimal condition | Inferior condition |
|---|---|---|
| Program Duration | > 7 weeks | ≤ 7 weeks |
| Weekly frequency | 3 sessions/week | < 3 sessions/week |
| Total sessions | > 14 sessions | ≤ 14 sessions |
| Effect size | ES = 0.54 (95% CI: 0.46–0.62; p < 0.001) | |
Plyometric progression to improve RSI
Progression is essential to maximize RSI improvement while minimizing the risk of injury:
- Phase 1 (weeks 1–3): low intensity jumps with emphasis on landing mechanics. Box jump, long jump, CMJ with focus on minimizing contact time.
- Phase 2 (weeks 4–7): introduction of rebounds and hurdle jumps with progressively shorter contact times. Objective: < 200 ms of contact.
- Phase 3 (weeks 8–12): drop jumps from increasing heights. Use the RSI measured with the ADR Jumping to find the individual optimal drop height.
- Phase 4 (maintenance): High reactive speed work integrated with strength training and sport-specific context.
Use the ADR Jumping in each session to monitor contact time. If it consistently exceeds 250 ms, the stimulus is not working the fast SSC optimally.
The combination with strength training
Strength training does not directly improve RSI, but it creates the foundation necessary for plyometric work to be effective. A weak athlete cannot benefit from drop jumps because their tendons and muscles do not have the ability to absorb and transmit those reactive forces. The combination of strength + plyometrics produces the greatest increases in RSI and vertical jump (Hernández-Belmonte and Pallarés, 2022).
Common mistakes when training and measuring RSI
- Do not measure contact time: Many trainers only measure jump height. Without contact time there is no RSI, and without RSI you don’t know if the athlete is improving their reactivity or simply jumping higher with more stance time.
- Use mobile apps to calculate RSI: Video analysis apps do not accurately measure contact time. For a reliable RSI you need a contact or infrared platform like the ADR Jumping.
- Do drop jumps without progression: Starting at high altitudes without preparation increases the risk of injury and can generate reflexive neural inhibition, which worsens RSI.
- Ignore fatigue when measuring: RSI falls with fatigue. If you measure it at the end of an intense session, the values do not reflect the athlete’s real ability. Always measure it at the beginning, as a readiness test.
- Confusing CMJ improvement with RSI improvement: They are partially independent qualities. An athlete can improve their CMJ without improving their RSI, because the CMJ uses the slow SSC and the RSI measures the fast SSC.
Conclusion
The Reactive Strength Index is a must-have metric for any coach or athlete looking to optimize explosive performance. It measures what the vertical jump alone cannot measure: the quality of the rapid stretch-shortening cycle.
Measuring it correctly requires having a tool that captures both jump height and contact time. He ADR Jumping does exactly that: automatically, portable and with independent scientific validation. Integrating it into your testing routine—even your warm-up, as a readiness measure—can transform the way you make loading decisions and schedule plyometric training.
Measure RSI before training. Improve RSI with progressive plyometrics. Repeat. That’s data-driven training.
RSI FAQ
What is RSI?
The Reactive Strength Index (RSI) is the ratio between jump height and ground contact time. It measures the efficiency of the rapid stretch-shortening cycle (SSC < 250 ms), reflecting the athlete’s reactive explosive capacity.
How is RSI measured with ADR Jumping?
The athlete places himself between the two units of the ADR Jumping and perform bounce jumps or drop jumps. The device automatically detects the flight time and contact time, calculates the jump height and shows the RSI in real time on the display and in the ADR System app.
What is the RSI for beyond the jump?
The RSI predicts sprint performance, change of direction and agility. In addition, it is a sensitive marker of neuromuscular fatigue: a drop in RSI compared to the athlete’s baseline value is a sign that the nervous system has not recovered.
How long does it take for RSI to improve?
With a plyometric program of more than 7 weeks, 3 weekly sessions and adequate progression, significant improvements can be observed (ES = 0.54 in meta-analysis of 61 studies). The first detectable changes usually appear from week 4–6.
What is the difference between RSI and CMJ?
The CMJ measures countermovement jumping (slow SSC, > 250 ms contact). The RSI measures fast SSC (< 250 ms), such as in drop jumps or bounces. They are complementary but partially independent qualities that require specific differentiated training.
What is a good RSI value?
For the drop jump, RSI values ≥ 2.0 are suitable for trained athletes. Values > 3.0 are typical of elites (sprinters, jumpers). The most important thing is individual progress over time, not comparison with absolute percentiles.
Can I measure the RSI with my mobile?
Not reliably. Mobile apps that analyze video do not accurately measure contact time, which is essential to calculate RSI. To obtain a valid RSI you need a contact platform or an infrared system such as the ADR Jumping.
Literature
- Brooks, L.C. (2026). A Unified Mechanical Framework for Evaluating Stretch–Shortening Cycle Function. European Journal of Sport Sciences, 5(1). https://doi.org/10.24018/ejsport.2026.5.1.9272
- Flanagan, E.P., & Comyns, T.M. (2008). The use of contact time and the reactive strength index to optimize fast stretch-shortening cycle training. Strength and Conditioning Journal, 30(5), 32–38. Ver artículo
- Hernández-Belmonte, A., & Pallarés, J.G. (2022). Effects of Velocity Loss Threshold during Resistance Training on Strength and Athletic Adaptations. Applied Sciences, 12(9), 4425. https://doi.org/10.3390/app12094425
- Jukic, I., Pérez-Castilla, A., García-Ramos, A., Van Hooren, B., McGuigan, M.R., & Helms, E.R. (2023). The Acute and Chronic Effects of Implementing Velocity Loss Thresholds During Resistance Training. Sports Medicine, 53(1), 177–214. https://doi.org/10.1007/s40279-022-01754-4
- Knezevic, O.M. et al. (2022). The association between reactive strength index and RSI modified with approach jump performance. PLOS ONE, 17(2), e0264144. https://doi.org/10.1371/journal.pone.0264144
- Ramirez-Campillo, R. et al. (2023). Effects of Plyometric Jump Training on the Reactive Strength Index in Healthy Individuals Across the Lifespan. Sports Medicine, 53, 1077–1099. https://doi.org/10.1007/s40279-023-01847-8
- Refalo, M.C. et al. (2023). Influence of Resistance Training Proximity-to-Failure on Skeletal Muscle Hypertrophy. Sports Medicine, 53, 649–665. https://doi.org/10.1007/s40279-022-01784-y
- Young, W. (1995). Laboratory strength assessment of athletes. New Studies in Athletics, 10, 89–96.
