What Is the Reactive Strength Index?
The Reactive Strength Index (RSI) is one of the most informative metrics you can obtain from a jump assessment and, paradoxically, one of the least used in coaches' daily practice.
The formula is simple:
RSI = Jump height (m) ÷ Contact time (s)
What it measures isn't just how high the athlete jumps: it measures how efficiently they convert absorption force into propulsion force in a rapid stretch-shortening cycle. In other words, it measures the quality of their reactive neuromuscular system.
A high RSI means the athlete can absorb and reuse elastic energy efficiently, with short contact times and high jumps. A low RSI means they're "losing" energy at landing: the tendon and nervous system aren't working optimally.
Why RSI Matters More Than Jump Height Alone
Most coaches who use jump platforms stick with CMJ height as the main metric. That's fine, but it's incomplete.
Two athletes can have exactly the same CMJ height — say, 38 cm — and have completely different neuromuscular profiles:
- Athlete A: RSI of 2.8 → jumps 38 cm with very short contact time → high reactive efficiency.
- Athlete B: RSI of 1.4 → jumps 38 cm with long contact time → low reactive efficiency, compensates with more force application time.
These two athletes need completely different programming. Athlete A has a reactive foundation and can tolerate high plyometric load. Athlete B needs stiffness and reactivity work before adding plyometric volume. Give them the same plan and one of them will have suboptimal results or, worse, get injured.
"RSI doesn't tell you how high your athlete jumps. It tells you how they jump, and that changes everything that follows in programming."
How RSI Is Measured: The Drop Jump Protocol
RSI is primarily measured through the Drop Jump (DJ): the athlete drops from a set height (typically between 20 and 45 cm), contacts the ground, and immediately jumps upward trying to maximize height and minimize contact time.
The protocol steps:
- The athlete steps onto a box of the established height (start with 30 cm as standard).
- Steps off the box with feet parallel and knees slightly flexed.
- Upon ground contact, immediately activates extension without additional countermovement.
- Jumps as high as possible with the shortest possible contact time.
- Perform 3-5 jumps and average the RSI.
With the Nexo Jump platform, the system automatically measures flight time and contact time, calculates RSI in real time, and records it to the cloud. No stopwatch needed, no spreadsheet: the data appears in the app instantly.
How to Interpret RSI: Reference Values and What to Do With Each
RSI below 1.0: severe reactive strength deficit
The athlete has very little capacity to use the rapid stretch-shortening cycle. Contact times are long and force transfer is inefficient.
What to program: maximum strength work (the system can't be reactive without a strength base), low-intensity stiffness exercises, low box jumps.
What to avoid: high drop jumps, high plyometric volume, exercises demanding reactivity before having the neuromuscular foundation.
RSI between 1.0 and 1.8: developing reactive capacity
The athlete has basic reactive capacity but with significant room for improvement. Contact times are moderate.
What to program: combination of strength work and moderate-intensity plyometric work. Drop jumps from 20-30 cm, depth jumps, progressive stiffness work.
RSI between 1.8 and 2.5: good reactive capacity
The athlete has a developed reactive neuromuscular profile. They can tolerate higher plyometric loads and benefit from high-intensity specific work.
What to program: drop jumps from higher boxes (40-50 cm), sport-specific reactivity exercises, strength-plyometric combinations (complex method).
RSI above 2.5: elite reactive capacity
Typical values for speed, jump, and high-performance contact sport athletes. At this level, maintaining reactive capacity is as important as developing it.
What to program: controlled plyometric volume (injury risk increases with very high loads), high-specificity combinations, close fatigue monitoring.
RSI as a Fatigue Indicator: The Use Most Coaches Don't Know
Beyond being a capacity indicator, RSI is one of the best available markers of acute neuromuscular fatigue.
When an athlete is fatigued, the central nervous system reduces muscle activation as a protective mechanism. This manifests as longer contact times: the athlete can't be as reactive as usual. The result is an RSI drop that can be detected before the athlete subjectively feels tired.
RSI fatigue monitoring protocol:
- Establish the athlete's baseline RSI with the initial test.
- Measure RSI at the start of important sessions during the block.
- A drop of more than 10-15% from baseline indicates significant neuromuscular fatigue.
- Action: reduce the session load or replace with active recovery work.
This protocol, implemented consistently, allows preventing overtraining objectively, without depending on the athlete saying they're tired.
RSI and Programming Decisions: The Most Frequent Scenarios
Athlete has good CMJ but low RSI
Has power but not reactivity. Can generate force in long cycles (CMJ with wide countermovement) but not in short cycles. Needs specific stiffness and reactivity work. Adding more maximum strength work won't solve this deficit.
Athlete has high RSI but low CMJ
Has reactivity but not a strength base. Is efficient in short cycles but lacks power in long cycles. Needs maximum strength and high-load power work. Plyometric work alone won't significantly improve CMJ.
Both RSI and CMJ are low
Global neuromuscular function deficit. The priority is building the strength base before any plyometric or reactive work.
Both RSI and CMJ are high
Athlete with a good neuromuscular profile in both dimensions. The objective is to maintain and enhance, with sport-specific work and close fatigue monitoring to not compromise achieved adaptations.
How Nexo Kinetics Integrates RSI Into the Coach's Workflow
RSI is a powerful metric. But its real value appears when integrated into a system that connects it to programming, not when it sits isolated in a spreadsheet.
In Nexo Kinetics, the RSI obtained with Nexo Jump feeds directly into the platform's analysis engine. From the multiple jumps test, it calculates RSI and generates a specific prescription by level: RSI below 1.0 indicates severe deficit (priority on maximum strength and basic stiffness, avoid high drop jumps); 1.0 to 1.5, low reactive capacity (combined strength and moderate plyometric work); 1.5 to 2.0, moderate level (drop jumps from 20-30 cm, progressive stiffness); 2.0 to 2.5, good reactive capacity (drop jumps from higher boxes, complex method); 2.5 to 3.0, high capacity; and above 3.0, world-class level (controlled plyometric volume, close fatigue monitoring). The system also crosses RSI with ECR (CMJ/SJ difference) and arm swing contribution to generate a complete, integrated prescription.
Additionally, the athlete's RSI history is saved to the cloud and can be compared between sessions and blocks, turning RSI into a progress indicator as valuable as 1RM or CMJ.
"RSI isn't just an assessment metric. It's a programming compass. When you track it over time, it tells you exactly where to take the training."
Conclusion
The Reactive Strength Index is probably the most underutilized metric in strength and power training. Coaches who incorporate it into their assessments, and connect it to their programming, have an enormous advantage over those who only look at jump height.
It's not a complex metric to obtain. With the right tool, it's measured in seconds. What it does require is a system that connects that data to what comes next: prescription, monitoring, and continuous adjustment.