The Reactive Strength Index (RSI) is one of the most informative metrics you can get from a jump assessment, and paradoxically, one of the least used in coaches’ daily practice.
The formula is simple:
RSI = Flight time (s) ÷ Contact time (s)
What it measures isn’t just how high the athlete jumps, it measures how efficiently they convert braking force into propulsive force in a fast 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 long flight times. A low RSI means they’re “losing” energy on landing: the tendon and the nervous system aren’t working optimally.
Most coaches who use jump platforms stick with CMJ height as their main metric. That’s fine, but it’s incomplete.
Two athletes can have exactly the same jump height, say 38cm, and completely different neuromuscular profiles:
These two athletes need completely different programs. Athlete A has a reactive base and can tolerate high plyometric loads. Athlete B needs stiffness and reactivity work before adding plyometric volume. If you give them the same plan, one of the two will get suboptimal results, or worse, get injured.
RSI is mainly measured through the Drop Jump (DJ): the athlete drops from a set height (typically between 20 and 45cm), contacts the ground, and immediately jumps up, trying to maximize height and minimize contact time.
The protocol steps:
With Nexo Jump, the system automatically measures flight time and contact time, calculates RSI in real time, and logs it to the cloud. No stopwatch, no spreadsheet: the number appears in the app instantly.
The athlete has very little ability to exploit the fast stretch-shortening cycle. Contact times are long and force transfer is inefficient.
What to program: max 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 the neuromuscular base is in place.
The athlete has basic reactive capacity but significant room for improvement. Contact times are moderate.
What to program: a combination of strength work and moderate-intensity plyometric work. Drop jumps from 20-30cm, depth jumps, progressive stiffness work.
The athlete has a developed reactive neuromuscular profile. They can tolerate higher plyometric loads and benefit from specific high-intensity work.
What to program: drop jumps from higher heights (40-50cm), sport-specific reactivity exercises, strength-plyometric combinations (contrast method).
Typical values for high-performance sprint, jump, and contact-sport athletes (values above 3.0 are considered world class). 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), highly specific combinations, close fatigue monitoring.
Besides being a capacity indicator, RSI is one of the best markers of acute neuromuscular fatigue available.
When an athlete is fatigued, the central nervous system reduces muscle activation as a protective mechanism. That shows up as longer contact times: the athlete can’t be as reactive as usual. The result is a drop in RSI that can be detected before the athlete subjectively feels tired.
RSI fatigue monitoring protocol:
This protocol, implemented consistently, allows you to prevent overtraining objectively, without relying on the athlete saying they’re tired.
They have power but not reactivity. They can generate force in long cycles (CMJ with a wide countermovement) but not in short ones. They need specific stiffness work and progressive reactivity work. Adding more max strength work won’t solve this deficit.
They have reactivity but no strength base. They’re efficient in short cycles but lack power in long cycles. They need max strength and power work with heavy loads. Plyometric work alone won’t significantly improve the CMJ.
Global neuromuscular function deficit. The priority is building the strength base before any plyometric or reactive work.
An athlete with a good neuromuscular profile in both dimensions. The goal is to maintain and enhance, with sport-specific work and close fatigue monitoring so as not to compromise the adaptations achieved.
RSI is a powerful metric. But its real value appears when it’s integrated into a system that connects it with programming, not when it sits isolated in a spreadsheet.
In Nexo Kinetics, the RSI obtained with Nexo Jump feeds directly into the platform’s internal analysis engine, which classifies the athlete’s reactive strength level and generates a specific prescription for each level. For example, an RSI below 1.0 indicates a severe deficit, and the system prioritizes max strength and basic stiffness work, avoiding high drop jumps. The system also cross-references RSI with other assessment metrics to generate a complete, integrated prescription.
The athlete’s RSI history is stored in the cloud and can be compared across sessions and blocks, which makes RSI a progress indicator as valuable as 1RM or CMJ.
The Reactive Strength Index is probably the most underused metric in strength and power training. Coaches who incorporate it into their assessment, and connect it with 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 with what comes next: prescription, monitoring, and continuous adjustment.
Nexo Jump measures vertical jump, contact time and RSI with millimetre precision — and the software turns that data into programs for your athletes.