The reactive strength index is one of the most informative single measurements in athletic performance testing. A single number that captures the quality of an athlete’s fast stretch-shortening cycle, it tells coaches and athletes more about explosive athletic potential than vertical jump height alone, sprint time alone, or any other isolated measurement.
Yet RSI remains largely unfamiliar outside elite sport science settings, partly because measuring it accurately requires technology that most athletes do not have, and partly because the concept of the stretch-shortening cycle it measures is not as intuitively understood as strength or speed.
This guide explains what RSI is, why it matters more than most people realise, how to measure it with the tools available to most athletes, what good scores look like, and the specific training interventions that improve it most effectively.
What the Reactive Strength Index Is
The reactive strength index is a measure of the quality of the fast stretch-shortening cycle (SSC), the ability to rapidly absorb and redirect force during a brief ground contact. It is calculated from two variables measured during a drop jump or depth jump:
RSI = Jump height (metres) ÷ Ground contact time (seconds)
A depth jump that produces a jump height of 0.35 metres and a ground contact time of 0.20 seconds gives:
RSI = 0.35 ÷ 0.20 = 1.75
A higher RSI indicates better reactive strength: either a higher jump for the same contact time, the same jump height in less time, or ideally both.
Why the ratio matters more than either variable alone:
Two athletes can produce identical jump heights from a depth jump with very different RSI values:
- Athlete A: Jump height 0.35m, contact time 0.35s → RSI = 1.0
- Athlete B: Jump height 0.35m, contact time 0.20s → RSI = 1.75
These athletes are not equally explosive. Athlete B is producing the same jump height in significantly less time, which means their fast SSC is significantly more efficient. In a sprint or a jump-dominated sport, Athlete B will be faster and more explosive despite identical jump height.
The RSI ratio captures this distinction in a single number that neither variable alone would reveal.
The Science Behind RSI: The Fast Stretch-Shortening Cycle
To understand why RSI matters, it helps to understand what it is measuring physiologically.
The stretch-shortening cycle (SSC) is the sequence of muscle actions that occurs when a contracting muscle is rapidly stretched and then immediately shortens. The pre-stretch of the eccentric phase:
- Stores elastic energy in the tendons (primarily the Achilles tendon)
- Potentiates the subsequent concentric contraction through the myotactic stretch reflex
- Allows the muscle to produce greater force in the concentric phase than it could from a static start
The SSC is classified into two types:
Slow SSC (contact time greater than 250 milliseconds): Involves significant hip, knee, and ankle flexion during the eccentric phase. Trained by countermovement jumps, depth jumps with deep knee bend, and many conventional strength exercises. Relevant to jumping from a standing position and other moderate-speed athletic actions.
Fast SSC (contact time less than 250 milliseconds): Involves rapid, stiff ankle and knee loading with minimal joint excursion. Trained specifically by depth jumps, pogo jumps, and sprint contacts. Relevant to sprinting, high-frequency hopping, and rapid direction changes.
RSI specifically measures fast SSC quality. An athlete can have excellent slow SSC performance (high countermovement jump height) but poor fast SSC performance (low RSI). This combination is common in strength-trained athletes who lack reactive plyometric training. RSI testing identifies this gap that other tests would miss.
The neural and tendon components of RSI:
RSI improvement comes from two sources:
Tendon stiffness: Stiffer tendons store and return elastic energy more rapidly, allowing the contact phase to be completed more quickly without energy loss. Achilles tendon stiffness is the primary tendon property determining RSI in jumping and running.
Neural drive and coordination: The speed and magnitude of motor neuron activation during the brief contact window determines how much force can be generated in the available time. Better neural coordination produces higher force in a shorter contact, improving both the jump height and contact time components of RSI.
How to Measure RSI
Laboratory Measurement (Gold Standard)
The most accurate RSI measurement uses a force plate combined with a high-speed camera or motion capture system. The force plate provides precise ground contact time from the force-time curve, and the camera system provides precise jump height from displacement data.
This level of measurement is available at universities, elite sporting institutes, and some well-equipped sport science facilities.
Field Measurement Using a Contact Mat
A contact mat (or timing mat) is a pressure-sensitive surface that records the precise timing of foot contacts and flight phases. RSI is calculated from the flight time (which gives jump height through the flight time equation) and the contact time.
Several commercial contact mats are available at accessible price points (Fusion Sport Smartjump, Chronojump, and others). This represents the most practical high-accuracy RSI measurement available outside a laboratory.
Flight time to jump height conversion:
Jump height (m) = (flight time in seconds)² × 9.81 ÷ 8
Example: A flight time of 0.53 seconds gives:
Jump height = (0.53)² × 9.81 ÷ 8 = 0.281 × 9.81 ÷ 8 = 0.344 metres
Field Measurement Using a Smartphone App
Several smartphone applications can estimate RSI with reasonable accuracy using the phone’s microphone or camera to detect contact and flight times:
My Jump 2 (iOS): Uses high-speed video to detect jump height and contact time from a depth jump. Research has validated this app against force plate measurements with good agreement. This is the most accessible accurate RSI measurement tool for most athletes and coaches.
Reactive Strength Index Calculator apps: Several apps designed specifically for RSI calculation from video analysis are available. Quality varies and validation data is limited for most.
Practical field testing protocol with My Jump 2:
- Set up the phone on a stable surface at hip height, camera facing the athlete
- Mark a consistent starting height on the box (the same height for all tests)
- Perform 5 depth jumps from the box with full recovery between each
- Record jump height and contact time for each jump
- Average the 3 best attempts for the RSI score
- Test from the same box height consistently across testing sessions
Field Estimation Without Technology
If no technology is available, RSI can be estimated subjectively using the following observations:
Contact time estimate from sound: A very brief, sharp contact sound during depth jumps corresponds to a contact time of approximately 150 to 220 milliseconds. A longer, softer contact sound corresponds to 250 to 350 milliseconds.
Knee bend observation: Minimal knee bend (10 to 20 degrees) at landing corresponds to shorter contact times. Deep knee bend (more than 60 degrees) corresponds to contact times above 300 milliseconds, indicating slow SSC rather than fast SSC performance.
While these qualitative observations do not provide a precise RSI value, they allow coaches without technology to make reasonable judgements about reactive strength quality.
RSI Benchmarks by Athlete Level and Sport
Understanding what RSI values mean requires context. The following benchmarks are drawn from the sports science research literature and elite coaching practice.
RSI benchmarks by athlete level (drop jump from 30 to 45cm box):
- Recreational athletes: RSI 0.5 to 1.0
- Trained recreational athletes: RSI 0.8 to 1.5
- Competitive club athletes: RSI 1.2 to 2.0
- High-performance athletes: RSI 1.8 to 2.8
- Elite athletes: RSI 2.5 to 3.5
- World-class sprinters and jumpers: RSI above 3.5
RSI benchmarks by sport (elite level):
Sprinters: RSI 2.5 to 4.0. The highest RSI values of any sport, reflecting the extreme reactive strength demands of maximum velocity sprinting where contact times are 80 to 100 milliseconds.
High jumpers and long jumpers: RSI 2.5 to 3.5. Explosive jumping from a single-leg contact requires exceptional fast SSC performance.
Basketball and volleyball players: RSI 1.8 to 2.8. Repeated vertical jumping demands high RSI for performance and for the ability to recover between jump contacts in competition.
Football (soccer) players: RSI 1.5 to 2.5. Sprint acceleration and direction change demands are the primary drivers of RSI relevance in football.
Rugby and American football players: RSI 1.2 to 2.2. Explosive collision sport demands require reactive strength alongside the greater body mass that tends to limit RSI values relative to lighter sports.
Distance runners (elite): RSI 1.5 to 2.2. Elite distance runners have higher RSI than recreational runners, reflecting the relationship between Achilles tendon stiffness, ground contact time, and running economy.
Recreational runners: RSI 0.6 to 1.2. Significant room for improvement that directly translates to better running economy and reduced ground contact time.
What RSI Tells You That Other Tests Do Not
RSI versus vertical jump height:
Vertical jump height measures the maximum height achievable from a countermovement jump. It reflects slow SSC performance and leg power but does not distinguish between athletes who achieve the same height with different contact efficiencies. RSI reveals the fast SSC quality that vertical jump height misses.
RSI versus sprint time:
Sprint time over 10 to 30 metres reflects the integrated outcome of many qualities including strength, technique, and reactive strength. RSI provides a more isolated measure of the reactive strength component, which allows more targeted training decisions.
RSI versus strength measures:
Strength measures (one-repetition maximum, force plate peak force) reflect the maximum force production capacity without the time constraint of athletic contacts. RSI reflects the force production within the brief window available during fast SSC contacts, which is more athletically relevant than maximum force alone.
The RSI-specific training implication:
An athlete with high strength and moderate countermovement jump but low RSI has a specific fast SSC deficit that will not be addressed by more strength training or countermovement jump practice. The training prescription is plyometric-specific: depth jumps, pogo jumps, and sprint work targeting the fast SSC mechanism.
An athlete with high RSI relative to strength has the opposite situation: the reactive system is well developed relative to the force production capacity. More strength training and slower plyometrics would better serve this athlete.
How to Improve RSI
The most effective training interventions for improving RSI are those that specifically challenge and develop the fast SSC mechanism.
Method 1: Depth Jumps
The primary training tool for RSI improvement. As covered in the depth jumps article on this site, depth jumps specifically load the fast SSC by using the box drop to create a landing velocity that must be reversed within a brief contact window.
RSI-specific depth jump protocol:
- Use a box height that produces the highest RSI value (see box height selection in the depth jumps article)
- Focus on minimising contact time, not maximising jump height
- Track RSI with a smartphone app after each session to monitor adaptation
- Perform 3 to 5 sets of 4 to 6 reps with 2 to 3 minutes rest between sets
- Two sessions per week during a focused RSI development block
Method 2: Pogo Jumps
As covered in the pogo jump exercise article on this site, pogo jumps develop the ankle-level reactive strength and Achilles tendon stiffness that is the primary physical determinant of RSI in most athletes.
RSI-specific pogo jump protocol:
- Bilateral pogo jumps: 3 to 4 x 20 reps, focusing on minimal contact time
- Single-leg pogo jumps: 3 x 15 per leg
- Forward pogo jumps: 3 x 30 metres
- Perform 2 to 3 sessions per week
Method 3: Sprint Training
Maximum velocity sprint training develops the ground contact times (80 to 100 milliseconds at maximum speed) that represent the upper extreme of fast SSC performance. Regular sprint training improves the neuromuscular coordination and tendon stiffness that underpin RSI.
Sprint protocol for RSI development:
- 6 to 8 x 30 to 60 metre sprints at maximum effort
- Full recovery between sprints (3 to 4 minutes)
- One to two sessions per week
Method 4: Heavy Strength Training
Contrary to what might be expected given the speed specificity of RSI, heavy strength training produces meaningful RSI improvements through its effects on tendon stiffness and maximum force production capacity.
Research consistently finds that concurrent strength and plyometric training produces greater RSI improvements than plyometric training alone. The mechanism is that heavy strength training increases Achilles tendon stiffness and maximum force production capacity, which improves both the quality of elastic energy storage and the magnitude of force available during the brief contact window.
Strength training for RSI:
- Heavy single-leg Romanian deadlift: 3 to 4 x 5 to 6 at 80 to 85 percent maximum
- Heavy calf raise: 3 to 4 x 6 to 8 per leg
- Hip thrust: 3 to 4 x 5 to 6 at heavy load
- Frequency: 2 sessions per week alongside the plyometric work
A Structured 8-Week RSI Development Programme
This programme is designed for an intermediate athlete (current RSI approximately 1.0 to 1.5) looking to improve reactive strength for athletic performance. Testing should be performed at weeks 0, 4, and 8 using consistent box height and measurement method.
Structure: 4 training days per week
Day 1: Depth jumps and strength
- Depth jumps: 3 x 5 reps (at optimal RSI box height)
- Heavy single-leg Romanian deadlift: 3 x 6 per leg
- Heavy calf raise: 3 x 8 per leg
Day 2: Pogo jumps and sprints
- Bilateral pogo jumps: 4 x 20
- Single-leg pogo jumps: 3 x 15 per leg
- Sprint: 6 x 30 metres at maximum effort, full recovery
Day 3: Rest or easy aerobic activity
Day 4: Depth jumps and strength
- Depth jumps: 4 x 5 reps
- Hip thrust: 4 x 6 at heavy load
- Single-leg press: 3 x 6 per leg
Day 5: Pogo jumps and sprints
- Forward pogo jumps: 4 x 30 metres
- Single-leg pogo jumps: 3 x 12 per leg
- Sprint: 4 x 40 metres, full recovery
Days 6 and 7: Rest
Weeks 1 to 2: Follow the above at conservative loads. Focus on technique.
Weeks 3 to 4: Increase depth jump box height if RSI has improved. Increase strength loads.
Week 4: Test RSI and compare to baseline.
Weeks 5 to 6: Further progress box height and loads based on week 4 test.
Weeks 7 to 8: Peak intensity. Maximum effort in all sessions.
Week 8: Final RSI test and compare to baseline and week 4.
Expected improvement: Research suggests trained athletes can improve RSI by 15 to 30 percent over an 8-week focused programme. Recreational athletes who are less well-trained may see larger improvements.
RSI in Plyometric Periodisation
RSI is not just a performance metric. It is a monitoring tool that guides periodisation decisions throughout the training year.
Pre-season RSI assessment: Testing RSI at the start of pre-season establishes the baseline and identifies the athlete’s current fast SSC status. Athletes with low RSI relative to their sport’s benchmarks need a focused plyometric development block. Athletes with high RSI can progress to sport-specific power work sooner.
During-season RSI monitoring: RSI values decline during periods of high training load and competition stress. Monitoring RSI every 2 to 4 weeks during the season tracks the athlete’s reactive strength status and can signal when additional plyometric maintenance is needed.
Post-season RSI assessment: Comparing post-season RSI to pre-season values reveals whether the in-season maintenance was sufficient to preserve the reactive strength developed in pre-season.
RSI as a readiness indicator: Some elite programmes use daily or weekly RSI testing as a readiness indicator, similar to heart rate variability. A significant drop in RSI from an athlete’s established baseline (more than 10 to 15 percent) suggests accumulated fatigue and may indicate the need for reduced training load.
Frequently Asked Questions
What is a good RSI score?
This depends entirely on the sport and athlete level. For recreational athletes, an RSI above 1.0 indicates reasonable reactive strength. For competitive club athletes, values above 1.5 indicate good reactive strength. For high-performance athletes in speed and power sports, values above 2.5 are desirable. See the benchmark tables above for sport-specific context.
Can RSI be too high?
In theory, higher RSI is better for reactive performance. However, pursuing very high RSI values through extreme training loads carries significant injury risk to the Achilles tendon, patellar tendon, and tibial periosteum. RSI development should be progressive and accompanied by appropriate recovery.
Is RSI the same as reactive power?
Reactive power is a related but different concept. Reactive power refers to the power produced during the concentric phase of an SSC action and is typically measured in watts. RSI is a dimensionless ratio that captures the efficiency of the overall SSC cycle including both the contact time and the height achieved. RSI is the more commonly used field measurement.
Does RSI predict sprint speed?
RSI is significantly correlated with sprint performance in the research literature, particularly for sprints of 10 to 30 metres where the acceleration phase dominates. However, sprint speed is determined by multiple factors including stride length, technique, and strength, of which RSI is one component. RSI is a better predictor of reactive qualities within sprinting than of overall sprint performance.
How often should RSI be tested?
During a focused RSI development programme, testing every 2 to 4 weeks tracks adaptation and guides programming decisions. Outside of a specific development block, testing RSI at the start and end of each training phase (typically every 6 to 8 weeks) is sufficient to monitor reactive strength status.