Measuring Explosive Power with RSI (Reactive Strength Index)
- Apr 9
- 8 min read
What Is RSI?
Picture a basketball player catching a rebound and immediately leaping back up for a dunk. Or a soccer player changing direction -- the foot is on the ground for less than 0.2 seconds. How much force can be produced in that brief moment? RSI is the number that shows you.
RSI (Reactive Strength Index) is a metric that evaluates an athlete's reactive strength by measuring jump height relative to ground contact time. First proposed by Young (1995), this concept is now one of the most widely used plyometric ability assessment tools in sports science.
RSI = Jump Height (m) / Ground Contact Time (s)
A high RSI means the athlete can produce large forces in a short time. This is important in virtually every sporting action -- sprint starts, changes of direction, and re-takeoff after landing.
At a Glance RSI measures not "how high you jump" but "how quickly you react and jump high" An RSI of 2.0 or above indicates excellent reactive strength -- a competitive level for most team sports Finding each athlete's optimal drop height maximizes training efficiency A 10% or greater decrease in RSI is a clear signal of neuromuscular fatigue, making monitoring essential
Why RSI Matters
1. Stretch-Shortening Cycle (SSC) Assessment
RSI directly measures the muscle's stretch-shortening cycle utilization ability. According to Komi (2000), athletes with a good SSC:
Efficiently convert landing impact into elastic energy
Achieve maximum jump height with minimal ground contact time
Excel at consecutive jumps and rapid direction changes
2. Sport Performance Prediction
According to a review by Flanagan & Comyns (2008), RSI shows strong correlations with:
Athletic Ability | Correlation (r) |
10m Sprint | 0.65-0.75 |
Agility Test | 0.60-0.70 |
Change of Direction Speed | 0.55-0.65 |
Consecutive Jump Ability | 0.75-0.85 |
3. Injury Risk Assessment
Athletes with low RSI absorb impact for a longer duration during landing. In Hewett et al. (2005):
Greater stress on knee and ankle joints
Increased ACL injury risk
Accelerated fatigue accumulation
RSI monitoring is a core component of injury prevention programs.
4. Training Program Design
Training direction differs based on RSI level (Young et al., 2002):
RSI Level | Male | Female | Training Direction |
Low | <1.5 | <1.2 | Basic strength and plyometric introduction |
Average | 1.5-2.0 | 1.2-1.6 | Moderate-intensity plyometrics |
Good | 2.0-2.5 | 1.6-2.0 | High-intensity reactive training |
Elite | >2.5 | >2.0 | Sport-specific training |
RSI Variants
RSI-modified (RSImod)
A variant proposed by Ebben & Petushek (2010), measured from the CMJ:
RSImod = Jump Height (m) / Time to Takeoff (s)
Suitable for athletes who find drop jumps difficult or are in rehabilitation.
Dynamic Strength Index (DSI)
Concept from Sheppard & Young (2006):
DSI = CMJ Peak Force / IMTP Peak Force
Values closer to 1.0 indicate that maximum strength is being well utilized.
RSI Testing Methods
Drop Jump Test (Standard)
Protocol from McClymont & Hore (2003):
Stand on a box 30-40cm high
Step off the box (do not jump -- drop naturally)
Immediately upon landing, jump to maximum height
Minimize ground contact time while aiming for maximum height
Key Points:
Do not excessively bend the knees on landing
Think of the ground as "hot" and bounce off quickly
Use the best result from 3-5 repetitions
Finding the Optimal Drop Height
According to Byrne et al. (2010), the optimal drop height varies by individual. The optimal drop height is the height at which RSI is highest -- the point where the athlete's SSC ability is utilized most efficiently.
Optimal Drop Height Test Protocol:
Preparation: Set up boxes at 20cm, 30cm, 40cm, and 50cm heights
Execution: Perform 3-5 drop jumps at each height, recording the best RSI
Rest: 2-3 minutes between heights, at least 30 seconds between attempts
Analysis: The height with the highest RSI = the most effective training height for that athlete
Drop Height | RSI Result Example | Interpretation |
20cm | 1.8 | Insufficient stimulus |
30cm | 2.1 | Optimal drop height |
40cm | 1.9 | Ground contact time starting to increase |
50cm | 1.5 | SSC unable to be utilized (overload) |
In this example, 30cm is the optimal height. RSI declining at 40cm means the landing impact is too great for SSC conversion.
Periodic reassessment is important. As SSC ability improves through training, the optimal drop height also changes. Retest every 4-6 weeks and adjust the training height.
Consecutive Jump Test (10/5 RSI)
Protocol from Harper et al. (2011):
Perform 10 consecutive jumps
Calculate the average RSI of the middle 5 jumps
Assesses sustained reactive strength and fatigue resistance
Measuring RSI with Point Go
The Point Go sensor is attached to the waist to provide accurate RSI data.
Measurement Workflow
Sensor connection: Firmly attach the Point Go sensor to the lower back (sacral area)
Test selection: Select RSI measurement in the Coach app and specify drop jump or consecutive jump test
Calibration: The sensor establishes a baseline while stationary. The athlete stands still for 2-3 seconds
Start measurement: Measurement begins after the countdown. For drop jumps, step off the box; for consecutive jumps, begin from standing
Real-time feedback: RSI, jump height, and ground contact time are displayed immediately for each jump
Save results: Results are automatically saved to the athlete's profile after measurement, with comparison to previous records available
Measured Variables
Jump height: Calculated from flight time
Ground contact time: Time from landing to takeoff
RSI: Automatically calculated
Jump-by-jump trend: Fatigue monitoring
Interpreting Results
RSI for each jump can be viewed in real time in the app. If RSI drops sharply during consecutive jumps, fatigue management or basic fitness improvement is needed.
According to Cormack et al. (2008), a 10% or greater decrease in RSI signals neuromuscular fatigue.
RSI Data Utilization Tips
In-season monitoring: Measure RSI once per week under identical conditions (time, warm-up, drop height) to objectively track athlete fatigue status
Left-right comparison: Comparing left-right RSI with single-leg drop jumps can reveal asymmetry. Differences of 15% or more indicate elevated injury risk
Training effect validation: Compare RSI before and after a plyometric program to quantitatively evaluate program effectiveness
RSI Improvement Training
Beginner (RSI < 1.5)
Basic reactive ability development:
Bilateral Pogo Jumps 3x10
Low box Drop Jumps (20cm) 3x5
Ankle Hops 3x15
Skipping drills 3x20m
Intermediate (RSI 1.5-2.0)
Increased plyometric intensity:
Drop Jumps (30cm) 4x5
Hurdle Jumps 3x5
Bounding 3x20m
Box Drills 3x6
Advanced (RSI > 2.0)
Sport-specific reactive training:
Drop Jumps (40-50cm) 4x5
Depth Jump to Box 3x5
Single-response reactive jump training
Complex jump sequences
RSI Improvement Timeline
RSI improvement does not happen overnight. Below is a realistic improvement timeline with consistent training.
Weeks 1-4: Neural Adaptation Phase
During this period, nervous system adaptations rather than muscle mass changes are the primary drivers.
RSI improvement: 5-10%
Primary change: Decreased ground contact time (improved landing reaction speed)
Jump height may not change significantly yet
Coordination and timing are improving
Weeks 5-8: Musculotendinous Adaptation Phase
SSC mechanisms start to genuinely improve.
RSI improvement: Additional 10-15%
Primary change: Increased tendon stiffness improving elastic energy storage capacity
Both jump height and ground contact time improve
Optimal drop height may change during this period, so retesting is recommended
Weeks 9-12: Performance Expression Phase
The neuro-muscular-tendon system begins to function in an integrated manner.
RSI improvement: Cumulative 20-30% (vs. baseline)
Ground contact time decreases notably while jump height is maintained or increases
Perceivable changes in sport movements (sprinting, direction changes)
Consider transitioning to sport-specific training from this point
After 12 Weeks: Maintenance and Specialization
It is normal for the rate of improvement to slow down
Periodically vary training variables (drop height, exercise type, load)
Test every 4-6 weeks and adjust programming
Realistic expectations: An untrained athlete improving RSI from 1.2 to 1.6-1.8 after a 12-week program is entirely achievable. For athletes already at RSI 2.0 or above, even 5-10% improvement is meaningful progress.
Using RSI for Return-to-Sport Decisions
Deciding when to return to competition after a lower-extremity injury (ACL, ankle sprain, Achilles tendon, etc.) is a challenge for both coaches and medical staff. RSI provides objective data for this decision.
Why RSI Is Valid as a Return Criterion
RSI directly measures landing-takeoff ability, reflecting actual sport performance capability
It captures reactivity and confidence that simple strength tests (leg press, etc.) cannot detect
Bilateral comparison allows quantification of the injured side's recovery level
RSI-Based Return Criteria
Stage | RSI Criterion | Permitted Activity |
Early rehab | Unable to measure bilateral RSI | Isometric exercises, ROM recovery |
Mid rehab | Injured side RSI < 70% of uninjured | Low-intensity plyometrics, straight-line jogging |
Late rehab | Injured side RSI = 70-85% of uninjured | Moderate-intensity plyometrics, direction changes |
Return readiness | Injured side RSI = 85-90% of uninjured | Partial team training participation |
Return to competition | Injured side RSI ≥ 90% of uninjured | Full match participation |
Considerations for Return Decisions
Do not base return decisions on RSI alone. Strength tests, functional tests, and psychological readiness should be comprehensively evaluated
Single-leg tests are more sensitive than bilateral tests. In bilateral tests, the uninjured side may compensate, masking weaknesses on the injured side
Continue weekly monitoring for at least 4 weeks after return to confirm RSI does not decline again
Having preseason baseline data makes setting return targets much easier. Pre-injury data collection is important
Volume Management
Guidelines from Ebben et al. (2010):
Training Experience | Contacts/Session | Weekly Frequency |
Beginner | 60-80 | 1-2 sessions |
Intermediate | 100-120 | 2 sessions |
Advanced | 120-140 | 2-3 sessions |
Precautions
RSI training places high loads on the joints:
Thorough warm-up is essential
Limit to 2 sessions per week or fewer (beginners)
Do not train in a fatigued state
Stop immediately if pain occurs
Ensure strength base is established before progressing to high-intensity training
Frequently Asked Questions (FAQ)
Q. RSI vs. jump height -- which should I prioritize?
It depends on the objective. For sports where pure jump height (high jump, blocking) matters, jump height is the priority. However, for most team sports requiring sprinting, direction changes, and consecutive jumps, RSI is the more useful metric. If ground contact time decreases from 0.5 seconds to 0.3 seconds, game performance improves significantly even if jump height remains the same.
Q. Drop jump RSI vs. CMJ RSI (RSImod) -- which test should I choose?
The two tests measure different abilities. Drop jump RSI assesses fast SSC (ground contact time <250ms) and is closely related to sprinting and rapid direction changes. RSImod assesses slow SSC (ground contact time >250ms) and relates to vertical jump ability. Ideally, perform both tests, but if you must choose one, decide based on sport characteristics. For rehabilitation contexts, RSImod is safer.
Q. How often should RSI training be performed?
Beginners should start with 1-2 sessions per week. Plyometrics place high loads on joints and tendons, requiring a minimum of 48 hours recovery between sessions. Intermediate athletes (RSI 1.5+) can increase to twice weekly, but avoid scheduling it on the same day as lower-body weight training. If RSI is 15% or more below the previous session, recovery is incomplete -- replace with low-intensity technique work for that day.
Q. Can older athletes do RSI training?
Yes, but the approach needs to be modified. Athletes 30+ or with limited training experience should start with low drop heights (20cm) and bilateral landings. Since tendons and joints need more time to adapt, allow a 4-6 week foundation adaptation period before progressively increasing height and difficulty. A strength base is critical -- being able to squat at least 1.5x bodyweight is necessary for safe high-intensity RSI training.
Related Articles
Jump Training Guide for Athletes - Various jump tests including CMJ and SJ
Scientific Understanding and Application of Isometric Training - Isometric training for landing stability
VBT Training Basics - Improving explosiveness through velocity-based training
References
Young, W.B. (1995). Laboratory strength assessment of athletes. New Studies in Athletics, 10, 89-96. PDF
Komi, P.V. (2000). Stretch-shortening cycle: a powerful model to study normal and fatigued muscle. Journal of Biomechanics, 33(10), 1197-1206. DOI
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. DOI
Hewett, T.E., et al. (2005). Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes. American Journal of Sports Medicine, 33(4), 492-501. DOI
Ebben, W.P., & Petushek, E.J. (2010). Using the reactive strength index modified to evaluate plyometric performance. Journal of Strength and Conditioning Research, 24(8), 1983-1987. DOI
Byrne, P.J., et al. (2010). Identifying the optimal resistive load for jump squats in recreational athletes. Strength and Conditioning Journal, 32(2), 67-72. DOI
Cormack, S.J., et al. (2008). Neuromuscular and endocrine responses of elite players to an Australian rules football match. International Journal of Sports Physiology and Performance, 3(3), 359-374. DOI
RSI reveals not simply jump height, but how efficiently force is produced. A high RSI is the core of athletic performance.
Comments