Reactive Strength Index (RSI) Explained: Testing, Calculation & Training

The reactive strength index (RSI) is one of the most powerful metrics in sports science for quantifying an athlete's ability to tolerate and utilize stretch-shortening cycle (SSC) loads. Originally developed by Warren Young at the University of Ballarat in the 1990s, RSI measures how effectively an athlete can transition from an eccentric (landing) phase to a concentric (jumping) phase during a plyometric movement. Unlike jump height alone, RSI accounts for both the output (how high the athlete jumps) and the input (how quickly they produce that jump), making it an essential metric for any sport that demands rapid, reactive movements. Related: Countermovement Jump: Proper Form & Performance Tips

The reactive strength index is defined as the ratio of jump height to ground contact time during a plyometric task — most commonly a depth jump (also called a drop jump). It was introduced to solve a fundamental limitation of using jump height as a standalone performance metric. Consider two athletes who both achieve a 40 cm jump height from a depth jump: if athlete A achieves this with a ground contact time of 180 milliseconds while athlete B requires 350 milliseconds, their reactive strength capabilities are vastly different despite identical jump heights. Athlete A produces the same output in roughly half the time, demonstrating far superior reactive strength.

Physiologically, RSI reflects the efficiency of the stretch-shortening cycle — the mechanism by which muscles and tendons store elastic energy during rapid eccentric loading and release it during the subsequent concentric action. A high RSI indicates that the neuromuscular system can rapidly absorb impact forces, efficiently store elastic energy in the muscle-tendon unit, and redirect that energy into explosive upward propulsion with minimal time delay. This quality underpins performance in virtually every sport involving running, jumping, cutting, or rapid change of direction.

The SSC operates through three distinct phases: the eccentric (pre-stretch) phase where the muscle-tendon unit lengthens under load, the amortization phase representing the brief transition between eccentric and concentric action, and the concentric (shortening) phase where stored elastic energy is released alongside active muscle contraction. RSI essentially quantifies how effectively and quickly an athlete manages all three phases. A prolonged amortization phase — indicated by long ground contact times — dissipates stored elastic energy as heat, reducing the elastic contribution to the subsequent jump. See also: Standing Long Jump Test: Protocol, Norms & Horizontal Power Assessment

The reactive strength index is calculated using a simple formula:

RSI = Jump Height (m) / Ground Contact Time (s)

For example, if an athlete achieves a jump height of 0.35 m (35 cm) with a ground contact time of 0.200 s (200 ms), their RSI is:

RSI = 0.35 / 0.200 = 1.75

The resulting RSI value is expressed in meters per second (m/s), though it is typically reported as a dimensionless number in practice. Higher values indicate superior reactive strength. RSI values typically range from 0.5 (novice or fatigued) to 3.0+ (elite plyometric athletes).

Two measurement inputs are required: jump height and ground contact time. Jump height can be calculated from flight time using the formula h = (g x t²) / 8, where g is gravitational acceleration (9.81 m/s²) and t is flight time. Alternatively, jump height can be derived from take-off velocity using h = v² / (2g). Both methods produce equivalent results when measured accurately. Ground contact time is the duration between initial foot contact after the drop and the moment the feet leave the ground for the subsequent jump. It encompasses the entire eccentric-amortization-concentric sequence.

It is critical to note that RSI values are specific to the drop height used. Higher drop heights increase eccentric loading demands and typically increase ground contact time more than they increase jump height, resulting in lower RSI values. For this reason, RSI values should always be reported alongside the drop height from which they were obtained. Comparing RSI values obtained at different drop heights is not valid.

The standard RSI test uses a depth jump (or drop jump) from a prescribed box height. The protocol demands careful attention to technique, as improper execution invalidates the reactive strength measurement. Here is the step-by-step procedure:

1. Equipment setup: Position a sturdy plyometric box (typically 30 cm, 40 cm, or 60 cm height) on a firm, level surface. Set up your measurement system — force plate, contact mat, or IMU sensor — on the landing surface adjacent to the box.
2. Warm-up: Complete 5–10 minutes of aerobic activity followed by dynamic stretching. Perform 10–15 submaximal hops in place, then 3–5 submaximal depth jumps from the test height to prepare the neuromuscular system for the eccentric loading demands.
3. Starting position: Stand on the edge of the box with toes at the edge. Arms may be held akimbo (hands on hips) for standardization or free to swing for sport-specific assessment. Maintain an upright posture.
4. Drop phase: Step off the box by leading with one foot — do not jump up or step down. The goal is to drop with minimal vertical displacement of the center of mass before leaving the box. The landing should occur on both feet simultaneously.
5. Ground contact and jump: Upon landing, react as quickly as possible to jump maximally upward. The instruction to the athlete is critical: emphasize minimum ground contact time with maximum jump height. Cue the athlete to imagine the ground is a hot surface — touch down and get off as fast as possible while still jumping high.
6. Trials: Perform 5–8 maximal depth jumps with 30–45 seconds of rest between trials. Discard the first 1–2 as familiarization. Use the average RSI of the remaining valid trials or the best single trial, depending on your protocol.

Validity criteria for each trial: The athlete must land on both feet simultaneously, must not exhibit an excessively deep knee bend (ground contact time above 400 ms typically indicates a power-dominant rather than reactive strategy), and must not step off the box with an upward trajectory. Some protocols set a maximum allowable ground contact time of 250–300 ms, with longer contacts classified as "slow SSC" jumps rather than true reactive efforts.

RSI values are highly specific to the drop height, testing protocol, and population assessed. The following normative ranges are based on published research using a standard 30–40 cm drop height with hands-on-hips protocol:

Sport-specific contexts matter enormously when interpreting RSI. Sprint athletes and high jumpers typically demonstrate the highest RSI values (2.5–3.5) because reactive strength is a primary performance determinant in their events. Team sport athletes like basketball and soccer players generally range from 1.5–2.5. Endurance athletes and strength sport athletes (powerlifters, weightlifters) may have relatively lower RSI values (1.0–1.8) because their training does not prioritize fast SSC function.

Sex-based differences are also significant. Female athletes typically produce RSI values 15–25% lower than male athletes at the same competitive level, primarily due to differences in muscle-tendon stiffness, relative strength, and neuromuscular activation rates. Female-specific norms should be used for valid comparisons.

Longitudinal tracking of RSI is one of its most valuable applications. A sustained decline in RSI across multiple testing sessions — particularly when jump height is maintained but ground contact time increases — is a sensitive indicator of accumulated neuromuscular fatigue and may warrant a reduction in training load.

In recent years, a variation called the RSI modified (RSImod) has gained widespread adoption, particularly in team sport settings. RSImod is calculated from a countermovement jump (CMJ) rather than a depth jump, using movement duration instead of ground contact time:

RSImod = Jump Height (m) / Time to Take-off (s)

Here, time to take-off is the duration from the initiation of the countermovement (first downward movement) to the moment of take-off. This typically ranges from 0.5–0.9 seconds, much longer than the 0.15–0.30 second ground contact times seen in depth jumps.

RSImod offers several practical advantages over traditional RSI. First, it eliminates the need for a plyometric box and the associated eccentric loading demands, making it safer and more accessible for a wider range of athletes. Second, the CMJ is already the most commonly performed test in professional sport, so RSImod can be derived from existing testing data without additional trials. Third, RSImod has been shown to be sensitive to neuromuscular fatigue and is used extensively in daily athlete monitoring programs.

However, RSImod and traditional RSI measure fundamentally different qualities. RSI from a depth jump assesses fast SSC function (contact times under 300 ms), while RSImod from a CMJ assesses slow SSC function (contact times above 500 ms). Fast and slow SSC abilities do not always change in parallel — an athlete may have excellent fast SSC function but average slow SSC function, or vice versa. For this reason, both metrics provide unique and complementary information, and ideally, both should be included in a comprehensive testing battery.

Typical RSImod values range from 0.3–0.5 for untrained individuals, 0.5–0.7 for trained athletes, and 0.7–1.0+ for elite athletes. As with RSI, longitudinal within-athlete tracking is more valuable than cross-sectional between-athlete comparisons.

Improving the reactive strength index requires a systematic approach that develops both the capacity to absorb high eccentric forces and the ability to redirect those forces rapidly into concentric output. This is not simply about jumping higher — it is about jumping higher in less time. Training must therefore target muscle-tendon stiffness, rate of force development, and neuromuscular coordination specific to fast SSC function.

Progressive depth jump training is the most specific method for improving RSI. Begin with low box heights (20–30 cm) and focus on minimizing ground contact time while maintaining jump height. As competency improves, gradually increase box height to 40–60 cm. Perform 3–5 sets of 3–5 repetitions with full recovery (2–3 minutes) between sets. The focus must always be on quality — once ground contact time increases noticeably due to fatigue, the set should end.

Ankle stiffness exercises play a critical role because the ankle joint is responsible for much of the elastic energy storage and return during fast SSC movements. Pogo hops (continuous small bounces with stiff ankles, minimal knee bend), single-leg pogo hops, and hurdle hops with an ankle-dominant strategy all develop the ankle stiffness component. Research by Flanagan and Comyns (2008) identified ankle stiffness as a primary contributor to RSI performance.

Eccentric strength training builds the force absorption capacity that enables an athlete to tolerate higher drop heights without excessive amortization time. Eccentric-focused squats and calf raises at supramaximal loads (110–130% of concentric 1RM) develop the structural and neural qualities needed to rapidly decelerate the body on landing.

Maximal strength training underpins all reactive qualities. Athletes with higher relative strength (1RM squat / body mass > 2.0) consistently demonstrate higher RSI values than weaker athletes, even before plyometric training. If relative squat strength is below 1.5 times body mass, prioritizing strength development will likely produce larger RSI improvements than plyometric training alone.

Tendon conditioning: Heavy isometric holds at long muscle lengths and heavy slow resistance training stimulate tendon remodeling, increasing tendon stiffness over 8–12 week training blocks. Stiffer tendons store and return elastic energy more efficiently, directly enhancing RSI. Calf raises with 3–5 second isometric holds at end range are particularly effective for the Achilles tendon, which is the primary elastic structure during depth jump landings.