The Reactive Strength Index (RSI)

Why RSI Matters and How to Use It

Reactive strength is a key strength and power ability driving athletic performance. Strength and power abilities include maximal muscle strength (how much force an athlete can generate irrespective of time) and rate of force development (RFD-how fast an athlete generates force). However, reactive strength is unique in that it involves the ability to couple movements that lengthen musculotendinous tissue (eccentric movement) followed those in which musculotendinous tissue shortens (concentric movement). These movements are called stretch-shorten-cycles (SSCs) and they occur readily in all kinds of human activities like running, jumping and change of direction movements. Movements that involve SSCs are often referred to as plyometrics and reactive strength and plyometric ability are sometimes used interchangeably.

The Stretch Shorten Cycle (SSC)

Reactive strength can be assessed using the reactive strength index (RSI) obtained from variations of the vertical jump.

When the RSI is obtained from a vertical drop jump, it is calculated as the ratio of the vertical jump height over the ground contact time or the ratio of the flight time and ground contact time. The RSI can also be assessed during a cyclic hop test in which the athlete attempts to minimize the ground contact time while maximizing the jump height over multiple jumps. A modified RSI can be computed from a countermovement jump (CMJ) by calculating the flight time to jump contraction time ratio (i.e. the time frame from the initiation of the jump to the point of takeoff).

The modified Reactive Strength Index (RSI)

The RSI is unique from the other strength and power abilities. This means that there may be a low correlation between those athletes who have a high RSI, those who are strong, those who are fast and those who are powerful. RSI is correlated with change of direction ability and in sports like volleyball, the ability to jump high and fast is a key performance indicator. Consequently, assessing an athlete’s RSI is a great indicator of their plyometric or SSC ability and because jumping fast and changing direction are important for so many sports, it is a foundational test used by many sport performance practitioners. Reactive strength needs to be developed with specific training methods, including plyometric training.

The graph below shows the modified RSI for a group of elite level, development level and anterior cruciate ligament (ACL) reconstructed (ACLR) athletes. Note that the modified RSI was calculated differently than what is typically shown in the literature. The RSI is often calculated from the flight time, which is measured from the point of takeoff to touchdown.

The modified RSI for ACLR, Control, and Developmental athletes

But athletes can inflate the raw flight time by holding their feet off the ground at the point of landing. The modified RSI shown above was calculated using the takeoff velocity, which was obtained from the impulse-momentum method. The flight time of the body centre of mass was then determined from the takeoff velocity. The modified RSI in Figure 4 ranges from a low of 0.43 for one of the development-level athletes to a maximum of 0.75 for one of the elite-level athletes.

We can obtain a rough estimate of the RSI that would have been observed in the drop jump by multiplying the modified RSI shown in the graph above for the CMJ by afactor of 3. With this approach, an RSI in the drop jump greater than 2.5 can be considered excellent whereas an RSI below 1.5 identifies athletes requiring better reactive strength. Athletes with a low RSI require progressions and a gradual increase in load when it comes to plyometric training. This is one of the applications of an RSI test — with a short test taking no more than a minute, we can screen athletes to identify those that require a slower progression of plyometric training and more attention on reactive strength.

However, a challenge with tracking improvements in the RSI also presents coaches with an opportunity for monitoring athlete readiness. The RSI is highly sensitive to increases in training load and neuromuscular fatigue. This is problematic as a coach who is wanting to evaluate the efficacy of their training program for improving reactive strength ability using the RSI test will have to time the exit testing properly to ensure that lingering fatigue from the previous training cycle has dissipated. An RSI can continue to improve over several days as lingering fatigue effects from a heavy training cycle diminish. This also provides us with an opportunity to assess an athlete’s reaction to a training program, sometimes referred to as the reaction curve by my good friend Derek Evely.

The main function of coaches, trainers and sport science practitioners is to prescribe training to bring about peak performance in their athletes when it counts the most. However, it is nearly impossible to characterize an athlete’s reaction to training in many sports because of the complexities of the competition environment itself. Further, fatigue is a complex process involving many physiological and psychological systems. To simplify our language around fatigue, Professor Roger Enoka, a well-known neuromuscular physiologist, introduced the concept of performance fatigability or the effects of fatigue on performance. With this framework, we use standardized and repeatable performance tests to evaluate performance fatigability.

Neuromuscular performance can be characterized using variations of the vertical jump, a simple and repeatable performance test. The reliability of common vertical jump performance variables obtained from the CMJ is shown in the table below. The flight time to contraction ratio (FT:CT) shown was calculated using the raw flight time whereas the Velocity Index was obtained using the predicted flight time of the body centre of mass from the takeoff velocity as described above. The RSI obtained from the vertical jump variants has been used broadly in high performance sport as a component of athlete monitoring systems to evaluate performance fatigability and neuromuscular readiness.

Coefficients of variation for common CMJ outcome measures

Every time we design a training program, we are running a mini science experiment. Our training hypothesis is that if we prescribe program x, response y will occur over a particular time course. But how often do we verify that the training hypothesis actually manifested?

Weekly RSI testing is a key anchor point allowing coaches to verify their training hypotheses. By incorporating the RSI throughout normal training alongside measures of workload or training load, we can better assess our athletes and ensure we are bringing about peak performance when it matters most. We can also ensure that our athletes do not suffer from maladaptation like non-functional overreaching.

In the set of graphs below, we can observe an overall trend from the start to the end of the training period, namely that the jump contraction time is decreasing (the athlete is jumping faster) and mechanical power is increasing. Within each of the training cycles we can also see a decreasing trend for the modified RSI (velocity index) but an overall improvement from the start to the end of the training period. Taken together, not only can we see what is happening on the macro scale (from the start to the end of the training period) but also on the meso scale (what is happening within each training cycle). This provides key information to help inform training program design both on an ongoing basis and retrospectively as we aim to determine what worked and what didn’t.

Reaction curve to training for an elite athlete. Top panel shows the training loads through three training cycles. Bottom panel shows vertical jump outcome measures and the velocity index (modified RSI). Red circles show the same time periods in each panel.

In summary, the RSI can be used to help inform training program design, assess reactive strength or plyometric ability, and monitor performance fatigability and athlete readiness. Here are a few tips for using RSI:

  • Assess RSI at the same point each week, like after a rest day
  • Make sure that the technology and jump test used are not overly affected by measurement error as this can obscure our ability to detect the signal through the noise — the Plantiga insole system provides a portable and high-fidelity assessment of the RSI comparable to the gold standard, force plate methodology
  • Remember that RSI will not fluctuate like some of the other strength abilities — RSI will naturally decrease through training cycles and with load, and it will gradually recover as training load dissipates so test timing is crucial
  • Use RSI to evaluate reactive strength ability in athletes and to identify those who require a slower progression in plyometric training, like athletes recovering from lower body injury
  • A modified RSI is obtained from a CMJ and the RSI is often measured in the vertical drop jump or a cyclic jump test — thresholds presented in this blog post can be helpful for coaches and trainers who wish to evaluate reactive strength ability across performance levels

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