Using Reactive Strength Index (RSI) in a Team Setting
Using Reactive Strength Index
(RSI) in a Team Setting
High Performance Director
What is RSI?
RSI is shorthand for “Reactive Strength Index”. The Reactive Strength Index is a measure of lower body explosiveness and is the combination of two metrics derived from a plyometric movement: flight time (FT) or jump height in meters (JH) and ground contact time (CT).
RSI Calculation (Option A) = Flight Time/Contact Time
RSI Calculation (Option B) = Jump Height in meters/Contact Time
Converting Flight Time to Jump Height (meters):
Excel or Sheets formula: =((POWER((A1/2),2)*9.81)*0.5)
A1 represents the cell with Flight Time value
Converting Flight Time to Jump Height (inches):
Excel or Sheets formula: =((POWER((A1/2),2)*9.81)*0.5)*39.37
A1 represents the cell with Flight Time value
To establish the RSI of a depth jump, one would divide flight time by contact time. For example, an athlete with a flight time of .40s (jump height of .20 meters (20cm)) and a contact time of .15 seconds would have an RSI value of: .40/.15 = 2.67. This easily derived metric can be used to monitor long term explosive development, identify strengths and weaknesses in dynamic movement, and monitor CNS fatigue. RSI allows the coach or practitioner to begin to evaluate not just how high an athlete jumped, but the strategy with which they performed the movement. Using a tool like the Plyomat, simple but effective metrics such as RSI can be easily calculated and acted upon.
The Reactive Strength Index is a tool that expresses more than just the outcome measure of a depth jump, i.e., jump height. It also exposes strategy, meaning it provides a quantifiable measurement of how the athlete absorbs and expresses energy in a short contact, dynamic fashion. Jump strategy; in this case how quickly an athlete can land and “rebound” off of the ground, will tell a lot about the athlete’s ability to utilize the stretch shortening cycle (SSC) in dynamic movement.
Because RSI is the combination of both contact time and jump height, the resulting score will be influenced by both variables. If for instance, the athlete that scored an RSI of 2.67 from the combination of a 20cm height and .15second contact time, subsequently improves their jump height to 22cm in the same timeframe, they will have achieved an RSI of 2.8. This would indicate that the athlete has improved their power output while maintaining the same movement strategy. They are effectively “more explosive”. On the other hand, if the same athlete maintained their 20cm jump height, but improved their contact time to .137, they have also achieved an RSI of about 2.9. However, in this case, the improvement has manifested due to a decrease in contact time. This example also illustrates a “more explosive” athlete, however in this case it is due to an improved stretch shortening cycle.
Why use RSI
Because RSI takes two related but separate variables from the depth jump into consideration, we can use the information to begin to better understand how our athletes produce force, and where their strengths and weaknesses are when it comes to dynamic, explosive activities. This additional information can have profound implications in both fatigue monitoring and performance profiling. RSI allows us to move beyond “what did they do”, and ask “how do they do it”. This begins to form the foundation of a performance profiling and fatigue monitoring program.
A common goal of sport science is to be able to identify fatigue in athletes prior to the onset of significant performance decrements. Amongst most team sports, CNS or neurological fatigue is especially important to monitor, as speed and power are often the most sought-after physiological traits in these sports. The old adage, “Speed kills” is usually true. Making sure our athletes are staying fast and powerful is key during a long in-season period.
Utilizing the basic countermovement vertical jump has long been a simple way to keep tabs on not only improvements in power development, but acute decreases in output that might be associated with CNS fatigue. If an athlete consistently jumps 50cm in the countermovement jump, but suddenly can’t break 40cm, you can be fairly confident something is going on. While consistently performing vertical jumps as a performance and monitoring tool is a simple way to keep tabs on big swings in explosive performance, a problem often arises especially in more athletically gifted players when trying to identify more nuanced changes in performance.
Often times, an athlete will subconsciously alter how they jump while they are fatigued, in an effort to maintain their normal jump height during the vertical jump. This typically manifests as a longer eccentric loading phase, and/or a deeper descent during the eccentric loading portion of the jump. In effect, an athlete that might consistently jump 50cm during a countermovement jump, will still be able to reach 50cm, or at least something close to that, even though they are starting to accumulate fatigue from training, competition, lack of recovery, etc. The countermovement jump height as a standalone metric isn’t sensitive enough to pick up small changes in fatigue because the athlete will change how they jump to attempt to “mask” the onset of CNS fatigue. By the time you identify a significant decrease in jump height alone, you may be behind the proverbial 8 ball when it comes to fatigue management. This is where RSI comes in.
Because RSI is the combination of jump height and contact time, any change in either variable will effect the final score. Opposite the earlier example of the athlete who improved their contact time while maintaining their jump height, a neurologically fatigued athlete will often maintain their jump height but spend more time on the ground in an effort to create more force. They are subconsciously “loading” their jump more, in an effort to make up for the decrease in neural output. Their strategy changes from their typical expression of force and reactivity, and often shifts more toward concentric muscular force output, away from the more neurologically dependent stretch shortening cycle contribution to movement.
This change in movement signature can be very hard to see during the countermovement jump with the naked eye, and unless you have access to expensive force plates, it’s all but impossible to measure. However, the depth jump provides opportunity to collect ground contact time information with a tool like the Plyomat, which then becomes the indicator of jump strategy. A longer than usual ground contact time upon descending from the depth jump will alter the RSI score, demonstrating a change in technique or strategy. Thus, RSI becomes a powerful tool in fatigue monitoring as it allows you to quickly measure and understand how the athlete completes the task.
Another common goal within the realm of sport science is performance profiling. In short, performance profiling is simply an attempt to utilize various metrics to identify strengths and weaknesses of an athlete. In the case of defining jump performance, either as a specific task within a given sport, or as a proxy for “explosiveness”, RSI can serve as a helpful metric. Very much the “other side of the coin” from fatigue monitoring, we can look at the total RSI value as well as its constituent parts to paint a broader picture of how an athlete expresses force, and therefore develop a roadmap or decision tree as to what/how to improve or maintain specific qualities. For example, if you have identified an RSI score of 3.0 to be significant with your group, then all of the athletes who meet this goal can be further evaluated by how they get there…do they have moderate jump heights, but exceptionally fast ground contact times? Or do they have very impressive jump heights, but less than stellar ground contact times? Either way, you have now identified their individual strengths and weaknesses within this movement and can program accordingly. The athlete who lacks short ground contacts but is able to “muscle” their way into a high jump is demonstrating high concentric output but might benefit from improving their stretch shortening cycle abilities by working on more short contact plyometric activities. On the other hand, the athlete that seems to be very reactive and demonstrates short contact times but lacks significant jump heights may need more force production work. This could take the form of longer response plyometric activities or loaded jumping.
There is no “one way” to evaluate and subsequently program based on this information. The key is to be able to identify through trial and error, what is “good” in your sport and population, and then look under the hood of the athlete’s bio motor ability and make more informed decisions about how to train them. The starting point is to consistently collect data so that you have an idea of what is normal, what is better than normal, and what is below standard. Once this starts to become clear, you can utilize the information from RSI to make changes, if changes need to be made.
How I’ve implemented RSI
The Reactive Strength Index is a tool to identify reactive abilities in athletes. By objectively measuring not only how high an athlete can jump during a simple depth jump, but also how quickly they can effectively “rebound” off of the ground to achieve this outcome, we can begin to surmise helpful information on performance abilities, increases or decreases in fatigue, long term training adaptations, and movement strategies that form part of the unique movement signatures of our athletes. I have used RSI for all of these purposes at various times during my career.
As a low-cost readiness tool, I’ve used the Plyomat pre-training to identify CNS fatigue states, and then adjusted training based on where my athletes were at on that specific day. By having simple options available based on how an athlete scored relative to their norm, they either stayed the course with the prescribed training, decreased volume to account for an elevated level of fatigue or pushed the gas pedal in our primary lifts for the day because they were performing well above their norm. If for instance, as our primary lower body pushing strength exercise, we have Rear Foot Elevated Split Squats prescribed, and the set/rep scheme during this phase of training calls for 3x3x80%, then the group who shows signs of fatigue might only perform 1 set at the prescribed intensity. The group who scores close to their “normal” would simply perform the previously prescribed 3x3x80%, and the group who had a higher-than-normal RSI score would have the option of either adding a set, or adding load. Once the ground rules were established and communicated, and a simple chart showing individuals “High, Normal, Low” thresholds was created, the athletes simply adjusted training based on where they were that day.
In other instances, I’ve used RSI scores from depth jumps along with jump height data from the standard countermovement jump to create a jump profile on my players. This information helped shape the subsequent training blocks, bucketing athletes within the group setting into more reactive work, more strength-based work, eccentric overload work, etc. based on where their strengths and weaknesses were. For example, if an athlete had a very good countermovement jump relative to their peers (say, 50cm), but a relatively slow contact time in the depth jump (.3seconds), this indicated that they were capable of producing high levels of concentric force but needed to focus on the rate at which they could produce it. This athlete might perform our plyometric work with a double-bounce as opposed to a single contact hurdle jump that the rest of the group was performing in an effort to focus on short ground contacts. In an opposite example, an athlete with a very fast ground contact, but relatively low countermovement jump might perform more loaded ballistic jumps in an effort to improve their force output abilities. In either case, we were simply “looking under the hood” at their jump output and jump strategy and making decisions about what variable of dynamic explosiveness we needed to emphasize in training.
By individualizing work within the team training setting, we were able to maintain the all-important logistics and training flow in the group dynamic, while continuing to find areas of improvement over long periods of time with high level athletes. RSI and the Plyomat have been instrumental in allowing us to gather relevant information and make more informed decisions in our training process quickly and efficiently.