UNDERSTAND THE REACTIVE STRENGTH INDEX AND ITS USE IN ATHLETIC DEVELOPMENT by Kyle O’Toole

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The field of strength and conditioning is evolving as we know it. Athlete monitoring, analysis, and testing tools are changing the way we do things. At times balancing the scientific principles with the real-world components that shape our coaching methods seems like an impossible feat. But don’t let that stop you from providing the best training methods for your athletes that you can. My intent with this article is to help explain why reactive strength index (RSI) is being used, examine some of the key testing protocols, and provide you with the formulas to calculate RSI with your athletes. When it comes down to it, RSI is much easier to understand than you may think. It is a great tool used by coaches to measure how well an athlete is coping with stress on their body and it guides the direction of future training.

WHAT IS REACTIVE STRENGTH INDEX

RSI is one component of the Strength Qualities Assessment Test (SQAT) created by the Australian Institute of Sport. The SQAT assesses the strength capabilities of the leg extensor muscles and is intended to specifically gauge the progression of running and jumping movements (7). The RSI has since been taken from the SQAT because of its influence on training variables when performing running and jumping. RSI is considered a valuable tool within the sport of track and field for sprinters and jumpers alike.
RSI is used to measure performance during a plyometric exercise. It was traditionally used to measure the performance of drop jumps from various box heights (30cm, 45cm, 60cm, 75cm) to dictate which height provided the optimal level of training stimulus for the athlete. It has since been adapted and used with a wider range of plyometric exercises. Ultimately, RSI measures muscle-tendon stress capabilities and reactive jump capacity (3). It tells you if/when an athlete can tolerate more training.

The RSI metric looks at how high you can jump and how quickly you do it. That’s it! It is a ratio between your jump height and the amount of time you remain in contact with the ground (Jump Height/Ground Contact Time). To improve your RSI and performance potential the jump height increases, or the contact time decreases.

IMPORTANCE IN ATHLETE DEVELOPMENT

Reactive strength represents the ability of an athlete to effectively make use of their fast stretch shortening cycle (SSC). Athletic movements that cause an athlete to utilize their fast SSC are characterized by ground contact times that occur within 0.25 seconds. Tendon stiffness is a critical component of a high-level fast SSC (6). Stiffer tendons shorten at a much higher velocity than more compliant ones and this causes them to react more powerfully (6). Higher levels of reactive strength are commonly seen in faster athletes compared to slower athletes. This means faster athletes possess the ability to tolerate higher amounts of eccentric loads and turn them over into concentric forces in shorter periods of time. These reactive strength qualities are best built upon during training when the athlete can maintain ground contact times below 0.25 seconds. Coaches use RSI to track progress and assess the athlete’s ability to tolerate more training. RSI offers coaches an objective testing measure they can use to evaluate the overall readiness of the athlete and track their progress over the course of training.

TESTING PROTOCOLS

RSI was originally developed using an incremental drop jump (IDJ) as the testing exercise. At the time this was the only plyometric exercise that offered identifiable ground contact times for research (1). With advancements in technology, other exercises have since been used and research and testing protocols have expanded. The three testing protocols we will focus on are:

  1. the Incremental Drop Jump Test
  2. the Countermovement Jump Test
  3. the 10-5 Jump Test

The Incremental Drop Jump Test

The IDJ test was developed to measure how an athlete copes and performs during plyometric activities by measuring the muscle-tendon stress and their reactive jump capacity (3). The test involves the use of a range of plyometric boxes (30cm, 45cm, 60cm, and 75cm) and is meant to test the ability of the athlete to utilize their fast SSC. This test can be performed using force platforms, jump mats, accelerometers, or liner transducers. The athlete performs 2-3 drop jumps from the 30cm box with minimal ground contact time and maximal intent of jump height. Depending on the equipment being used, jump height, flight time, ground contact time, and time to take-off are recorded. The athlete will follow the exact same procedure at the next highest box height. This would continue all the way up to the 75cm box height until a ground contact time greater than 0.25 seconds occurs or the athlete displays decreased jump height performance. The IDJ test helps coaches build a jumping profile of an athlete. With these numbers you can objectively steer the direction of the training to ensure the proper stimulus is being placed on the athlete.

The Countermovement Jump Test

The countermovement jump (CMJ) test is a popular test to administer because of its ease and versatility. It can be performed using force platforms, jump mats, accelerometers, or liner transducers. To perform the test, have the athlete stand on the platform/mat in an upright position. They will then eccentrically lower their center of mass and attempt a vertical jump with the intent of getting as high as possible and landing where they took-off from. It is important to note that depending on the performance variable being looked at you will need to give your athlete specific directions on how to perform the test. Particular attention should be given to the depth and time of the countermovement prior to the vertical jump. The countermovement acts as the pre-stretch or eccentric action that occurs immediately before amortizing and concentrically completing the vertical jump. Because of the nature of the CMJ ground contact time cannot be calculated. Instead, using the time to takeoff in place of ground contact time will provide you with a reliable measure to calculate RSI (1). Using the CMJ test you can actively track your athlete’s progress and gauge their ability to tolerate further training.

The 10-5 Jump Test

The 10-5 Jump Test is commonly referred to as the repeated jumps test (RJT). This test has the athlete perform ten consecutive vertical jumps (rebounds) on a jump mat or force platform. Their goal is to attain maximal elevation and minimal ground contact time with each jump (5). An average of the five jumps with greatest height or the average of the last five total jumps, that exhibited ground contact times lower than 0.25 seconds, are used to calculate the RSI (2,4,5). A key difference between the RJT compared to the IDJ or CMJ tests is the built-in jump repeatability. With the RJT the drop height of the subsequent jump is constrained by the jump height of the previous repetition. I agree with Greg Gustin, when he implies the RJT may be better suited for assessing tendon stiffness. If an athlete were able to reach their maximum height over the ten jumps, the only way to improve their RSI would be to get off the ground faster while maintaining that jump height (4). This kind of data is much more difficult to attain with single effort tests like the IDJ or CMJ.
Each of these RSI testing protocols assists both the coach and athlete while providing insight into the overall effectiveness of the training. In order to properly use RSI, there are formulas that will need to be used. Below is a cheat sheet that provides the most commonly used equations for calculating RSI, in one spot. Use it for yourself and share with your friends.

REFERENCES

  • Ebben WP, Petushek EJ. Using the reactive strength index modified to evaluate plyometric performance. J Strength Cond Res. 2010; 24(8).
  • Flanagan EP. The reactive strength index revisited. 2016. Available at: https://trainwithpush.com/blog/reactive-strength-index-revisited.
  • Flanagan EP, Ebben WP, Jensen RL. Reliability of the reactive strength index and time to stabilization during depth jumps. J Strength Cond Res. 2008; 22(5): 1677-1682.
  • Gustin G. Reactive strength as a performance measure. 2017. Available at: https://elitetrack.com/reactive-strength-performance-measure.
  • Harper DJ, Hobbs SJ, Moore J. The ten to five repeated jump test. A new test for evaluation of reactive strength. Bases Student Conference. 2011.
  • Hirayama K. Plyometric training favors optimizing muscle-tendon behavior during depth jumping. Frontiers in Physiology. 2017; 8.
  • Young W. Laboratory strength assessment of athletes. New Stud Athl. 1995; 10: 88–96.

Kyle O’Toole is a graduate assistant strength and conditioning coach at George Mason University. He is a certified strength and conditioning specialist, a corrective exercise specialist, certified personal trainer, and a USA weightlifting level 1 coach. Kyle completed the Mentorship Program at Athletic Lab in 2018 and is currently pursuing his master’s degree. He can be reached on Instagram @coach.kyleotoole

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