Guide To Reactive Strength Index

1 //  Introduction

The ability to develop force quickly is a necessary requirement in most sports. The majority of coaches and researches appear to agree that plyometric training is most appropriate when aiming to improve vertical jump ability and leg muscle power (Markovic 2007). This type of training targets the stretch shortening cycle (SSC), which is a natural form of muscle function, which occurs when active muscle lengthening (eccentric) is immediately followed by active muscle shortening (concentric) (Flanagan and Comyns 2008).

There are two types of SSC – slow and fast. Slow SSC (>250 ms) exercises (eg; box jump, standing long jump, single leg vertical jump) are suitable for teaching athletes about plyometrics and proper landing mechanics. Fast SSC (<250 ms) exercises (eg; hopping, sprinting, skipping, hurdling) are represented by reactive strength, and require the athlete to have adequate plyometric training to avoid injury (Flanagan and Comyns 2008). The reactive strength index (RSI) has been established as a measure of this fast SSC explosive strength.

2 //  What Is Reactive Strength?

The reactive strength index (RSI) has been established as a measure of explosive strength and demonstrates how athletes perform plyometric activities. It is a simple ratio involving two metrics: How high can you jump? How fast can you jump?

As mentioned, the athlete’s capability to rapidly and successfully change from an eccentric to a concentric contraction represents their RSI. Reactive strength is a representation of the fast SSC function, which shows athletes’ ability to change quickly from an eccentric to a concentric contraction and their ability to develop maximal forces in minimal time (Flanagan et al 2008). An example of this may be acceleration and change of direction in a Rugby or Soccer match. RSI was also found to be important for agility in Australian rules footballers, especially for attacking agility, which essentially outlines that RSI can potentially be central to physical performance (Young et al. 2017).

3 //  Measurement Of RSI

Devices/Technology

• Force plates are considered the gold standard for measuring RSI, but do not directly measure RSI and therefore must be calculated from the jump height and ground contact time measures.
• Inertial Measurement Units (IMU), like the Output Sports system, have been shown to be a valid and reliable alternative to force plates and provide an instantaneous RSI value (see image below).

Methods

The reactive strength index has many valid and reliable tests including drop jumps, repeated jumps, rebound jump, and the 10-5 test.  The two most popular tests are outlined below.

1.     Drop Jumps

• Typically RSI has been measured using plyometric drop jumps, as it was deemed as the exercise with a recognizable ground contact time (Flanagan et al. 2008).
• It involves the athlete performing 2-3 drop jumps across a range of “drop” heights (for example: 15, 30, 45, 60 cm) with RSI being recorded for each jump at each height. The optimal height then coincides with the best RSI score.

2.     The Rebound Jump Test

• This test involves the athlete performing a single countermovement jump, immediately followed by an additional jump.
• It is a quick and simple alternative to the incremental DJ-RSI test.

3.     10-5

• This test involves the athlete performing a single countermovement jump, immediately followed by a series of 10 repeated, bilateral hops.
• The best 5 RSI scores are then averaged to give the athlete’s true RSI.

4 //  RSI Measurement Guidelines

As RSI is a ratio between both jump height and contact time, small changes in either measure have the potential to result in significant changes in RSI. Thus meaning it is very a sensitive measure.

Despite the increase use of reactive strength testing, there is limited published normative data. Eamonn Flanagan has previously outlined a brief normative data guide, which combines data from his own experience of testing RSI across a variety of sports, with limited data available in the scientific literature. The values are based off drop jumps, and under the assumption that the tests are performed with hands on hips and that RSI is calculated as jump height (m) divided by contact time (s). These have been adapted and are outlined below.

Fair: Athlete is unprepared for moderate intensity plyometrics. Focus should be on their strength development and plyometric technique.

Good: Athlete is prepared for moderate intensity plyometrics. Focus should be on improving their reactive strength, which will in turn lead to performance enhancement.

Very Good: Athlete is prepared for more intensive plyometrics.

Excellent: Athlete may begin to plateau in terms of their RSI score and the question should be asked; will further improvements in RSI improve performance?

NOTE: The above data is only a rough guide. Establishing a baseline and normative data for your own athletes and sport is recommended, as the demands of all sports/athletes differ (Flanagan 2016; Beattie and Flanagan 2015).

5 //  Interpretation Of The Data

RSI was developed to measure how an athlete copes and performs during plyometric activities (Flanagan et al. 2008). It represents an athlete’s ability to utilize the stretch­ shortening cycle and their explosive capabilities during dynamic jumping activities, by measuring muscle-­tendon stress and their reactive jump capacity. An athlete’s ability to quickly and effectively move through the stretch­-shortening cycle is important for a variety of sports. Therefore monitoring and improving an athlete’s RSI will potentially allow for improvement across a wide range of movements, and reduce injury risk (Flanagan 2016).

Primarily RSI, in conjunction with other markers, is used as an athlete-monitoring tool because of its sensitivity and ability to assess an athlete’s:

• Neuromuscular fatigue
• Readiness to train
• Adaptation to the training and competition process
• Efficacy of tapering or “peaking” strategies
• Proximity to overtraining
• Optimal drop height for drop-jump training

(Flanagan and Comyns 2008)

6 //  Conclusion

• Essentially, an athlete’s RSI score is represented by their capability to rapidly and successfully change from an eccentric to a concentric contraction, which in turn relates to their fast SSC function.
• There is limited published normative data for RSI, and it is recommended to establishing a baseline and normative data for your own athletes and sport, as the demands of all sports/athletes differ.
• RSI is primarily used as an athlete-monitoring tool.
• RSI should be measured by a test that is valid and reliable.

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//  References

Beattie, K. and Flanagan, E.P., 2015. Establishing the reliability & meaningful change of the drop-jump reactive strength index. J Aust Strength Cond, 23(5), pp.12-18.

Flanagan, E., 2016. The Reactive Strength Index Revisited - Part 2. [online] PUSH // Train with Purpose. Available at: https://www.trainwithpush.com/blog/reactive-strength-index-revisited-2 [Accessed 3 April 2020].

Flanagan, E.P. and Comyns, T.M., 2008. The use of contact time and the reactive strength index to optimize fast stretch-shortening cycle training. Strength & Conditioning Journal, 30(5), pp.32-38.

Flanagan, E.P., Ebben, W.P. and Jensen, R.L., 2008. Reliability of the reactive strength index and time to stabilization during depth jumps. The Journal of Strength & Conditioning Research, 22(5), pp.1677-1682.

Markovic, G., 2007. Does plyometric training improve vertical jump height? A meta-analytical review. British journal of sports medicine, 41(6), pp.349-355.

Young, W.B. and Murray, M.P., 2017. Reliability of a field test of defending and attacking agility in Australian football and relationships to reactive strength. The Journal of Strength & Conditioning Research, 31(2), pp.509-516.