[7 minute read]

Introduction /

Explosive strength is a vital aspect of athletic performance, as it dictates both the amount of force developed and the rate that force is developed at. Explosive strength is important for various athletic tasks such as acceleration, change of direction, and sprinting. Without explosive strength, athletes would be less able to both produce and absorb forces in competition.

A physical quality associated with explosive strength is ’reactive strength’ and is measured by the Reactive Strength Index. Reactive strength is a physical quality that encompasses the stretch-shortening cycle (SSC) and refers to the muscle’s ability to absorb, amortize (stabilize or control), and finally, produce force.

Explosive strength = Producing force at a high velocity

Measuring Reactive Strength /

A method of measuring reactive strength is through calculating the Reactive Strength Index (RSI) of an athlete. RSI is the ratio between jump height & ground contact time and is calculated by dividing the former by the latter.

Reactive Strength Index is the measure of an athlete’s reactive strength. The RSI Score directly relates to the rate of force production, i.e explosive strength.

When we speak of RSI, we must also understand the Stretch-Shortening Cycle (SSC). The SSC is the muscle’s ability to absorb (eccentric), amortize (stabilize or control), and finally, produce (concentric) force. The competency of the muscle to engage in this cycle is a prerequisite of engaging in explosive movements. By measuring RSI, we are also measuring the function of the SSC mechanism.

The Stretch-Shortening Cycle is the ability of the muscle to absorb force, amortize/stabilize it, and finally, concentrically produce force.

As RSI is a measure of reactive strength, it must therefore be tested while performing an explosive/reactive movement. To calculate RSI, we require the athlete to perform a maximal plyometric movement and measure jump height and ground contact time. RSI is then calculated by dividing ground contact time by jump height.

Reactive Strength Index Testing Procedure /

The use of technology in athletic testing is necessary to gather valid, reliable and accurate data. Fortunately, there are many methods for testing RSI, and many options available for gathering data. For this blog, we will use the example of a drop jump as the performance test, using an Inertial Measurement Unit (IMU) to gather data:

• Athlete stands on a box with hands on hips and the IMU attached to the foot.
• Athlete hangs one foot off the box before dropping down with both feet.
• The focus for the athlete is to jump as high as they can while spending as little time as possible on the ground.
• The athlete will complete 2-3 reps of a drop jump across a range of box heights.
• RSI is calculated for each height, and the height at which the best RSI score was achieved will also inform the coach which height is optimal for that athlete to develop their RSI score.

Reactive Strength Index is calculated using the time spent on the ground after dropping from the box (secs) and the jump height achieved (cm).

Interpretation of the Data /

Once you have collected the Jump Height (cm) and Contact Time (secs), RSI is a simple calculation as shown in image 3 above.

The research surrounding the interpretation of RSI data is limited, however, some work does exist from Flanagan et al (2016). From this work, we have some guidelines for RSI scores in a drop jump test, as outlined in image 4 below.

We want athletes to be able to both control & produce force in sporting contexts. Consequently, an athlete who has an RSI of below 2.0 for example, may indicate a window of opportunity for a plyometric programme to be introduced. However, it’s advised that all team sport athletes include forms of plyometrics within training.

We also know that with a better RSI score, performance in various athletic tasks may improve. An athlete who can cycle through the SSC quickly and efficiently and still produce a high level of force will be better able to navigate the complex movements involved in their sport safely.

Training to Improve RSI Scores /

Similar to in our previous blog Training for Strength and Power Development, we also recommend considering the Force-Velocity Curve when designing training to enhance reactive strength. Whilst a range of loads is always advised to facilitate athletic development, typically plyometric type activities are required to develop the elastic qualities needed to improve RSI scores.

Developing Reactive Strength

We must also ensure we are training the body’s fast SSC (<250ms) ability. So, we must ensure the tasks the athlete undertakes are:

• A range of both extensive and intensive plyometric training; and,
• Sprinting

An training progression example can be seen below:

Conclusion /

RSI is an important element of the testing procedure, and an athlete’s RSI score is something that coaches in all sports should test, monitor, and develop as it is directly related to their performance in reactive/explosive tasks. An athlete without the ability to react efficiently is an athlete with a higher risk of injury.

As mentioned above, the production of force is not the only element at play in explosive movements. The absorption, control, and stabilization of this force are as important, if not more important, in the development of explosive strength. If the athlete can produce maximal explosive force, but cannot control & absorb it, the risk of injury increases exponentially.

While more research is required to provide different sports & positions normative data, the current research gives a good standing point. While there may not be peer-reviewed data for your particular sport, you can still work with the data from your own team and ensure that your athletes are following your own averages and/or position-specific norms.

References

1. Science for Sport (2016): Walker, Owen: Stretch-Shortening Cycle Article. https://www.scienceforsport.com/stretch-shortening-cycle/
2. Science for Sport (2016): Walker, Owen: Reactive Strength Index Article.
   https://www.scienceforsport.com/reactive-strength-index/
3. 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.
4. Flanagan, E., 2016. The reactive strength index revisited. Train With Push.
5. 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.
6. Kinsella-Kent, E., Guide to Reactive Strength Index.