Understanding the Reactive Strength Index (RSI)

Learn how to calculate the Reactive Strength Index with three different jump testing methods.

Reactive strength is an essential physical attribute for various movement demands including sprinting, jumping, and changing direction. It's crucial not only for sports performance but also from an injury risk standpoint.

We can assess an individual's reactive strength capacity via the Reactive Strength Index (RSI). In this blog post, we will explore the concept of RSI, understand how it is calculated, and discuss some of the methods used to assess it.

Reactive Strength

Reactive strength refers to an individual's ability to absorb and reproduce force quickly. It specifically involves the neuromuscular system's capability to quickly transition from a lengthening (eccentric) muscle action to a shortening (concentric) muscle action. This attribute is particularly critical for explosive movements, helping athletes generate force rapidly in activities such as jumping, sprinting, and changing direction.

Calculating RSI

The Reactive Strength Index (RSI) is a valuable tool for assessing reactive strength. RSI is commonly calculated by dividing an individual's jump height by their contact time with the ground.

Sometimes, the flight time is used instead of jump height, although depending on the method used to calculate jump height, these may also be the same. Therefore, the outcome depends on which calculation is used, so make sure you know which is used when comparing data in the literature.

Put simply, the greater an individual's reactive strength, the higher they can jump or the more air time they achieve for the same contact duration. The RSI can be tracked over time to assess an athlete's progress.

But remember, just as we discussed in relation to both the DSI and EUR, it's critically important when using ratios to understand the individual components that comprise them—in this case, the jump height and contact time. As I demonstrate in the video below with three different athlete examples (from ~3:20 onwards), we get a clearer picture of athlete qualities when we consider their RSI within the context of the underlying component parts.

Methods for Assessing RSI

Several methods exist to assess RSI, each with its benefits as I explore further in the video above:

1. Drop Jump Test: This involves stepping off a box at a fixed height and jumping as high as possible after minimal ground contact. The goal is to have minimal knee bend upon landing and to utilise reactive strength effectively.

2. Countermovement Rebound Jump: This test starts with a countermovement jump followed by a rebound jump, assessing an athlete's ability to utilise both slow and fast stretch shortening cycles.

3. Ten Five Hop Test: This test involves an initial jump followed by ten repetitive hops, evaluating the athlete's ability to maintain reactive strength over multiple jumps. From the 10 hops, the best 5 are used for analysis.

To see more on any or all of these methods, take a look at the demonstrations in the video above.

Normative Data for RSI

Normative data help gauge an individual's RSI within the context of trained populations, although it's crucial to remember that RSI values can widely vary based on factors like training age and test protocols.

The values below have been shared by Eamonn Flanagan, who compiled them through a mixture of published research as well as his own anecdotal experience. Female athletes are shown in Figure 1 and male athletes in Figure 2.

Figure 1. RSI normative data for female athletes, compiled by Eamonn Flanagan

Figure 2. RSI normative data for male athletes, compiled by Eamonn Flanagan

Final Thoughts

In conclusion, RSI provides a simple ratio to assess and track an individual's reactive strength capacity. As always, we should view it in the context of the component parts and be cautious of the limitations of a ratio. The approaches to assess RSI range from the drop jump to the ten five hop test and now the countermovement rebound jump, each offering unique insights into an athlete's performance capabilities.

For further exploration of force plate testing, check out VALD's free practitioner guides:

• Practitioner's Guide to Force Plates

• Practitioner's Intermediate Guide to Force Plates

These guides offer valuable insights into force plate technology and assessing athletic performance.

FAQs about the Reactive Strength Index (RSI)

What is Reactive Strength Index (RSI)?

RSI is a metric that measures an individual's reactive strength by calculating the ratio of jump height to contact time.

How is RSI calculated?

RSI is calculated by dividing the jump height by the ground contact time during a jump test. Sometimes flight time may replace jump height in the calculation.

What are common methods to assess RSI?

Common methods include the drop jump test, countermovement rebound jump, and ten five hop test, each providing unique insights into explosive strength capabilities. Which you use in your own setting most likely depends on the resources you have available, the practical and logistical constraints, as well as the strength and familiarity of your population.

Why is reactive strength important in sports?

Reactive strength is vital as it relates directly to the ability to perform rapid, explosive movements which are common in many sports, enhancing both performance and injury prevention.

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