Why A CrossFit Athlete Should Track Reactive Strength

What is Reactive Strength Index?

Reactive Strength Index (RSI) is a ratio between the height (or flight time) an athlete jumps and the speed in which the athlete jumps. RSI is calculated by dividing the height jumped (centimeters) by the ground contact time (milliseconds). If an athlete jumped 36cm with a contact time of 180ms then they would have an RSI of 2. The athlete could improve their RSI by increasing the height of their jump, decreasing their ground contact time, or both.

The RSI was developed to measure how an athlete copes and performs during plyometric activities by measuring muscle-­tendon stress and their reactive jump capacity (2). It demonstrates an athlete’s ability to rapidly change from an eccentric motion into a concentric muscular contraction and is an expression of their dynamic explosive vertical jump capacity (1).

How can you measure RSI with Smartjump?

A Drop Jump RSI Test or a Modified RSI Test are two tests that can be completed quickly with the use of the Fusion Sport Smartjump system. Before completing the specific protocol an athlete or coach can program a desired jump height and ground contact time so that Smartjump can provide feedback while the athlete performs the drill.

• An athlete could perform a Drop Jump RSI Test by stepping off of a box and then quickly performing a jump for maximal height after landing on the Fusion Sport Smartjump mat. The system would then instantly calculate the athlete’s RSI and cue the athlete on if they achieved the minimum jump height and ground contact time.
• An athlete could perform a Modified RSI Test by performing a series of repeated jumps on the Fusion Sport Smartjump mat focusing on maintaining a maximal jump height and a low ground contact time. Once again the system would instantly calculate the athletes RSI on each jump and provide feedback to the athlete by flashing a green (good) or red (bad) light, signaling to the athlete if they achieved the minimums or not.

How can RSI be applied in a CrossFit setting?

RSI is a great way to gauge an athlete’s readiness before entering a training session – a tool that can be utilized while completing plyometric training, and a repeatable data­ backed metric that can demonstrate how an athlete is improving over time.

The RSI represents an athlete’s ability to utilize the stretch-­shortening cycle and their explosive capabilities during dynamic jumping activities (2). An athlete’s ability to quickly and effectively move through the stretch­-shortening cycle is important for a variety of sports, including CrossFit. For example, completing an olympic lift, performing a rebounding box jump or sprinting requires the athlete to rapidly move through the stretch-­shortening cycle. As a result, monitoring and improving a CrossFit athlete’s RSI is going to allow for improvement across the wide range of movements tested in the sport of CrossFit.

Young, W. (1995). Laboratory strength assessment o f athletes. New Study Athletics. 10, pp.88–96. [Link ]

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