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Explaining Plantiga's RSI metric
The reactive strength index (RSI), calculated from Cyclic Jumps or Consecutive Countermovement Jumps, measures people's reactive jump capacity by reflecting their ability to turn deceleration into acceleration in a short period of time. This is often referred to as the stretch-shortening cycle. Reactive strength is related to acceleration in running, change of direction, and agility.
Tip: People being assessed using RSI should have a sound level of plyometric proficiency. In addition to its ability to characterize strength abilities, RSI can also be used in conjunction with a measure of workload, such as Plantiga’s g-force metric to optimize the prescription of training.
Why is the Reactive Strength Index important?
RSI measures someone’s ability to quickly and effectively change from an eccentric to a concentric contraction. It represents their ability to effectively utilize the stretch-shortening cycle and explosive capacity during dynamic jumping activities. A person’s ability to quickly and effectively move through the stretch-shortening cycle is important for a variety of athletic activities.
For example, taking off in the long jump or even changing direction in football both require the athlete to rapidly move through the stretch-shortening cycle. RSI has been shown to have a strong relationship with both change of direction speed and acceleration speed. A dip in RSI score is a good indicator of fatigue, and an increase in RSI score is a good indicator of performance readiness. Reactive strength is an important physical competency measure for acceleration, agility, and change of direction speed.
How to interpret Plantiga’s RSI Measure:
Note that RSI ranges will be different for Cyclic Jumps and Consecutive Countermovement Jumps. A higher RSI means either a shorter ground contact time or a longer flight time (higher jump), or a combination of the two. Below are means and standard deviations of consecutive countermovement RSI values for young men and women from NCAA Div I basketball teams and from the community.
Men:
- 0.86 + 0.42
Women:
- 0.66 + 0.39