Reactive strength index (RSI) measures reactive jump capacity by reflecting a person's 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 an important physical competency measure for acceleration, agility, and change of direction speed. A drop in RSI is a good indicator of fatigue, and an increase in RSI is a good indicator of performance readiness.
For cyclic jumps, RSI is reflective of the stretch-shortening cycle of the gastroc-soleus complex.
RSI is calculated as the ratio of flight time to ground contact time, so a higher RSI means either a shorter ground contact time or a longer flight time (higher jump), or a combination of the two.