Being “elastic” or “reactive” refers to being able to have a good ability to quickly develop force and transfer one movement’s energy into another. The reactive strength index (RSI) is one of the most commonly used field tests for assessing these qualities. The RSI is the jump height of the movement divided by ground contact time. In other words, the higher you jump and faster you get off the ground the better your RSI will be.
Common Traits of Elastic/Reactive Movements
All elastic/reactive movements have two traits in common. They have low ground contact times and they have minimal changes in joint angles (small range of motion).
**The strength of the elastic components have to do with the force production following the usage of the elastic components. Someone can still be elastic and not produce much force (long distance runners).** – this is why high force production is not a “common” trait
The two traits go hand in hand. In order to get off the ground quickly, one needs to make sure that the joints of the body do not travel through a large range of motion.
Large range of motion = Greater distance to travel = large ground contact times
How To Use This?
Knowing that the small ranges of motion are required to be elastic, we need to make sure athletes understand this is the goal of training. People often use verbal cues, such as “the ground is hot”, but this does not always ensure that the athlete will perform the movement properly. A good way to teach the athlete what the movement should feel like, is to limit the degrees of freedom.
A commonly seen mistake when performing a reactive movement is that the athlete may collapse at the waste, lower their chest, and go into too much hip flexion. A way we can get around this is by using a stick to “lock” them in an upright position. By having a stick on their back, the will be force to keep an upright torso. We limit the amount of forward flexion they can obtain, “freezing some of the degrees of freedom”.
The video below shows two alternative methods, one using a med ball and one using a stick. This can also be done by holding stick overhead, or any other similar method the coach sees fit.
When training for a specific type of stimulus, it is imperative that stimulus itself is fully understood. By knowing that a reactive movement is comprised of minimal ground contact times AND small changes in joint angles, we can devise methods that best allow this stimulus to be trained. Its the movement that matters.