“Plyometrics refers to exercise that enables a muscle to reach maximum force in the shortest possible time (3). The muscle is loaded with an eccentric (lengthening) action, followed immediately by a concentric (shortening) action.”
In part one of this series on plyometrics training for increase vertical jump I will focus on how plyometric exercises work. Plyometrics concentrates on the phenomenon of when a muscle that is stretched before a concentric contraction, will contract more forcefully and more rapidly. A quick example in relation to vertical jumping is when you do a “dip” before performing a vertical jump. By lowering your center of gravity you are in essence stretching the muscles initiating a more powerful counter movement like stretching a rubber band.
Why does this happen?
I will explain this phenomenon by using the following two models:
Mechanical Model
In this model, elastic energy is created in the muscles and tendons and stored as a result of a rapid stretch. This stored energy is then released when the stretch is followed immediately by a concentric muscle action. The effect is like that of stretching a spring or rubber band which wants to return to its natural length. The spring is this case a component of the muscles and tendons called the series elastic component.
Neurophysical Model
A response know as the stretch reflex occurs when a quick stretch is detected in the muscles, and an involuntary, protective response occurs to prevent overstretching and injury. The stretch reflex increases the activity in the muscles undergoing the stretch or eccentric muscle action, allowing it to act much more forcefully. The result is a powerful braking effect and the potential for a powerful concentric muscle action.
If the concentric muscle action does not occur immediately after the pre-stretch, the potential energy produced by the stretch reflex response is lost. (i.e. if there is a delay between dipping down and then jumping up, the effect of the counter-dip is lost).
It is thought that both the mechanical model (series elastic component) and the neurophysical model (stretch reflex) increase the rate of force production during plyometrics exercises.
Stay tuned for part 2 where I will talk about the stretch shortening cycle..
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