Plyometrics

The stretch-shortening cycle (SSC) occurs in fast movements such as jumping and sprinting. It involves a pre-stretch of a muscle and then a counter-movement where the muscle contracts.

As we're going down for example for a vertical jump, as we are lowering down the quads and glutes muscles are lengthening meaning we are going through an eccentric muscle contraction. The quadriceps and glutes are controlling the motion of lengthening then on the way up we're going through a concentric muscle action and we're shortening the muscle and jumping up. Between the eccentric and then the concentric contraction there's another phase called the amortization phase. 

This phase is important for strength, power, and speed. The amortization phase is really where the stretch-shortening cycle starts to give us our recoil. During the amortization phase there's stored energy as we stretch the tendon we begin to store up energy and depending on the length of the stretch or depth of the amortization phase determines the amount of recoil or propulsion.  

The SSC works in two ways the first is the neurophysiological component. As skeletal muscles are stretched muscle spindle signals a reflex contraction of the muscle. So in the case of a vertical jump, we are stretching the muscle spindle fibers within the quadriceps.  As we enter into the amortization phase the signal is sent causing the quadriceps muscles to shorten even more rapidly on the concentric contraction.  
The second aspect of the cycle is mechanical. When we're going through quick movements like jumping and stretching and we're stretching our tendon As the tendon stretches it is going to recoil and give us energy during the concentric portion of the motion. 

How does this apply to exercise?  In a vertical jump we can do it one of two ways. We can do a static vertical jump where we just squat down and hold the squad for three seconds and then we jump straight up and we might be able to elevate eight inches for a static vertical jump. Those eight inches were a result of just the active muscle contribution to the jump and did not involve the stretch-shortening cycle because they held that three-second isometric hold before jumping.  The common way to test vertical jump is with a counter-movement meaning throwing the arms down and then throwing the arms up and jumping as high as you can because that is more realistic to how genuine athletic movements work. We are combining the mechanical and neurophysiological components together which will aid in increasing the vertical jump height that will allow us to instead jump eighteen inches for example. Depending on the type of movement that you're doing we can double the force from the muscle tendon and the muscle spindle activation, contributing to increased speed and power movements.