Plyometric Mechanics and Physiology
Below, as well as in future blogs, I will detail the important steps and safety considerations when designing and implementing plyometric training into any program.
Mechanical Model of Plyometric Exercise
If you were to take a rubber band and stretch it out, the band would develop "potential energy". If you suddenly release a stretched rubber band, the potential energy would then release very rapidly. Similar type principles can be applied to the human body. Muscles, tendons and ligaments all contain elastic properties that can be utilized in powerful, explosive, athletic actions.
The main powerhouse or driving force behind plyometric movement is known as the "series elastic component" (SEC). The SEC is partly composed of muscular elasticity, but is mainly driven by the elastic components in the tendons. Although the SEC is very similar to stretching a rubber band, there are some differences as well.
During the "loading" or eccentric phase of a plyometric action (muscle lengthening), there is potential elastic energy stored in the tendons and muscles. If there is a quick transition to the concentric or "explosive" phase, then the elastic, potential energy stored in the tendons can be utilized in the explosive action.
If, however, the eccentric phase takes too long, or the transition to the concentric phase is not fast enough, much of the stored elastic potential energy ends up dissipating as heat. This is where the plyometric action is different than a rubber band. As long as the rubber band is not stretched too far, the stored energy will remain there until released. This is not the case with stored elastic energy in the muscles/tendons however, as the muscles will simply just "release" and stretch their fibers instead.
Neurophysiological Model of Plyometric Exercise
Inside each muscle, there are proprioceptive organs called "muscle spindles". There are similar-type organs in tendons known as "Golgi tendon organs". The job of these proprioceptors are to essentially protect the muscles and ligaments of the body. They detect sudden changes in length of the muscles and tendons and will reflexively shorten in order to protect the tissues.
For example, when you go to visit the doctor for a physical, often times, they will have you sit on the edge of the examination table and have you hang your leg over the edge, relaxed. They will then take a small hammer and knock your patellar tendon just below your kneecap causing the "knee-jerk response". This is caused by the muscle spindles detecting a quick, but relatively small, lengthening of the patellar tendon, causing an immediate concentric contraction out of the thigh muscles. This is mainly an involuntary action that your body does automatically based on an external stimulus.
It is this principle that is mainly utilized when doing plyometric exercises. You are essentially training your muscles to react as fast and explosively as possible by using its own natural reflex and elastic components.
The stretch-shortening cycle (SSC) is essentially the main system employed during the series elastic component (SEC). The SSC is broken down into 3 main phases that I will discuss below.
Phase 1 is the eccentric or stretching phase. This is also known as the preloading phase where the elastic components of the muscles and tendons are stimulated. As the muscles and tendons go through their quick shortening phase, the proprioceptors (muscle spindles) are stimulated and the elastic energy is stored.
Phase 2 is known as the amortization or transition phase between phase 1 and 3. This is the time it takes for the signal to be sent from the proprioceptors to the central nervous system and back again to the necessary muscles to create a neuromuscular response. Ideally, this phase should be as short as possible (more on this later).
Phase 3 is the concentric or muscle shortening phase. This is when the action happens and the elastic energy is released and the muscle contracts creating a powerful, explosive action.
By adding plyometric exercises to your athletic training program, you will not only develop powerful, explosive muscles, but you can also expect an improvement in the response time. What I am referring to is making the phase 2 or the amortization period as short as possible with as quick of a transition from phase 1 to 3. The faster the transition between phases 1 and 3, the more the elastic components of the muscles and tendons are utilized and not lost as heat.
A great example of this would be the studies that have shown that adding plyometric training to distance runner training programs can improve times quite dramatically. The plyometric training will not only help the muscles improve in strength and exlosiveness, but will shorten that amortization stage, losing less potential energy to heat, and improving running efficiency.