Science of Plyometrics
By Drew Bodette, M.A, C.S.C.S
Plyometric exercises refer those activities that enable a muscle to reach maximal force in shortest amount of time. Mechanical and neurophysiological components of plyometrics There are two major physiological components which plyometric is based upon. One is mechanical in nature and explains how energy is store within and released by the musculotendinous complex or unit. The other is neurophysiological in nature and explains reflexive muscle action releases stored energy. Mechanical component The Series Elastic Component (SEC) is the workhorse of plyometric exercise. The SEC is made up mostly of tendons although some muscle tissue is included. How the SEC works is when the musculotendinous unit or complex is stretched or lengthen (as in an eccentric muscle action) elastic energy is stored. If the muscle begins a concentric action immediately after the eccentric action the stored energy is released
The SEC contributes to total force production by naturally returning the stretched or lengthen musculotendinous unit to its unstretched state. If the concentric action doesn't immediately follow the eccentric action or is the eccentric phrase is too long the stored energy is lost as heat.
The stretch reflex is the means by which an "immediate" concentric action can follow an eccentric action. A stretch reflex is the body's natural and involuntary response to an external stimulus that stretches the muscle. It is triggered or stimulated by muscle spindle activity. Muscle spindles are proprioceptors within the musclotendinous complex that respond to the rate, degree and magnitude of a stretch. When the muscle spindles are stimulated by a rapid stretch (such as caused by a plyometric exercise) a reflexive muscle action is created. This reflexive response potentiates or increases the activity in the agonist muscle thereby increasing the force the muscle produces.
The exact degree to which both of these components (mechanically and neurophysiological) contribute remains speculation. It seems likely both contribute to increase the production of force seen in plyometric exercise.
Stretch-Shortening Cycle (SSC)
The Stretch-Shortening Cycle (SSC) uses the energy storage abilities of the SEC and stimulation of the stretch reflex to attain maximal force production in a muscle in the shortest amount of time. The SSC involves three distinct phases. These phases are: eccentric phrase, amortization (transition) phase, concentric phase.
- preloading the agonist muscle group(s)
- SEC stores elastic energy
- Muscle spindles are stimulated
- time between eccentric and concentric phrase
- most crucial phase of the SSC, its duration must kept short
- as soon as movement begins the amortization phase ends
- body's response to the eccentric and amortization phases
- the stored energy in the SEC is either used to increase the force of the subsequent movement or is dissipated as heat.
- the stored elastic energy increases force production beyond that of an isolated concentric muscle action
Plyometric training like resistance training can be programmed into a periodization cycle. The progression of plyometric within a periodized model depends on two major factors: intensity and volume.
Factors which determine intensity level:
- points of contact
- height of drill
- participants weight
Factors which determines volume:
- reps and sets
- lower body: foot contacts (each time a foot or feet together contact the surface)
- upper body: throws or catches per workout