The basis of plyometric exercises is that they toughen muscles and condition nerve cells, which trigger a pattern of muscle contractions. Specifically, the exercises engage the myotatic reflex -- the release of power -- when muscles are stretched to their maximum. This reflex in turn stimulates neurons called stretch sensory receptors. The ultimate goal is achieving as strong a muscle contraction as possible in the shortest amount of time possible. Plyometrics uses a key concept of exercise science: Muscle contractions that last the least amount of time produce more energy than a slower contraction or release regardless of the size or bulk of the muscle.
Another key element of plyometrics is concentric contraction. For a muscle to act, it first must "coil up," which is actually stretching out to its maximum pre-action length. This loads the muscle, like cocking a gun. Then, as the muscle is held in this state, energy is stored, and energy grows as the muscle is held. That brief storage -- lasting a second or less -- allows for maximum storage and use of the energy.
There are three phases of the plyometric muscle contraction. The first is the eccentric phase, the rapid muscle-lengthening movement -- that initial stretch or loading action. Next is the amortization phase, the very brief, power-building period of muscle rest or holding period. Finally comes the concentric phase, in which muscles are released to create the explosive muscle shortening movement. It's the big payoff.
There's also a neurological component: Plyometrics train the brain to treat muscles differently, to the athlete's advantage. The brain is hardwired to limit force when a muscle is stretched out; the brain tells the muscle to hold, and the muscle can waver slightly, resulting in a loss of energy as it's slowly released in a series of small contractions. Plyometric exercises can condition the brain to tell the muscles something different -- they train neurons to learn that in sports situations, the needs of a muscle are quick muscle contractions and precise, powerful, singular releases that produce a surge of energy.
Read on to learn about the benefits and risks of plyometrics.