Plyometric training was first introduced in the United States by a Track and Field coach, Fred Wilt in 1975. The core idea behind plyometrics is the idea of reactive power; essentially, an aggressive lengthening of muscle fibers (eccentric phase), followed by a powerful shortening of muscle fibers (concentric phase). Reactive power is another way of saying stretch shortening cycle. The stretch shortening cycle utilizes sensory receptors in the belly of muscles called muscle spindles. Muscles spindles are sensitive to change in length and the speed of the change in length. The faster the muscle spindles are stretched, the faster and more powerfully the muscles contract in response to the stretch. The stretch shortening cycle has three primary phases: (Baechle, 2008)