Plyometric exercise is defined by the National Strength and Conditioning Association (NSCA) as, “those activities that enable a muscle to reach maximal force in the shortest possible time” (Essentials of Strength and Conditioning). These movements are geared toward increasing maximum power output as opposed strength or endurance. Power reflects both force and speed. Thus, plyometric exercises must be performed as quickly as possible. If executed correctly, plyometrics can result in noticeable gains in maximal power output.
How do they work?
There are two proposed models which explain the physiology behind plyometrics. The first is the Mechanical Model , where elastic energy in the musculotendinous unit (a muscle and its corresponding tendons) is increased with a rapid stretch and then stored. This means that when a muscle and its tendons are rapidly stretched, they act as a spring and lengthen. When the muscle and its tendons lengthen, a small amount of elastic energy is created and stored. When this movement is immediately followed by a concentric muscle action, the stored elastic energy is released and combined with the force of the concentric contraction to create a more powerful contraction. When the stored energy is released, the muscle and its tendons begin to return to their normal lengths. This natural return to a shorter length, when combined with the concentric contraction creates a much more rapid (thus more powerful) overall movement.
The second model is referred to as the Neurophysiological Model. In this explanation, the body takes advantage of natural stretch reflexes to incorporate them into a stronger contraction. A stretch reflex is the body’s involuntary response to an external stimulus that stretches the muscle. When a muscle is rapidly stretched, a proprioceptive organ within the muscle is triggered. This organ is called the muscle spindle, and it is sensitive to the rate and magnitude of a stretch. When a rapid stretch takes place in a muscle, the muscle spindle reflexively contracts in order to safeguard the muscle from potential tearing. When this reflex is combined with a concentric contraction, the result is a more powerful overall contraction.