Plyometrics is a type of exercise that uses explosive movements to develop muscular power. It may be used, for example, to improve the effectiveness of a boxer's punch, or to quicken the throwing ability of a baseball pitcher.

Power is when strength is converted into speed. The ability to convert strength to speed in a short time allows for athletic movements beyond what raw strength will allow. For example, an athlete with strong legs can perform a free-weight squat with extremely heavy weights, but may not be able to jump as high as another athlete who is not as strong, but can generate more force more quickly.

Plyometrics allows an athlete to develop more power because of the way muscles work. For a muscle to cause movement, it contracts; this is known as a concentric contraction. There is a maximum amount of power with which a certain muscle can contract. However, if the muscle is lengthened (eccentric contraction) just prior to the contraction, it will actually produce more power. This effect requires that the time between eccentric contraction and concentric contraction is very short. The theory is that this extra power is due to the muscle gaining potential energy much like a stretched rubber band. This energy dissipates rapidly, so the action must be quick. The process is called the "stretch shortening cycle," and is the underlying mechanism of plyometric training.

Most exercises involve a muscular contraction that starts off rapidly, but decelerates suddenly before the end of the repetition. For example, lifting a barbell involves jerking the weight quickly into the air, then bringing this motion to a sudden halt. Plyometric exercises are characterized by the lack of such a decelerative phase. They are open-ended movements into free space. Other animals take advantage of this effect; one is the kangaroo. If a kangaroo needed to use 100% new energy to contract its leg muscles every time it jumped, it would not be able to jump very far consistently. However, because of the muscles' ability to store energy from the previous jump like a spring, the kangaroo only needs to use a fraction of the total energy in the jump.

Physical educators have long used various plyometric apparatus – including medicine balls. One plyometric exercise involves catching and tossing a medicine ball to an assistant while the exerciser lies on his back. The triceps and chest muscles work both while they are lengthening (catch phase) and while contracting (toss phase). The clap pushup is another example of a plyometric exercise