Plyometrics – originating from the Greek words plythein or plyo (meaning to increase) and metric (meaning to measure) – means to increase the measurement. Plyometric training assists with the development of power, injury prevention and rehabilitation, improving strength and conditioning, and training specificity. Lower extremity plyometrics increase sprinting speed, velocity, and jump height. Upper extremity plyometrics increase throwing velocity. For any athlete requiring power, force production, and/or velocity, plyometrics will be an important training component.
Plyometric movements use the stretch-shortening cycle – the muscles lengthen and then quickly shorten to produce the movement. This occurs in three phases: eccentric, amortization, and concentric. The eccentric, or pre-stretch, phase enhances the concentric muscle contraction that follows. Three variables during an eccentric phase (how much the muscle is stretched, how fast it is stretched, and how long it is stretched) impact the amount of energy stored during the eccentric movement. The amortization phase, also called “time to rebound”, is the delay between the eccentric and concentric contractions – or the delay between the stretching of the muscle and the force produced by the muscles shortening again. The shorter this phase is, the more effective and powerful the movement, as energy is used more efficiently. If this phase is delayed, energy stored during the eccentric phase is wasted as heat and the stretch reflex is not activated, resulting in a less powerful contraction. The final phase – the concentric phase – is the shortening of the muscles to produce power. The greatest forces are produced during this phase, though it is reliant on the eccentric phase and a short amortization phase for optimum power development.
When performing plyometrics training, it is important to focus on quality over quantity and observe increased recovery times because of the specificity of muscle fiber recruitment required. Fast twitch IIb muscle fibers – the fibers crucial for power development – are only recruited when performing at above 80% maximal intensity. By focusing on the quality of movement and allowing the muscles ample time to recover, it ensures that the required intensity can be achieved during each set.
Common plyometric exercises are box jumps, squat jumps, medicine ball throws, and clap pushups. Let’s break down squat jumps using the principles outlined previously:
The eccentric phase occurs as you begin to squat down, preparing your body to jump into the air. The amount of energy stored will depend on how far you squat, how quickly you squat, and how long you remain in a squat. This will, in turn, impact the concentric phase later. The further and faster you squat, the greater the energy that is stored, which means there is more energy to be transferred to force production.
The amortization phase during a squat jump is how long you remain in a squat before exploding off the floor into a jump. To produce the greatest amount of force, this phase should be as short as possible. The longer you remain in the squat, the more energy you burn through isometric contraction and the less energy you have to use for the concentric contraction.
The concentric phase occurs when you extend your knees and hips. Your muscles shorten, concentrically contracting, thus producing the force and power necessary to push your entire body off the floor and into the air.
At this point you have a choice to make: land, absorb the shock, and then rest and reset; or land, absorb the shock, and then immediately explode into another jump squat. After a couple consecutive repetitions, you’ll find that your force production has decreased and your energy is depleting – you aren’t able to reach that 80% maximal intensity anymore. Rest, recharge, and reset. Remember: quality over quantity.