Information about Plyometrics

Plyometrics is a type of exercise that utilizes a rapid eccentric movement, followed by a short amortization phase, and then followed by an explosive concentric movement, which enables the synergistic muscles to engage in the myotatic-stretch reflex during the stretch-shortening cycle. Plyometric exercises use explosive movements to develop muscular power, the ability to generate a large amount of force quickly. Plyometric training acts on both the musculotendinous and neurological levels to increase an athlete's power output without necessarily increasing their maximum strength output. Plyometrics are used to increase the speed or force of muscular contractions, often with goals of increasing the height of a jump or speed of a punch or throw.

Physics of plyometrics

Muscular power is determined by how long it takes for strength to be converted into speed. The ability to convert strength to speed in a very short time allows for athletic movements beyond what raw strength will allow. Thus an athlete who has strong legs and can perform the freeweight squat with extremely heavy weights over a long duration may get less distance on a standing long jump or height on a vertical leap than a weaker athlete who is able to generate a smaller amount of force in a shorter amount of time. Though the plyometrically trained athlete has a lower maximal force output and may not squat as much, training allows them to compress the time required to reach their maximum force output, allowing them to develop more power with each contraction.

Musculotendinous component

For a muscle to cause movement, it must shorten; this is known as a concentric contraction. There is a maximum amount of force with which a certain muscle can concentrically contract. However, if the muscle is lengthened (eccentric contraction) just prior to the contraction, it will produce greater force through the storage of elastic energy. This effect requires that the transition time between eccentric contraction and concentric contraction (amortisation phase) be very short. This energy dissipates rapidly, so the following concentric contraction must follow the eccentric stretch before this occurs. The process is frequently referred to as the "stretch shortening cycle", and is one of the underlying mechanisms of plyometric training.

Neurological component

In addition to the elastic-recoil of the musculotendonous system there is a neurological component. The stretch shortening cycle affects the sensory response of the muscle spindles and golgi tendon organs (GTO). It is believed that during plyometric exercise, the excitatory threshold of the GTO's is increased, meaning they become less likely to send signals to limit force production when the muscle has increased tension. This facilitates greater contraction force than normal strength or power exercise, and thus greater training ability.

The muscle spindles are involved in the stretch reflex and are triggered by rapid lengthening of the muscle as well as absolute length. At the end of the rapid eccentric contraction, the muscle has reached a great length at a high velocity. This may cause the muscle spindle to enact a powerful stretch reflex, further enhancing the power of the following concentric contraction. The muscle spindle's sensitivity to velocity is another reason why the amortisation phase must be brief for a plyometric effect.

A longer term neurological component involves training the muscles to contract more quickly and powerfully by altering the timing and firing rates of the motor units. During a normal contraction, motor units peak in a de-synchronized fashion until tetanus is reached. Plyometric training conditions the neurons to contract with a single powerful surge rather than several disorganized contractions. The result is a stronger, faster contraction allowing a heavy load (such as the body) to be moved quickly and forcefully.

Therefore, a plyometric exercise involves:

1) An eccentric contraction

2) A brief amortisation phase (no change in muscle length)

3) A short concentric contraction delivering maximum force in a short period of time

Safety Considerations

Plyometric exercises carry increased risk of injury due to the powerful forces generated during training and performance, and should only be performed by well-conditioned individuals who are under supervision. Good levels of physical strength, flexibility and proprioception should be achieved before commencement of plyometric training.

The specified minimum strength requirement varies depending on where the information is sourced and the intensity of the plyometrics to be performed. Chu (1998) recommends a participant should able to perform 5 repetitions of the squat exercise at 60% of their bodyweight before doing plyometrics. Core body (trunk) strength is also important.

Flexibility is required both for injury prevention and to enhance the effect of the stretch shortening cycle.

Proprioception is an important component of balance, coordination and agility, which are also required for safe performance of plyometric exercises.

Further safety considerations include:

• Age - low-intensity and low-volume only for athletes under the age of 13 or for athletes who squat less than 1.5 times their bodyweight.
• Surface - some degree of softness is needed. Gymnastics mats are ideal, grass is suitable. Hard surfaces such as concrete should never be used.
• Footwear - must have adequate cushioning and be well fitting.
• Bodyweight - athletes who are over 240 pounds (109 kg) should be very careful and low-intensity plyometric exercises should be selected.
• Technique - most importantly, a participant must be instructed on proper technique before commencing any plyometric exercise. They should be well rested and free of injury in any of the limbs to be exercised.

Plyometrics is not dangerous, but the potential for high intensity and stress on joints and musculo-tendonous units makes safety a strong prerequisite to this particular method of exercise. Low-intensity variations of plyometrics are frequently performed in various stages of injury rehabilitation, indicating that correct performance is valuable and safe for increasing muscular power in all populations.

Additional Information

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 its previous jump before 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, and Indian clubs. 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 press up is another example of a plyometric exercise.

References

• Brooks, G.A, Fahey, T.D. & White, T.P. (1996). Exercise Physiology: Human Bioenergetics and Its Applications. (2nd ed.). Mountain View, California: Mayfield Publishing Co.
• Chu, D. (1998). Jumping into plyometrics (2nd ed.). Champaign, Illinois: Human Kinetics.