Plyometrics (also known as "plyos") is a type of exercise training designed to produce fast, powerful movements, and improve the functions of the nervous system, generally for the purpose of improving performance in sports. Plyometric exercises may also be referred to as explosive exercises.[1] [2] Plyometric movements, in which a muscle is loaded and then contracted in rapid sequence, use the strength, elasticity and innervation of muscle and surrounding tissues to jump higher, run faster, throw farther, or hit harder, depending on the desired training goal. Plyometrics is used to increase the speed or force of muscular contractions, providing explosiveness for a variety of sport-specific activities. Plyometrics has been shown across the literature to be beneficial to a variety of athletes. Benefits range from injury prevention, power development and sprint performance amongst others.
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Plyometric training involves and uses practicing plyometric movements to toughen tissues and train nerve cells to stimulate a specific pattern of muscle contraction so the muscle generates as strong a contraction as possible in the shortest amount of time. A plyometric contraction involves first a rapid muscle lengthening movement (eccentric phase), followed by a short resting phase (amortization phase), then an explosive muscle shortening movement (concentric phase), which enables muscles to work together in doing the particular motion. Plyometric training engages the myotatic reflex, which is the automatic contraction of muscles when their stretch sensory receptors are stimulated.
Muscular power and muscular strength are two different things. Muscular strength refers to how much force can be applied (The ability to lift a heavier weight as opposed to a lighter one). Strength alone is not indicative of speed. Although muscle strength is correlated to sprint performance, research has shown that combining both resistance training and plyometric training will have better effects on training. While plyometrics assists in rapid force development (power), weight training assists in maximal force output (strength). Power refers to the combined factors of speed and strength. Performance in many sports is based on different types of power. In American Football, a lineman and a receiver may have the same power, but they have different limitations in how their power is delivered. The lineman would be speed-limited, whereas the receiver would be strength-limited. The purpose of plyometrics is to emphasize speed-based power. One activity that requires speed-favored power is high jumping: ultimately, jump height is determined by how fast one is moving once one's legs have left the ground. Good jumpers may not have exceptional leg strength, but they can produce it at exceptional speeds.
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 while loaded (eccentric contraction) just prior to the contraction, it will produce greater force through the storage of elastic energy. The quick transition from the eccentric to the concentric phase is known as the stretch shortening cycle (SSC), and is one of the underlying mechanisms of plyometric training.The force created by the muscled-tendon during the SSC is determined by the muscle's length and compliance. To increase power through plyometrics two integral controlling aspects are required. These aspects include “a more rapid initial stretch, which generates more power in the muscle group moving in the opposite direction in the second phase of the action; and a shorter time between eccentric and concentric contractions (SSC)".
Nearly all animals have different types of muscles to produce different types of reactions. Humans have three different types of muscle fibers: slow-twitch (Type I), fast-twitch A (Type IIa), and fast-twitch B (Type IIb). Type I muscle fibers are recruited for aerobic activities, so therefore contract slowly, but are very resistant to fatigue. Type IIa muscle fibers have both endurance and power characteristics and are recruited for long anaerobic activities. They lie in the middle of Type I and Type IIb muscle fibers as they are more fatigue resistant than Type IIb muscle fibers, but less fatigue resistant than Type I muscle fibers. Type IIb muscle fibers are recruited only for short intense activities like lifting heavy objects, sprinting and jumping. Exercising fast twitch muscles to produce quicker reactions is the basis of plyometrics.
Although each type of muscle fiber cannot be converted into another (i.e. fast-twitch to slow-twitch), specific types of training can increase the area occupied by the targeted muscle fiber. Fast twitch muscle fibers have a greater ability to hypertrophy, therefore specified training targeting fast twitch muscle fibers can increase the area in the muscle that fast twitch muscle fibers will occupy. For example if the occupancy of fast twitch to slow twitch muscle fibers is 50-50, with the right training regiment, fast twitch muscle fibers may increase to occupy 75% of that muscle.
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 (GTOs). It is believed that during plyometric exercise, the excitatory threshold of the GTOs is increased, making them 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 tetany 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.
Repeated use of plyometric exercises will gradually increase the efficiency of neuromuscular connections between brain and muscle. However, a fine balance must be used if one wishes to build strength and power through plyometrics. It is often recommended that plyometric repetitions be no higher than 75-100 reps. Also, training with plyometric exercises more than three or four times per week can cause muscular degeneration if proper nutrition and rest are not taken into account.
Plyometrics have been shown to have benefits for reducing lower-extremity injuries in team sports while combined with other neuromuscular training (i.e. strength training, balance training, and stretching). Plyometric exercises involve an increased risk of injury due to the large 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 that a participant be able to perform 5 repetitions of the squat exercise at 60% of his 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:
Plyometrics are not inherently dangerous, but the highly focused, intense movements used in repetition increase the potential level of stress on joints and musculo-tendonous units. Therefore safety precautions are a strong prerequisite to this particular method of exercise. Low-intensity variations of plyometrics are frequently utilized in various stages of injury rehabilitation, indicating that the application of proper technique and appropriate safety precautions can make plyometrics safe and effective for most people.