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Plyometrics 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 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, often with the goal of increasing the height of a jump.
[edit] Procedure
Plyometric training involves 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, followed by a short resting phase, then an explosive muscle shortening movement, which enables muscles to work together in doing the particular motion. Plyometric training engages the myostatic-reflex, which is the automatic contraction of muscles when their stretch nerve receptors are stimulated.
Plyometric exercises use explosive movements to develop muscular power. Plyometric training acts on the nerves, muscles, and tendons to increase an athlete’s power output without necessarily increasing their maximum strength.
[edit] Physics of Muscular power
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 but in a shorter amount of time. The plyometrically trained athlete may have a lower maximal force output, and thus may not squat as much, but his training allows him to shorten the amount of time required to reach his maximum force output, leading to more power from each contraction.
[edit] Muscle-tendon 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 while loaded (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 concentric contraction must rapidly follow the eccentric stretch. The process is frequently referred to as the “stretch shortening cycle“, and is one of the underlying mechanisms of plyometric training. Usually after plyometric exercise of the legs the tendons stretch and the thighs and quadriceps feel tender and rips can possibly occur when overworked.
[edit] 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, 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.[1]
[edit] Safety considerations
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 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.
- 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 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 populations.
[edit] 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.
Plyometrics refers to exercise that enables a muscle to reach maximum force in the shortest possible time (3). The muscle is loaded with an eccentric (lengthening) action, followed immediately by a concentric (shortening) action.
This article outlines the physiology behind how and why plyometrics works. It also examines the research that demonstrates why, as a form of power training, plyometric training is very effective.
Practical guidelines for designing a plyometric training program along with animated drills can be found in the main plyometric training section
How Plyometric Exercises Work
A muscle that is stretched before a concentric contraction, will contract more forcefully and more rapidly (4,5). A classic example is a “dip” just prior to a vertical jump. By lowering the center of gravity quickly, the muscles involved in the jump are momentarily stretched producing a more powerful movement. But why does this occur? Two models have been proposed to explain this phenomenon. The first is the…
Mechanical Model
In this model, elastic energy is created in the muscles and tendons and stored as a result of a rapid stretch (6,7,8). This stored energy is then released when the stretch is followed immediately by a concentric muscle action. According to Hill (9) the effect is like that of stretching a spring, which wants to return to its natural length. The spring is this case a component of the muscles and tendons called the series elastic component. The second model is the…
Neurophysical Model
When a quick stretch is detected in the muscles, an involuntary, protective response occurs to prevent overstretching and injury. This response is known as the stretch reflex. The stretch reflex increases the activity in the muscles undergoing the stretch or eccentric muscle action, allowing it to act much more forcefully. The result is a powerful braking effect and the potential for a powerful concentric muscle action (10,11,12).
If the concentric muscle action does not occur immediately after the pre-stretch, the potential energy produced by the stretch reflex response is lost. (i.e. if there is a delay between dipping down and then jumping up, the effect of the counter-dip is lost).
It is thought that both the mechanical model (series elastic component) and the neurophysical model (stretch reflex) increase the rate of force production during plyometrics exercises (6,7,8,10,11,12).
The Stretch-Shortening Cycle
All plyometric movements involve three phases. The first phase is the pre-stretch or eccentric muscle action. Here, elastic energy is generated and stored.
The second phase is the time between the end of the pre-stretch and the start of the concentric muscle action. This brief transition period from stretching to contracting is known as the amortization phase. The shorter this phase is, the more powerful the subsequent muscle contraction will be.
The third and final phase is the actual muscle contraction. In practice, this is the movement the athlete desires – the powerful jump or throw.
This sequence of three phases is called the stretch-shortening cycle. In fact, plyometrics could also be called stretch-shortening cycle exercises (1).
How to Increase Your Vertical Jump
One very quick and simple way to demonstrate the effect of the stretch-shortening cycle is to perform two vertical jumps. During the first vertical jump the athlete bends the knees and hips (eccentric muscle action or pre-stretch) and holds the semi-squat position for 3-5 seconds before jumping up vertically (concentric contraction) as high as possible. The 3-5 second delay increases the amortization phase.
On the second jump the athlete bends the knees and hips to the same degree but immediately jumps up without a delay. This keeps the amortization phase to a minimum and makes best use of the stored elastic energy. The second jump will be higher.
Is Plyometric Training Really That Effective?
By making use of the stretch-shortening cycle, movements can be made more powerful and explosive. Plyometrics is simply a set of drills designed to stimulate the series elastic component over and over again – preferably during movements that mimic those is the athlete’s sport. But what long-term effect does practising plyometrics have on the body and performance?
A wide variety of training studies shows that plyometrics can improve performance in vertical jumping, long jumping, sprinting and sprint cycling. It appears also that a relatively small amount of plyometric training is required to improve performance in these tasks. Just one or two types of plyometric exercise completed 1-3 times a week for 6-12 weeks can significantly improve motor performance (13,14,15,16,17,18,19). Additionally, only a small amount of volume is required to bring about these positive changes i.e. 2-4 sets of 10 repetitions per session (14,16) or 4 sets of 8 repetitions (15).
While upper body plyometrics has received less attention, three sessions of plyometric push ups a week has been shown to increase upper body power as measured by medicine ball throws (20).
Using a variety of plyometric exercises such as depth jumps, counter-movement jumps, leg bounding and hopping etc., can improve motor performance (13,22,23,24,25,26,27,28). While the majority of studies have focused on untrained subjects, trained athletes such as soccer and basketball players have improved their performance with plyometrics (16,23,28).
