The Physiology of Plyometrics
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? 
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 athletes 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).
Plyometrics & Concurrent Strength Training
A conditioning program consisting of both plyometric training andresistance training can improve power performance in the vertical jump (13,14,29,30,31,32) and 40yard sprint time (33).
It appears that concurrent resistance and plyometrics training can actually improve power to a greater extent than either one alone (13,29,30,33). However, the overall program should be carefully planned as heavy weight training and plyometric training are not recommended on the same day (3). One way around this is to alternate upper body and lower body exercises as follows:
Plyometrics & Injury
Strength and conditioning specialists are often cautious in their prescription of plyometrics due to what they believe is an inherent risk of injury. However, there is limited data to either confirm or reject this claim.
Several researchers have explicitly stated that no injuries occurred during their plyometric studies (13,33,34). Most do not mention whether injuries occurred or not or to what extent.
As a precaution it has been suggested that athletes have a substantial strength training background. The criteria often cited is that the athlete should be able to back squat 1.5-2x bodyweight (2,3,35) for lower body plyometrics and bench press 1x bodyweight for upper body plyometrics (3,35).
If injuries are more likely to occur with this form of training it may be due to improper landing, landing surface or depth jumps from too great a height (1). Several studies have measured the height of depth jumps on vertical jump performance. Depth jumps from both 50cm (19.7) and 80cm (31.5in) both improved power to the same extent (13). The same results were found between jumps of 75cm and 110cm (31) and between jumps of 50cm and 100cm (16). This suggests that there may be little or no added benefits of jumping from heights above 50cm (19.7in) even though the risk of injury is likely to rise.
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