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? Notice the counter-movement 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 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 and resistance 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 and strength training concurrently



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.

Finally, landing surface is an important component of the plyometrics session. It should posses adequate shock absorbing properties such as grass, rubber mats and a suspended floor. Concrete, tiles, hardwood and crash mats are not suitable (35).

Return from this physiology of plyometrics article
to the main plyometric training section




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References
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3) Baechle TR and Earle RW. 2000 Essentials of Strength Training and Conditioning: 2nd Edition. Champaign, IL: Human Kinetics
4) Bosco C and Komi PV. (1980) Influence of countermovement amplitude in potentiation of muscualr performance. Biomenchanics VII proceeding (pp129-135). Baltimore:University Park Press
5) Schmidtbleicher D. Training for power events. In Komi PV (ed) Strength and Power in Sport (pp381-395). Oxford, UK: Blackwell Scientific
6) Asmussen E, Bonde-Petersen F. Storage of elastic energy in skeletal muscles in man. Acta Physiol Scand. 1974 Jul;91(3):385-92
7) Bosco C, Ito A, Komi PV, Luhtanen P, Rahkila P, Rusko H, Viitasalo JT. Neuromuscular function and mechanical efficiency of human leg extensor muscles during jumping exercises. Acta Physiol Scand. 1982 Apr;114(4):543-50
8) Hill AV. (1970) First and last experiments in muscle mechanics. Cambridge: University Press
9) Hill AV. (1970) First and last experiments in muscle mechanics. Cambridge: University Press
10) Guyton AC and Hall JE. (1995) Textbook of medical physiology, 9th ed. Philadelphia: Saunders
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12) Bosco C, Komi PV, Ito A. Prestretch potentiation of human skeletal muscle during ballistic movement. Acta Physiol Scand. 1981 Feb;111(2):135-40
13) Bartholomeu SA. Plyometrics and vertical jump training. University of Noth Carolina, Chapel Hill
14) Blackey JB, Southard D. The combined effects of weight training and plyometrics on dynamic leg strength and power. J Appl Sport Sci Res. 1987 1:14-16
15) Gehri DJ, Richard MD, Kleiner DM and Kirkendall DT. A comparison of plyometric training techniques for improving vertical jump ability and energy production. J strength Cond Res. 1998 12:85-89
16) Matavulj D, Kukolj M, Ugarkovic D, Tihanyi J, Jaric S. Effects of plyometric training on jumping performance in junior basketball players. J Sports Med Phys Fitness. 2001 Jun;41(2):159-64
17) The effects of plyometric training on the vertical jump performance of adult female subjects. British J Sports Med. 1982 16:113-15
18) Scoles G. Depth Jumping! Does it realy work? Athletic J. 1978 58:48-75
19) Steben RE, Steben AH. The validity of the stretch shortening cycle in selected jumping events. 1981 Mar;21(1):28-37
20) Comparison of dynamic push-up training and plyometric push-up training on upper-body power and strength. J Strength Cond Res. 14:248-53
21) Adams K, O'Shea JP, O'Shea Kl and Climstein M. The effects of six weeks of squat, plyometric and squat-plyometric training on power production. J Appl Sport Sci Res. 1992 6:36-41
22) Bosco C and Pittera C. Zur trainings wirkung neuentwicker sprungubungen auf die explosivkraft. Leistungssport 12:36-39
23) Diallo O, Dore E, Duche P, Van Praagh E. Effects of plyometric training followed by a reduced training programme on physical performance in prepubescent soccer players. J Sports Med Phys Fitness. 2001 Sep;41(3):342-8
24) Fatouros IG, Jamurtas Az, Leontsini D, Taxildaris K, Kostopoulos N and Buckenmyer P. Evaluation of plyometric exercise training, weight training and their combination on vertical jump in performance and leg strength. J Strength Cond Res 14:470-476
25) Ford HT Jr, Puckett JR, Drummond JP, Sawyer K, Gantt K, Fussell C. Effects of three combinations of plyometric and weight training programs on selected physical fitness test items. Percept Mot Skills. 1983 Jun;56(3):919-22
26) Potteiger JA, Lockwood RH, Haub MD, Dolezal BA, Almuzaini KS, Schroeder JM and Zebras CJ. Muscle power and fiber characteristics following 8 weeks of plyometric training. 1999 13:275-79
27) Rimmer E, Sleivert G. Effects of plyometrics intervention program on sprint performance. J Srength Cond Res. 2000 14 295-301
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29) Adams K, O'Shea JP, O'Shea Kl and Climstein M. The effects of six weeks of squat, plyometric and squat-plyometric training on power production. J Appl Sport Sci Res. 1992 6:36-41
30) Bauer T, Thayer RE and Baras G. Comparison of training modalities for power development in the lower extremity. J Appl Sport Sci Res. 1990 4:115-121
31) Clutch D, Wilson C McGown C, Bryce GR. The effect of depth jumps and weight training on leg strength and vertical jump. Research Quarterly. 1983 54:5-10
32) Hunter JP, Marshall RN. Effects of power and flexibility training on vertical jump technique. Med Sci Sports Exerc. 2002 Mar;34(3):478-86
33) Olhemus R, Burkhart E, Osina M, Patterson M. The effects of plyometric training with ankle and vest weights on conventional weight training programs for men and women. National Strength Coaches Association J 2:13-15
34) Blattner SE, Noble L. Relative effects of isokinetic and plyometric training on vertical jumping performance. Research Quarterly. 1979
35) National Strength and Conditioning Association. Position statement: Explosive/plyometric exercise. NSCA.J. 1993 15(3):16


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But it's impossible right?

Actually it's not...

The Vertical Project

Read how and why here...

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