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

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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).

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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).

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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.

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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).

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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:

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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.

Power Training for Sport

Power training enables an athlete to apply the greatest amount of their maximal strength in the shortest period of time.

This is crucial for many sports men and women who will rarely be required nor have the time to produce maximal forces.

Most athletic activities involve far faster movements and far higher power outputs than are found in maximal strength exercises (4,5). An athlete can be exceptionally strong but lack significant explosive power if they are unable to apply their strength rapidly.

This article outlines the various methods of power training, their parameters and how they can be used to convert maximal strength into sport-specific power. But before examining how power training should fit into the overall strength program, it's important to have a basic understanding of the relationship between the force of movement and the velocity of movement...

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The Force-Velocity Relationship

Power is intimately related to force and time, which can be expressed in the simple formula:

Traditional strength training typically alters the top half of this equation - increasing the ability to apply a maximum amount of force. But for power to be maximized the time component must also be altered. This is the aim of power training - to reduce the amount of time it takes to apply a set amount of force.

Maximum force production occurs when the speed of movement is very low (i.e. performing a one repetition maximum lift) or zero such as performing a static or isometric exercise.

Conversely, as the speed of movement increases, force decreases and at very high speeds force production is very low. Between these two extremes is an optimal point for power development. In fact, maximal power occurs at intermediate velocities when lifting moderate loads (6,7). Peak power output is typically seen when loads of 30% one repetition maximum (1-RM) are used (6,8).

This relationship between force and velocity and its affect on power explains why an athlete can be exceptionally strong but lack significant power if they are unable to apply much of their strength over a short period of time.

Assuming an athlete has maximized his or her ability to apply force (through maximal strength training), it would be beneficial if they could train to increase the rate of force production. Increasing the rate at which strength can be generated positively alters the time aspect of the power equation above.

The goal of power training is to increase the rate of force production and there are several methods that have been devised to do this...

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The Different Types of Power Training

Below are four methods of power training. A prerequisite to starting one of these routines is the development of a solid base of functional strength. Power training, particularly plyometrics and ballistics, becomes less effective and the risk of injury is increased if a phase of anatomical adaptation has not already been completed.

Heavy Strength Training

Strength training alone can increase explosive power by positively affecting the top half of the power equation or the peak force production (9,10,11). Most athletic movements also start from a stationary position and it is this early phase of moving a resistance (be it a medicine ball or bodyweight) that requires the most effort. Therefore the greater an athlete's strength is, the more explosive this initial phase of motion will be. However, once this initial inertia has been overcome less force and more speed is required to continue the movement and heavy strength training becomes less suitable.

Additionally, lifting weights of 70-100% 1-RM has also been shown to reduce the rate of force production which is counter-productive to power development (12). This may explain why in strength trained individuals heavy resistance training is less effective at increasing vertical jump performance compared to ballistics or plyometrics for example (11,13,14).

For an athlete who already has a solid base of strength training (+6 months) gains in power are minimal with further weight training (15,16). Of course, untrained individuals can significantly improve their power with weight training (15,17) and this is a safer and more favorable mode of training than some of the advanced techniques that follow.

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Explosive Strength Training

Once a plateau in strength has been reached, more sport-specific types of power training are required. One of these training methods is a variation of traditional resistance training. As mentioned earlier, maximal power production occurs when moderate loads of about 30% 1-RM are used.

Completing traditional weight lifting exercises as fast as possible with relatively light loads produces in theory, the greatest power output. Unfortunately there is a problem with this approach...

Lifting a bar rapidly loaded with 30% 1-RM is difficult to execute, particularly in the final phase of the movement. The athlete must decelerate and stop the bar in order to keep it under control (18,19). This deceleration activates the antagonist muscles negatively affecting power output and hinders the required adaptations (11,20).

Ballistics and plyometrics avoid this problem, as there is no deceleration. The athlete is free to jump as high as possible or throw an object as far as possible without restricting the movement.

If free weights exercises are used for power training, loads of 75-85% are recommended (1,8,11) for sets of 3-5 repetitions. The parameters for explosive strength training can be seen in the table below:

For single power efforts such as the throwing events in athletics, a higher load (80-90% 1-RM) can be used for a smaller number of repetitions (1-2). A multiple power effort sport includes sprinting,team sports or any event that requires repeated efforts.

Sets are not performed to exhaustion as the quality and speed of each lift is the most important factor. Rest intervals are also kept high for the same reason.

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Ballistics

During a ballistic action, the force far outweighs the resistance so movement is of a high velocity. The resistance is accelerated and projected. Examples include a medicine ball throw and a jump squat. The aim is to reach peak acceleration at the moment of release projecting the object or body as far as possible.

While there is no definitive guidelines for the resistance used with ballistics, Fleck and Kraemer (3) suggest a load of 30-35% 1-RM should be used for exercises that include free weights such as jump squats. For many ballistic exercises the weight of the objects themselves dictate the load i.e. medicine balls ranging from 2-6kg (4.4-13lbs) and kettlebells ranging from 10-32kg (22-70lbs).

Parameters for ballistic power training are summarized in the table below:

Repetitions can be reasonably high as the nature of some exercises means there can be up to 20 seconds between efforts - for example when a medicine ball has to be retrieved. A set should stop however, the moment the speed and quality of movement can no longer be maintained.

For exercises such as jump squats that use 30% 1-RM loads, Fleck and Kraemer (3) recommend up to 5 sets of 3 repetitions with 3 minutes rest between sets.

Ballistics can place considerable eccentric forces on joints, ligaments and tendons when landing from a jump squat for example. Athletes should always progress gradually from unloaded to loaded exercises and must not be fatigued before starting a ballistic power training session.

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Plyometrics

Plyometric drills involve a quick, powerful movement using a pre-stretch or counter-movement that involves the stretch shortening cycle (1). Classical plyometric exercises include various types of jump training and upper body drills using medicine balls.

Plyometrics is a suitable form of power training for many team and individual sports. While many might see it simply as jumping up and down, there are important guidelines and program design protocols that need to be followed if plyometrics is to be as safe and effective as possible. For this reason, and due to its popularity plyometrics has its own section of the website...

For full plyometric guidelines and sample sessions see the plyometric training section of the website.

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Which is The Best Form of Power Training?

The type of power training employed must be the most specific to the sport or event. Olympic lifts, such as power cleans, may be suitable for sports such as football and rugby. Some plyometric exercises are suitable for soccer and hockey. Ballistic exercises with medicine balls fit well with basketball and volleyball.

But many sports would benefit from a combination of power training methods. Take basketball for example - explosive strength training such as power cleans, plyometric exercises such as depth jumps and ballistics such as jump squats and overhead medicine ball throws would all be suitable choices.

Interestingly, a study measuring the effects of three types of power training found that all of them increased vertical jump performance. However, while traditional weight training lead to a 5% increase and plyometrics a 10% increase, the most effective was ballistic jump squats, which lead to an 18% improvement in jump height (11). This confirmed the findings of a similar earlier study (14).

Does this mean ballistics is superior to other forms of power training? Not necessarily. In this case it may be that jump squats was the most specific to the performance outcome.