Plyometrics

Plyometrics, also known as "jump training" or "plyos", are exercises based around having muscles exert maximum force in as short a time as possible, with the goal of increasing both speed and power. This training focuses on learning to move from a muscle extension to a contraction in a rapid or "explosive" way, for example with specialized repeated jumping. Plyometrics are primarily used by athletes, especially martial artists and high jumpers, to improve performance, and are used in the fitness field to a much lesser degree.

Introduction
The term plyometrics was coined by Fred Wilt after watching Soviet athletes prepare for their event in track and field. He felt this was a key to their success. It is a poor term to describe what happens but it has since been accepted and is now well established. When Wilt learned of the work being done by Michael Yessis on Soviet (Russia) training methods, they quickly collaborated to help disseminate information on plyometrics.

Since its introduction in the early 1980s, two forms of plyometrics have evolved. In the original version of plyometrics created by Russian scientist Yuri Verkhoshansky, it was defined as the shock method. In this, the athlete would drop down from a height and experience a “shock” upon landing. This in turn would bring about a forced, involuntary eccentric contraction which was then immediately switched to a concentric contraction as the athlete jumped upward. The landing and takeoff were executed in an extremely short period of time, in the range of 0.1- 0.2 seconds. The shock method is the most effective method used by athletes to improve their speed, quickness and power after development of a strong strength base.

Rather than using the term plyometrics to indicate exercises utilizing the shock method, it may be preferable to use the term explosive or true plyometrics which can be considered the same as the plyometrics originally created by Verkhoshansky. The shock method that he created was the result of studying the actions that occur in running and jumping. He found that the landings and takeoffs in these two skills involved high ground reaction forces that were executed in an extremely quick and explosive manner. For example, time of execution of the landing and takeoff in jumping was close to 0.20 seconds and in sprinting it was approximately 0.10 seconds.

Since one of the main objectives of the Soviet research was to develop practical methods of training to improve athletic performance, Verkhoshansky tackled the task of how these forces in explosive execution could be duplicated in an exercise. By doing exercises such as the depth jump, that he created, the athlete would enhance his ability in the takeoff and his resultant performance in the running or jumping event. He experimented with many different exercises but the depth jump appeared to be the best for duplicating the forces in the landing and takeoff.

The second version of plyometrics, seen to a very great extent in the United States, relates to doing any and all forms of jumps regardless of execution time. Such jumps cannot be considered truly plyometric (as described by Verkhoshansky) since the intensity of execution is much less and the time required for transitioning from the eccentric to the concentric contraction is much greater. The term plyometrics became very popular with the publication of many books on the subject matter. It now appears impossible to go back to its original meaning and method of execution.

As a result, it is important to distinguish which type of “plyometric” exercise is used in order to determine its effectiveness and potential to receive the stated benefits. Understand that even though the name plyometrics is given to all jumps, not all jumps are plyometric.

History
Fred Wilt, a former US Olympic long-distance runner, is credited with coining the term plyometrics. He admits that it is not a very good term but it was the best he could come up with after watching the Russians execute jumps in their warm-ups prior to their event in track and field. He could not understand why the Russians were doing all of these jumps while the Americans were doing multiple static stretches. But he firmly believed it was one of the reasons why they were so successful in many events. From its beginnings in the early 1980s, the term plyometrics gained greater popularity and is now well established. When Fred Wilt learned of the work being done by Michael Yessis in the field of Russian training methods, they quickly teamed up to help disseminate information on plyometrics.

In collaboration with Yessis who visited and worked with Verkhoshansky in the former Soviet Union the early 1980s, the term plyometrics and how it is used (practiced) was gradually disseminated in the US. Yessis brought this information on plyometrics back to the US and in the following years was able to create even more ways of using this method to train and improve explosive power. Plyometrics (the shock method) was created by Yuri Verkhoshansky in the late 1960s, early 1970s. Since then the shock method of plyometrics is still being practiced for improvement of athletic performance by what appears to be a relatively limited number of athletes. These athletes still do depth jumps, the key exercise in the shock method, according to the guidelines established by Verkhoshansky.

Most athletes execute simple and complex jumps and call them plyometrics rather than jump training as it was called in the past. This includes the depth jump which was executed in ways different from what was recommended by Verkhoshansky. This form of jump training but called plyometrics is very popular and is a buzzword for any and all types of jumps, regardless of how long it takes to execute the jump. Its use is so pervasive that it is even possible to find push-ups described as being plyometric.

Due to the wide use and appeal of the term plyometrics, the true meaning plyometrics as developed by Verkhoshansky has for the most part, been forgotten. Verkhoshansky was well known and respected worldwide in both the scientific as well as in the coaching arenas. He was relatively unknown in the United States except for some of his articles that were translated and published in the “Soviet Sports Review”, later called the “Fitness and Sports Review International”.

In addition to creating the shock method, Verkhoshansky is credited with developing the stretch-shortening concept of muscle contractions and in the development of specialized (dynamic correspondence) strength exercises. Note that plyometrics or more specifically the shock method, is considered a form of specialized strength development.

Thus before undertaking plyometric training it is necessary to distinguish jumps that are commonly called plyometric and true plyometric jumps as exemplified in the depth jump which is illustrative of the shock method. Since its inception in the former Soviet Union as the shock method, there have been other forms of true or explosive or the plyometric exercises created by Yessis that do not involve jump exercises. For details and illustrations of these exercises see “Explosive Running” and “Explosive Plyometrics”. These exercises involve the stretch-shorten concept that underlies the shock method.

Methodology
In the depth jump, the athlete experiences a shock on landing in which the hip, knee and ankle extensor muscles undergo a powerful eccentric contraction. In this, the muscles are forcibly tensed in an eccentric contraction. For the muscles to respond explosively, the eccentric contraction is then quickly switched to the isometric (when the downward movement stops) and then the concentric contraction, in a minimum amount of time. This allows the athlete to jump upward as high as possible. For simplicity, rather than always mentioning the isometric contraction, which always occurs in the transition from the eccentric to the concentric contraction, it is usually omitted. In the eccentric contraction, the muscles are involuntarily and forcefully lengthened while in the concentric contraction the muscles are shortened after being pre-tensed. Most of the stretching and shortening takes place in the tendons that attach to the muscles involved rather than in the muscles. To execute the depth jump, the athlete stands on a raised platform, usually not greater than 20-30 inches high, and then steps out and drops down in a vertical pathway to make contact with the floor. The exact height used by most athletes is usually quite low in the early stages of training. The key is how high the athlete jumps in relation to the height of the takeoff platform. Technique and jump height are most important at this time. While the body is dropping the athlete consciously prepares the muscles for the landing impact by pre-tensing the muscles. The flooring upon which the athlete drops down on should be somewhat resilient, mainly for prevention of injury. Upon making contact with the floor, the athlete then goes into slight leg flexion to absorb some of the forces for safety. However, the main role played by the muscles and tendons is to withstand the forces that are experienced in the landing. These forces are withstood in eccentric contraction. When muscle contraction is sufficiently great, it is able to stop the downward movement very quickly.

This phase is sometimes called the phase of amortization in which the athlete absorbs some of the forces and stops downward movement by the strong eccentric contraction of the muscles. The strong eccentric contraction prepares the muscles to switch to the concentric contraction in an explosive manner for takeoff.

When the athlete drops down to the floor, the body experiences an impact upon landing. The higher the height of the step-off platform, the greater are the impact forces upon landing. This creates a shock to the body which the body responds to by undergoing a strong involuntary muscular contraction to prevent the body from collapsing on the ground. This in turn produces great tension in the muscles and tendons which is then given back in a return upward movement. The faster the change in the muscular contractions, the greater is the power created and the resulting height attained.

More specifically, the muscles and tendons undergo a stretch (eccentric contraction) on the landing which is needed to absorb some of the forces generated but most importantly, to withstand the forces that are produced by the shock that occurs on the landing. The greater the shock (forces experienced on landing), the stronger is the eccentric contraction which in turn, produces even greater tension. This tension which is potential force, is then given back in the return movement when the muscular contractions switch to the concentric or shortening regime.

However, for maximum return of energy, minimum time must elapse from when the forces are received to when they are returned. The greater the time between receiving the forces and giving them back, the less is the return and the less the height that can be achieved in the jump. Most of the lengthening and shortening occurs in the respective muscle tendons which have greater elasticity.

Another way of saying this is that the faster the switching from the eccentric to the concentric contraction, the greater will be the force produced and the greater will be the return movement. The speed of the switching is extremely fast; 0.20 seconds or less. For example, high-level sprinters execute the switch from the eccentric contraction that occurs when the foot hits the ground to the concentric contraction when the foot breaks contact with the ground, in less than 0.10 seconds. In world-class sprinters, the time is approximately 0.08 seconds. The exact platform height used by most athletes in the depth jump should be less than 30 inches in the early stages of training. Most athletes start at approximately 12 inches after doing some jump training. They then gradually work up to 20 inches and then to 30 inches depending upon how well the jumps are executed. The main criterion is to make sure that the athlete is jumping as high as possible on every jump.

If the athlete gradually improves his jump height, the same platform height is continued until increases in jump height are no longer observed. At this time takeoff height is increased by a few inches. If the athlete continually fails to jump very high, the height of the drop-down is lowered somewhat. Most important here is how high the athlete jumps after the drop-down.

The maximum platform height used by a high level athlete is no more than 40 inches. Rather than developing greater explosive power this height leads to more eccentric strength development. Because of this, going higher than 30 inches is usually counterproductive and may lead to injury. This occurs when the intensity of the forced involuntary eccentric contraction upon landing is greater than the muscles can withstand. In addition, the athlete will not be able to execute a quick return (fast transition between muscular contractions) which is the key to successful execution of explosive plyometrics.

Because of the forces involved and the quickness of execution, the central nervous system is strongly involved. Because of this, it is important that the athlete not overdo using the shock plyometric method. Doing so will lead to great fatigue and according to Verkhoshansky, sleep disturbances. For example, athletes have great difficulty sleeping well if they execute too many depth jumps. This indicates that athletes must be well prepared physically before doing this type of training.

Technique of jumping is also very important when executing plyometric exercises. In essence, the athlete goes into a slight squat (crouch) upon landing in which there is flexion in the hip, knee and ankle joints. In the takeoff or jump upward, the jump is executed in a sequence initiated by hip joint extension followed by knee joint extension which begins during the hip joint extension. As the knee joint extension is taking place, ankle joint extension begins and is the only action that occurs as the takeoff (breaking contact with the ground) takes place. All three actions contribute force to the upward jump but the knee joint extension is the major contributor.

As simple jumping
The most common type of plyometrics used in the United States is simple and relatively easy jump exercises executed with little regard to execution time. These jumps are effective for athletes who execute skills in their sport that do not require explosive type muscular contractions. An example is long-distance running, in which the runners execute repeat actions of 20 to 30 consecutive jumps and other cyclic type activities such as leaping for multiple repetitions.

Such plyometric jumps are also used as a warm-up to doing explosive plyometric jumps and for initial preparation of the muscles prior to undertaking exercises such as depth jumps. In essence, they are effective in the early stages of learning how to do plyometric exercises and for preparing the muscles for explosive or quick jumps. These jumps are similar to those done by youngsters in the playground or in neighborhood games and as such, do not require additional preparation. Athletes, regardless of their level of expertise, can undertake such jumps in the initial stages of training.

When athletes who have been doing plyometrics without regard to time of execution first attempt to execute explosive plyometrics, they often fail because the time of execution is too long. This occurs quite often in the depth jump. The athlete usually sinks (drops) too low which takes too long to make the transition from the eccentric to the concentric contraction. As a result, the exercise becomes a jump strength exercise and not a true plyometric one.

Jump technique remains the same regardless of whether it is a true plyometric exercise or a jump exercise. There is flexion in the hips, knees and ankles on landing and extension in the joints on the upward return. The sequence and overlapping in the sequence is basically the same beginning with the hip extension followed by knee extension and ending with ankle plantar flexion. The major differences in execution are in the depth of the landing and in the time of executing the switch from the eccentric to the concentric contraction.

Studies have been conducted testing ten various plyometric exercises on overall performance during jumping examined by EMG, power, and ground reaction force (GRF). Of the 10 tested, the single leg cone hops, box jumps, tuck jumps, and two legged vertical jumps produced the highest EMG values alluding to greater motor recruitment. Power was examined by: dumb bell jumps, depth jumps, countermovement jumps, squat jumps, and tuck jumps which all produced the higher power scale readings. In terms of athletic performance and training, the plyometric movements that utilize total body vibration produced an overall increase in performance output. In one recent study examining two groups using the same plyometric protocol in combination with weight training, one using high loads and the other utilizing small loads, similar decreases in power were found. This shows that the plyometric exercises themselves had a greater effect in the decrease in power output rather than the type of weight training.

Safety considerations
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 50 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. In fact, some advanced training methods do combine Plyometrics and Intensive stretching in order to both protect the joint and make it more receptive to the plyometric benefits.

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 - should be taken into account for both pre-pubescent and the elderly because of hormonal changes.


 * 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 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.