Plyometrics

thumb|A [[US Marine performs plyometric jumps in Camp Foster, Okinawa]]

Plyometrics, also known as plyos, are exercises in which muscles exert maximum force in short intervals of time, and activating a rapid stretch-shortening cycle. While commonly associated with jumping, plyometrics can include explosive upper-body movements, such as plyometric push-ups and switch-ups, with the goal of increasing muscular power (speed-strength).

This training focuses on learning to move from a muscle extension to a contraction in a rapid or "explosive" manner, such as in specialized repeated jumping. The amortization phase (transition from eccentric to contraction) must be very short (milliseconds), as a slow transition will not fully release elastic energy stored in eccentric phase, and instead some stored energy leaks away as heat. SSC training loads tendons elastically and when done properly, it can yield benefits including improved landing mechanics, force absorption, joint stability and greater force and power output, with transferability to many athletic activities, such as sprinting, where similar repetitive movements are required. Progressive plyometric training can also help to reduce the chances of injury, especially to the lower leg, ankle and foot.

Plyometrics are frequently used by athletes, especially martial artists, sprinters and high jumpers, to improve performance, and are used in the general fitness field to a lesser degree.

Overview

Plyometrics include explosive exercises to activate the quick response and elastic properties of the major muscles. It was initially adopted by Soviet Olympians in the 1950s, and then by sportspeople worldwide. Sports using plyometrics include basketball, tennis, badminton, squash and volleyball as well as the various codes of football. The term "plyometrics" was coined by Fred Wilt after watching Soviet athletes prepare for their events in track and field. He began a collaboration with trainer Michael Yessis to promote plyometrics.

Since its introduction in the early 1980s, two forms of plyometrics have evolved. In the original version, 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 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 second. Several imaginary Greek words that would explain the y have been cited.

History

Fred Wilt, a former US Olympic long-distance runner, is credited with coining the term plyometrics 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. in the Soviet Union in the early 1980s, plyometrics 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. and "Explosive Plyometrics".

Tuck jump (tucked jump, tucked knee jump): with feet shoulder width apart, jump, tuck the legs in, extend them, and land.
Alternate leg bounding: run with long strides, placing emphasis on hang time.
Frog jumps: squat, place your hands on the floor, and jump forward explosively as far as possible, landing softly and immediately preparing for the next jump.
Straddle jump (similar to split jumps used by dancers, gymnasts and figure skaters).
Lunge jump

Upper-body

Plyometric push-up (plyo push-up): perform a push up, but exert enough upward force to lift the hands and body off the ground.
Medicine ball cross slams (one-arm diagonal slam): Hold the medicine ball in one hand, raise it overhead on one side, then explosively rotate the torso and hips to slam it diagonally across the body toward the ground.
Medicine ball slams: lift the medicine ball overhead and forcefully slam it straight down to the ground, engaging the core, shoulders, and arms.
Switch-ups (explosive pull-up/chin-up variations): An advanced plyometric exercise in which one uses alternating grips on a bar to perform a pull-up / chin-up, and at the top, explosively switches the grip or hand position before lowering. This heavily engages the shoulders, but is regarded comparatively safer to a muscle-up.

Full-body

Power skipping: On each skip, lift the upper leg as high as possible while driving the opposite arm upward to maximize stride height and explosive leg drive.
Burpee broad jumps: An advanced variation of the standard burpee, closer to a pure plyometric because it emphasizes rapid SSC use. It's like a standard burpee, but instead of jumping straight up, explosively leap forward as far as possible, land softly and immediately prepare for the next repetition.
Bounding: run with exaggerated, powerful strides, pushing off each leg forcefully and focus to maximize both forward distance and hang time.
Jump rope: Repeatedly skipping over a rope with knees slightly bent, engages the stretch-shortening cycle of leg muscles with each jump, making it a form of low intensity plyometrics.

Method

In the depth jumps, the athlete experiences a shock on landing in which the hip, knee, and ankle extensor muscles undergo a powerful 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.

In the eccentric contraction, the muscles are involuntarily lengthened, while in the concentric contraction, the muscles are shortened after being 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 high, and then steps out and drops down in a vertical pathway to make contact with the floor. The 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 impact by 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 flex to absorb some of the force for safety. However, the main role played by the muscles and tendons is to withstand the force that is experienced in the landing. This force is 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 force 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 the impact force 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 the power created and the resulting height attained. 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 . Rather than developing greater explosive power this height leads to more eccentric strength development. Going higher than 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. 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. 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.

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 force generated during training and performance, and should only be performed by well conditioned individuals under supervision. Good levels of physical strength, flexibility, and proprioception should be achieved before beginning plyometric training.

The specified minimum strength requirement varies depending on where the information is sourced and the intensity of the plyometrics being performed. Chu (1998) recommends that a participant be able to perform 50 repetitions of the squat exercise at 60% of his or her body weight before doing plyometrics. Core (abdomen) strength is also important.

Flexibility is required both for injury prevention and to enhance the effect of the stretch shortening cycle. Some advanced training methods 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 is also required for safe performance of plyometric exercises.

Further safety considerations include:

Age: should be taken into account for both prepubescent and the elderly because of hormonal changes.
Technique: a participant must be instructed on proper technique before commencing any plyometric exercise. He or she 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 and intense movements used in repetition increase the potential level of stress on joints and musculo-tendonous units. Therefore, safety precautions are strong prerequisites 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 many people.

Benefits

Many professional and Olympic athletes use plyometric training to improve muscular strength, power, and jumping performance. Plyometric exercises can be performed at varying levels of intensity, allowing adaptation to different training needs. Studies have also reported associations between plyometric training and improvements in muscular strength, endurance, metabolic rate, and cardiovascular responses.

As plyometrics places high mechanical loads transmitting through the bones, they stimulate bone remodelling and increase bone mineral density (BMD). A one year randomized clinical trial of moderately active men with low bone density reported that high intensity plyometric jump training had increased lumbar spine and whole body BMD.

Loaded plyometrics

Plyometric exercises are sometimes performed with an additional load, or weight added. In such cases, they are referred to as loaded plyometrics or weighted jumps. The weight is held or worn. It may be in the form of a barbell, trap bar, dumbbells, or weighted vest. For instance, a vertical jump whilst holding a trap bar or jumping split squats whilst holding dumbbells. In addition, a regular weight lifting exercise is sometimes given a plyometric component, such as is found in a loaded jump squat. Jumping onto plyo boxes or over hurdles whilst holding weights is not recommended for safety reasons. The advantage of loaded plyometric exercises is that they increase the overall force with which the exercise is performed. This can enhance the positive effect of the exercise and further increase the practitioner's ability to apply explosive power.

Unilateral plyometrics

Unilateral plyometrics are jumping exercises which involve only one foot being in contact with the ground at some stage. This can include jumping off of, and landing on, the same foot i.e. hopping, jumping from one foot and then landing on the other, jumping from one foot and landing on two, or jumping off two and landing on one. It typically makes more intense demands on the legs than bilateral plyometric training and can be used to further enhance explosive power. The intensity of the exercises can be manipulated through the adjusting of box and hurdle height, and any weight which is held or worn. The greatest intensity can be achieved whereby the height or the distance travelled is maximized.

A hop test involves a comparison between the hopping height or distance achievable by the left and right legs, considered separately. It is used to assess the relative strength levels of each leg and whether there is a muscle imbalance i.e. a strength discrepancy between the left and right sides which results in a significant variation in the results. If such an imbalance is found, unilateral plyometrics may be used to alleviate it. As the legs are used singly, and perform the same amount of work, the body and legs may be strengthened more evenly than bilateral plyometrics, which may involve one leg doing an excessively large amount of the work.

Some forms of unilateral plyometrics involve a cyclic alternation between the legs e.g. repeatedly jumping from one foot to the other. As runners perform a similar action of alternating between left and right legs, and each step has an acceleration phase like a jump does, then based upon this commonality, such unilateral plyometrics are considered to transfer effectively to running and sprinting and improve performance.