Plyometric Training: Leaps and Bounds above the rest

Plyometric Training: Leaps and Bounds above the Rest

Introduction/Overview:

Plyometric Training, or ‘plyometrics’, is a form of training which often utilises hops, jumps, bounds and skips. Plyometrics should not be confused with ‘ballistic’ training. Ballistic training involves the trajectory of objects and implements (e.g. barbells and medicine balls), whereas plyometric training uses the previously mentioned movements.

However, in some circumstances there is a degree of crossover between the two, where some movements are considered both plyometric and ballistic. Ultimately the differing factor between the two is that plyometric training typically involves rapid reactive contacts with a surface (e.g. foot contacts during sprinting and contact time during hops), whilst ballistic training involves the trajectory of objects/implements (11).

Plyometric training has been consistently shown to improve force production and power. A study by Hewett and colleagues (8) found that completing plyometric sessions 3 days a week, for 6 weeks, improved jump height by 10%. Similarly, a more recent intervention (1) showed plyometric training to significantly improve explosive power and maximum strength levels over a 9 week period.

When implementing plyometrics into a training programme, there are a number of factors to consider, including: volume, intensity, frequency and progressions.

Key Components of Plyometric Training:

The Stretch Shortening Cycle:

Plyometric training makes use of, and helps to develop the efficiency of what’s known as the ‘stretch-shortening cycle’, or SSC, of the muscles. It also takes advantage of utilising the muscles series elastic component (SEC).  Tendons constitute the majority of the SEC, acting as a spring like mechanism. The SSC involves three distinct stages:

• The Eccentric Phase: During this phase, the muscle is lengthening under load. Kinetic energy is stored, and the muscle spindles are activated. Muscle spindles detect changes in length of muscles, and convey this information to the central nervous system and brain.
• The Amortisation Phase: This is the transitional period between eccentric and concentric contractions. Or the change of direction from lengthening to shortening. Ideally, the amortisation phase is kept as short as possible as this prevents the loss of stored energy from the eccentric phase.
• The Concentric Phase: During the concentric phase, the muscles are shortening. If the amortisation phase was brief enough, the energy stored is then re-utilised, allowing for a more forceful contraction to occur.

Ground Contact Time:

Ground contact time is the term used for how long the foot is in contact with the floor during plyometric activities such as running, sprinting and jumping. When walking, ground contact times may be very long, whereas during a maximum effort sprint for example, ground contact times can be anywhere between 30-90 milliseconds (9).

Plyometric movements which utilise the SSC are classified as either ‘slow’ or ‘fast’. A ‘slow’ plyometric exercise has a ground contact time of greater than or equal to 251 milliseconds (0.251 seconds). A ‘fast’ plyometric exercise has a ground contact time of less than or equal to 250 milliseconds (0.25 seconds). Some examples of slow plyometric exercises are a countermovement jump or a depth jump from a high box. Fast plyometric exercises include repeated hops, long jump or sprinting.

Why is Plyometric Training Important?

As the SSC exists in all forms of human motion from changing direction in rugby to jumping in basketball, and even sprinting in the 100m, it becomes obvious that all of these movements can be deemed as plyometric activities. As all of these movements are classified as plyometric, its importance in sport suddenly becomes transparent (11).

Research in the area of plyometric training is constantly growing in popularity, with researchers attempting to discover its potency for improving athletic performance. So far, plyometric training has been shown to improve a number of physical qualities in both youth and adults, including (11):

• Strength
• Speed
• Power
• Change of Direction Speed
• Balance
• Jumping
• Throwing
• Kicking
• Bone density

How does it Improve Performance?

Exposure to plyometric exercise improves the body’s ability to utilise the SSC, which in turn can aid in improving performance. Research has investigated a large number of neurological and physiological mechanisms which may underpin and explain the impact of plyometric training on the SSC. These include: increases in the muscles active range (3, 4), enhanced involuntary nervous reflexes (5, 6) and enhanced motor coordination (3, 4).

However, the primary areas of significance are the improvement in storage and utilization of elastic strain energy (13, 10), and the increase in the muscles active state due to increases in the active working range (13, 10). Put simply, a muscle is able to be active over a longer range of motion, and is able to utilise more energy stored in the eccentric phase to provide a supramaximal concentric contraction. Improving these qualities will lead to an increase in leg stiffness during contact with the ground, and also force production during the concentric contraction. Increases in both leg stiffness and force production will likely lead to improvements in athletic performance (11).

Furthermore, plyometric training increases the proportion of Type IIB muscle fibres in the body (Type IIB muscle fibres are larger, contract much more rapidly, and produce more power and strength than Type I muscle fibres). This increase in Type IIB muscle fibres is crucial for many sporting and athletic endeavours and allows for faster and more powerful bursts of force production.

How to Implement Plyometrics into Your Training:

Plyometrics is an excellent training modality to improve athletic performance. However, there are a few important issues which should be taken note of before adding it into your training programme.

Potential Issues:

Firstly, plyometric movements are highly-coordinated and skilful movements. This means that athletes are required to produce high amounts of force, during very fast movements, in a very short amount of time. Thus, a fairly high degree of body coordination and skill should be developed before moving into intense plyometric training.

Furthermore, strength levels must be adequate before undertaking plyometric activities. Athletes have been shown to produce ground reaction forces during each foot contact of 3-4 times bodyweight (12, 2). Not only that, but they must apply these huge forces in a contact time of just 80-90 milliseconds (9). So during sprinting, athletes are required to move as quickly as possible, produce forces of over 3-4 times bodyweight, and do so in just 80-90 milliseconds (11). Without a pre-requisite level of strength, injury risk is higher, and performance may not be positively impacted. It has been recommended that athletes should be able to squat at least one to one and a half times bodyweight prior to commencing plyometric training.

Planning Plyometrics into your programme:

There are a number of variables to consider when planning how and when to implement plyometric training. These include: volume, intensity, frequency and progression.

Volume:

Volume for plyometric training tends to be fairly low when compared to other training modalities. This is due to the high demands it places on the body’s neuromuscular system. A commonly used measure of volume for plyometric training is ‘ground contacts’, simply meaning how many times the feet contact the ground. So, five hops would count as five ground contacts. Plyometric volume can also be expressed using distance in metres if running or bounding. It has been recommended that beginners perform between 80-100 ground contacts, intermediates perform between 100-120 ground contacts, and advanced between 120-140 ground contacts per session (7).

Intensity:

Plyometric intensity is the amount of stress placed on the involved muscles, connective tissues and joints. Plyometric drills cover a large range of intensities; skipping is relatively low intensity, while depth jumps are much higher intensity. In addition to the type of drill, there are several other factors to consider which can impact intensity levels (7):

• Points of Contact: Landing, hopping or jumping on one foot places more stress on the tissues than double-leg variations.
• Speed: Greater speed increases the intensity of the drill.
• Height of the Drill: The higher the bodies centre of gravity, the greater the force upon landing.
• Body Weight: The greater the athletes body weight, the more stress is placed on the muscles, connective tissues and joints.

Dependent upon these factors, the volume of the drill can vary dramatically away from the recommended figures. For example, an athlete who weighs 60kg and an athlete who weighs 100kg will be placing very different demands on their bodies during plyometric activities. It is crucial to take this into consideration when planning or prescribing any form of plyometric activity.

Frequency:

Frequency is the number of sessions per week. This typically ranges from one to three sessions depending on the athletes sport, experience, or time of the athletic year. As a general guideline, it is recommended to allow between 48-72 hours recovery between sessions (7).

Progression:

As with all forms of training, progression should be taken at a steady pace to ensure you are ready for the demands placed upon the body. Start off with low intensity drills such as two footed hops and submaximal jumps. Once these are comfortable, moving onto single leg variations, bounding and submaximal sprinting/fast runs is a good next step. Finally, high intensity drills such as depth jumps, weighted jump variations and maximum effort sprints can be added. It is important to give the muscles, tendons and joints adequate time to adapt to the forces placed upon them during plyometric training.

Take your time with progressions, keep the volume relatively low to start, and enjoy your new found power!

Stay Strong and Move Well,

George Barker

References:

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8. Hewett, T.E., Stroupe, A.L., Nance, T.A., Noyes, F.R. (1996) Plyometric training in female athletes. American Journal of Sports Medicine, Vol. 24, pp. 765-773.
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