Plyometrics

Plyometrics or plyometric training is a form of exercise which rplymetric trainingequires the muscles to contract quickly and deliver as much force (and therefore power) as possible. Plyometrics can be very useful to improve muscle contraction speed and the ability to generate power, which is useful in activitiets in which for example jumping or other explosive movements are important.

To achieve these specific training effects, the muscles should be stretched by a movement in the opposite direction also known as a counter movement. By doing a countermovement, energy is stored in tendons and ligaments which have elastic properties and helps to generate force more quickly. Plyometric training increases muscle power during specific movements by stretching both these elastic components and activating the stretch reflex or myotatic reflex.

Functional exercises and succes in a variety of sports are dependent on both well functioning muscles as the velocity with which they can generate force, in other words power. When applied right, plyometric training is very effective in increasing muscle strength en muscle power in particular.

A plyometric exercise consists of 3 phases. The eccentric phase, the isometric phase and the concentric phase. During the eccentric phase, the muscles are stretched by a counter movement. When this phase ends, the muscles are for a short time kept at a constant length while being fully activated. This is known as the isometric phase, in which the muscle does not change its length but is delivering force. Next the concentric phase starts, the muscles shorten to start the movement.

There are two reasons why more force and power are delivered. The first reason is of mechanical nature. Because a counter movement is used, energy is stored in the elastic parts of the muscle such as tendons. When this counter movement ends and the target movement starts this elastic energy is released adding to force production of the active muscles.

The second reason that more force is generatedPlyometric training explained is neurophysiological. Because the muscle is stretched during the counter movement, muscle spindles are activated. Muscle spindles protect the muscle from stretching too far and start activating the muscles stopping the counter movement and, since the muscle is already activated, force can be build up more quickly. Normally it takes some time for a muscle to achieve maximal activation when a movemnt is started. A counter movement causes muscle activation to be at a higher level at the start of a concentric phase than without a counter movement, so more power can be delivered as a result.

Although this seems like an ideal exercise method, this form of training is not suitable for beginners. The high impact forces during training cause a higher risk of injury than with regular resistance exercise. Proper technique is required and the athlete must already have an above average strength level. Another factor is the weight of the athlete. Most guidelines don’t advise it for athletes weighing more thatn 220lbs/100kg because of even higher risk of injury.

References:

-Baechle, T.R., Earle, R.W.(2008). Plyometric Training. Essentials of Strength Training and Conditioning.Third Edition.USA Human Kinetics.

-Wilmore, J.H., Costill, D.L, Kenney W.L. (2008). Principles of Exercise Training. Physiology of Sport and Exercise Fourth Edition. USA. Human Kinetics.