Plyometric training allows you to create a larger force used in the execution of skills such as jumping higher, and going faster. Isnít that what you want? SPEED and EXPLOSIVENESS. In order to achieve that SPEED and EXPLOSIVENESS, you must train for it. Imagine training for the snatch. You address the bar, set your body, shoulders tight, wide grip, back flat. Now, the snatch must be performed with maximum speed and explosiveness. So, if you donít explode out of the hole, will you make the lift? Doubt it. Your muscles respond to specifics. If you sport requires SPEED, and you train for a marathon, then your muscles will perform, in a marathon fashion. Are ya following me? In order to create SPEED and EXPLOSIVENESS, you must train with maximum SPEED and EXPLOSIVENESS. The Biomechanix of Plyometrics is not a complicated matter. What we want to do is create an opposite reaction for the previous action, just FASTER.
Now let's look at the most important muscular contraction involved with plyometrics.....
Past Soviet studies have found that in jumping movements your results are not limited by the amount of strength of takeoff, by the strength shown by the takeoff leg extensors and spinal extensors, but by the strength that these muscles show during their stretching (Eccentric contraction).
Jumpers use a kinetic energy reserve, acquired during the run-up and at the start of the take-off in what is called the Amoritization stage. In amoritization, there is some shock absorbing, which must take place when your foot first hits the ground and preparation for the following take-off by accumulating the energy of the hit.
The forces generated during the landing are significantly greater than during the take-off stage. Thus, the need for the most strength by jumpers is not for extension of the take-off leg, but to prevent excessive flexion of the hip and knee during amoritization. If excessive flexion of the support leg in this stage is prevented, then the final phase of the take-off, the push-off, is executed successfully.
The reason for this is that in this phase the external forces acting on the jumper's body are substantially decreased and the extensors begin to contract and return to their initial state just as all elastic bodies do. Keep in mind that the muscles are very elastic and resilient and when they are stretched and tensed, they exhibit great forces to return to the original state.
Thus, the key in plyometrics is to get MAXIMUM ECCENTRIC CONTRACTION, and then to have the central nervous system switch this contraction to concentric, which produces the desired movement. Keep in mind that the force developed during the eccentric contraction is greater than what can be generated volitionally.
In sports the entire skill is accomplished in most cases between one and two tenths of a second. Thus, the key to to get a maximally explosive push-off is to have the muscles respond with maximal force in the shortest amount of time. This can only be done by first getting a strong eccentric contraction, which must then be converted to a concentric contraction in the shortest amount of time.
Exactly what happens during a take-off and landing can be described by a rubber ball falling under the influence of gravity. At the moment of hitting the ground it flattens out somewhat due to its elasticity. Then it instantaneously takes on its former round form and bounces up. The height of the rebound depends not only on the level from which the ball was dropped, but also on the resilency of the ball. A steel ball will rebound higher from a cement floor than a rubber ball. In other words, the greater the resiliency of the material, then the more effective will be the utilization of the kinetic energy accumulated in the fall for the lift of the body upwards.
In the body, the ball is replaced by the extensor muscles, which act as resilient cords. Thus, when jumping from a height, your legs (knees, hips, and ankles) will, at the moment of landing, bend and stretch the resilent cords (muscles) of the respective joints. After some of the force of the landing action is absorbed by the eccentric contraction, the resilient muscles, in using the remaining force, will make you jump upwards. This is a very simplified view but it shows what happens when we execute a plyometric type exercise, especially in one of the methods called Depth Jumping.
A few words must be said about landing and the amount of flexion that takes place in the joints at this moment. If there is great flexion of the ankle, knee, and hip joints, the body will be in a very unfavorable anatomical position for work. If you want to become close to being like a resilient ball and to jump higher after landing, then you must withstand the load at the moment of landing with as little flexion of the joints as possible.
When there is great flexion of the legs because of too much landing force, it increases the time you spend on the ground and most of the forces are absorbed. The muscles undergo very little pre-loading or tensing.
Also involved is stretching of the muscles. It is well known from physiology that because the muscle possesses great elasticity, when it is stretched it will contract more forcefully and faster due to the tension ensuing from the stretching. This is known as the myotonic relex.
The key to developing explosive force, which is the main outcome from doing various forms of plyometrics, is the switch from the eccentric contraction to the concentric. This controlled by the nervous system and because of this, a good portion of training is directed to nervous system training, not just the muscles.
By having prior tensing of the muscle, there is a greater number of motor units involved in the contraction and the impulse frequency, the rate at which they are fired, is greater. In addition, through nervous sustem action there is greater synchronization of all the muscles involved and maximum mobilization of muscle fiber recruitment. In explosiveness there is simultaneous starting work of all the mechanisms involved to elicit a maximal contraction in the shortest amount of time.
To further illustrate the eccentric - concentric concept, we can use the example of a runner. Picture a runner running at maximum speed. At the moment his foot hits the ground he has a tremendous amount of force in the landing, which comes from the horizontal momentum and from the weight of his body when it hits the ground. The amount of force at this time is much greater than body weight and the faster the running, the greater the amount of force generated at touch down.
Some of this force must be absorbed and so the ankle and knee undergo sight flexion. However, the amount of flexion is kept at a minimum since you want to withstand most of these forces and switch them into the push-off quickly. As the flexion in the joints takes place, a strong eccentric contraction is elicited, which, when it becomes strong enough, stops the flexion. In the process, a great amount of tension is built up in the muscle.
At the moment maximum tension is developed, the central nervous system switches the eccentric contraction to a concentric and the push-off occurs. The total amount of time involved for this whole sequence is about one-tenth of a second (less in world class sprinters). The phase of amoritization (shock absorbing) takes approximately 0.05 seconds and the actual push-off approximately 0.05 seconds. The key in this case is to have as little flexion and stretching of the muscles as possible but to have maximum tension.
It takes many years to develop explosive power and as far as can be determined, it is not heredited. In order to develop explosive power you must train correctly.
Keep in mind that explosiveness is made up of two factors: Speed and strength. An increase in any one of these factors or both will lead to greater explosiveness and plyometrics can be adjusted to develop one or the other or both.