From: "Exercise and sports performance"
Author: Edo Patregnani - Edizioni Ermes 1990
energy for the athletic movement (as explained in fitness) is produced by the contraction of muscles. Contraction - broadly - can be categorized into three broad categories:
isotonic (contraction causing movement of the affected limb);
isometric (causing no movement);
plyometric, or an isotonic contraction preceded by stretching the muscle.
This type of contraction is the most common form of muscular activity seen in running, jumping and other exercises in many sports.
in the throws in track and field, the muscle groups involved in the movement suffered a final stretch before the action of the launch itself, stretching is caused by the force of inertia of some parts of the body is from of the various tools used (weight, disc, javelin and hammer). Jumps usually a leg or both are "stretched" by the combined mass of the athlete and his speed (kinetic energy) prior to contract again. In the race, when the foot hits the ground contracted muscles of the leg and thigh are stretched forcibly, before the next forward thrust-up.
exercises in which the concentric contraction of the muscle is preceded by a strong strain of the same muscle contract, fall under the name of plyometrics. The plyometric exercises are based on the following principles:
good job - when the muscle contracts to produce kinetic energy (acceleration of the body or a tool), takes a positive work;
negative job - when the muscle in a state of tension rather than shortening is stretched by a mass that moves and the force used is acting in the opposite direction to the movement of this mass (for example, is down the stairs), there is a negative work.
We have already seen that the positive work of the energy produced by the contraction is transformed into mechanical work and heat. In negative work, however, the energy produced is transformed into heat and some part in elastic energy, which can be immediately used to make a further positive work.
Like a bouncing ball, in which the forward displacement of the same is due to high elastic energy accumulated due to its deformation at the moment that hits the ground, the energy stored in the elastic component (muscle ) and the series elastic elements (tendons) is returned during the next thrust forward and upward.
During the phase in which the muscle performs a negative work is an accumulation of potential energy of elastic deformation, which after the transition from collapse to that of tolerance can be transformed into kinetic energy.
In physiology, the distinction between positive work and negative work is very important, as the force that a muscle can develop is very different if the same is shortened by the contraction is pre-stretched or a negative work.
We report briefly the fundamental characteristics of negative work:
- If the speed of muscle relaxation is relatively low there is production of thermal energy (heat) - if the rate of relaxation is relatively high you get an accumulation of elastic energy that can be returned when the muscle after being stretched, will shrink in the next contraction - muscle tension increases in proportion to the speed with which it is stretched.
Back to propose, now, the analogy between a sphere elastic and muscular action. If a ball is dropped on the ground the height of the rebound will depend on both the elasticity of the sphere, ie the resistance that opposes the surface deformation in the collision, both the speed with which the ball hits the ground, that is, its energy kinetics.
from all this is that to obtain optimum height of bounce we need both a strong kinetic energy of an elastic body is ideal. The kinetic energy depends on the height of fall (ie the speed of vertical or horizontal). The elasticity depends on the properties of the muscles (actomyosin bridges). One question now arises of how to maintain the right balance between the need to increase strength and improve the level of elasticity of the muscle, since it is established that a repetition of exercises with maximum loads (particularly isometric) reduce the possibilities elasticity of the muscle itself. The answer can not that come from the study of the characteristics of the object of sport training. In throwers in track and field, for example, where the increase in mass in relation to the increase of the force (absolute force then) is a necessary condition for achieving high results, we will work mainly in sectors and at different times: it first increase in absolute force and later the elasticity of the muscle groups that will give the character explosive technical movements. For the jumpers, however, as for many other sports, where the elastic nature of the muscle groups involved in the gesture (the leg) plays a crucial role both as the force increase on, then a training method that provides for the improvement of the characteristics mentioned muscle (strength and elasticity) in synchronous mode. It is also advisable to work through drills specific character: the angle of the leg thrust, the magnitude and the time of loading (negative work) and the elastic phase of restitution (posi-tive work) should be as similar to what occurs in the action of specific race. Furthermore, in addition to the plyometric type exercises, he will perform with other mechanical properties similar to the action racing technique.
But how to be a proper plyometric training?
We have seen that the work is manifested in plyometric exercises, which tend to an optimal and simultaneous increase of the capacity strength and elasticity. To do this you must: - complete specific exercises that mimic the key part of the action that you want to improve - look after a muscle relaxation exercises involved, or subject to conditions of gravity (for the lower limbs, for example , should lie down or sit with arms relaxed and resting on top), - perform 5-7 reps with a subsequent recovery of 10-15 minutes between sets, with actions to eliminate any residual tension of the muscles, the number of sets will be determined by the athlete's ability to maintain high responsiveness (ie ability to work positively) when returning the previously stored in the elastic force negative work - keep your muscles decontratta before the special measure of contracting with a maximum in the moment before impact (reactivity) for the effective absorption of kinetic energy. Type exercises for the lower limbs consist of the best jumps from a height and bounce in the vertical elevation.
The condition for an elastic action of the year is given by:
- fall in relaxed muscle, on the toes, - setting of the ankle, knee and even at the climax of the elastic phase of impact on the ground - bounce and immediate search for the maximum height of jump.
For optimum height of fall is the height from which it reaches the next hop higher. This height is determined by trial and error, plus 10 cm at a time until you reach one that will determine the next largest vertical leap. Obviously, the more the athlete will need to be stronger than optimal heights of fall high, as high strength indexes require more energy to stretch the muscles involved, the top high jumpers fall from 80-100 cm qualification with a subsequent rebound vertical the same number of centimeters.
This exercise can also be done with the use of one limb, and indeed this is a method that is inserted, usually in the training plan for the development of explosive power (of ' limb detachment) of the high jumper. The conditions for the creation of elastic action are the same as previous years, is the same as the real optimal height of fall. The flexible support limb detachment must take place at 2.10 to 2.50 m from the starting point, that is the last hold on the elevated area. The jumps on the steps down depict clearly the analogy between a bouncing ball and plyometric exercises. These exercises and, by analogy, those represented by leaps obstacles to equal standing together, pitch in and out, and the next alternate on a limb, are often used by jumpers in athletics and team sports athletes, such as volleyball, which require, at least in part, of a similar work.
The most common mistakes in training plyometric
In any plyometric exercise if the height (or length) is not reached maximum, or at least growing, this is mainly due to: - poor coordination Executive - enough to stretch the elastic potential due to a drop height of not very high - in excessive strain due to a drop too much (not compared to the capabilities of force the subject), in which case the voltage decreases, probably due to the partial influence Inhibiting the corpuscles of Golgi tendon that protect the muscle strain from excessive loads.
plyometric exercises in the activation of the CNS is very high. The system intervenes progressively inhibit the ability of elastic response of the muscle when the exercise involves a commitment not proportional to the real potential response.
In the annual cycle of plyometric training exercises should be included in the second half of the period of preparation.
During the period of notice they need to maintain the condition of special training: in this period must be completed, as a rule, once every 10-15 days. Noting, however, the executive precautions: - research optimal height of fall should have a gradual and must begin with a minimum height - you should initially make leaps forward in the direction of high-and then, after adequate preparation, those vertical direction - the plyometric exercise must be preceded by a good heating, to avoid damage to the muscles, tendons and joints - should use extreme caution in athletes with developmental stage, and generally with all those who have not reached a satisfactory level of maximal strength, - with the beginners, who have not mastered the exact specifications of the specialty practice, the specific plyometric exercises are dangerous and counterproductive.
The effectiveness of each exercise for explosive strength depends mainly on the ability to maintain high possibility of fast response to impact next spring (negative work) and optimal excitation of the CNS, must be thereby preventing any visible manifestation of fatigue, failure to do so will inevitably result in a slowdown of the exercise of executive action.