Saturday, 6 October 2012

Plyometric Science of Soccer

Unlike typical strength training exercises that involve long, slow movements designed to increase muscular strength and mass, plyometric exercises for quick explosive movements designed to increase speed and power.
A plyometric exercise consists of three phases. The first is a rapid muscle strengthen movement called the eccentric phase. Second comes a short resting period called the amortization phase. Finally, the athlete engages in an explosive muscles shortening movement called the concentric phase. The athlete repeats this three part cycle as quickly as he can.

The goal of the plyometric exercises is to decrease the amount of time in-between the eccentric and concentric movement. By reducing the time in-between these two movements, a man can become faster and more powerful.

Eccentric (Plyometric) Training:

1.Stretch-Shorteneing Cycles:
When a muscle is stretched prior to a concentric action, the tension within the
muscle is potentiated during the shortening phase. The elastic elements within the muscle store energy which is then released once the muscle begins to shorten. In activities such as jumping, the performance is enhanced when a
counter-movement precedes the jump. Exercises which employ such stretch–shortening cycles of muscle action are referred to as plyometric training.

    Hopping, on one leg or on both legs together (bunny hops), is a basic form of plyometric training. In this activity the hips should not sink below the level
where the femur is parallel with the ground in order to avoid placing undue
strain on the knee joint. The hops may be performed over a series of hurdles or can include diagonal movements over a low bench. Lateral movements can be designed so that the adductor muscles are activated. It is important that the muscles have experienced some conditioning work before formal plyometric training is attempted. Stretch–shortening exercises induce soreness which peaks about 48–72 h post-exercise. The soreness is linked to micro trauma within the muscle’s ultra structure causing disarrangement of the Z discs within the myofibrils. There is local inflammation and damage to the sarcolemma through which creatine kinase leaks into the bloodstream. This enzyme has been used as a marker of muscle damage caused by eccentric contractions. With training the damage is reduced, a phenomenon known as the ‘repeated bouts effect’.

   Delayed onset muscle soreness is therefore not a lasting problem, but players carrying an injury should be excused from intensive plyometrics. Bounding, hopping and jumping exercises can be included within any training system but formal plyometrics at high intensities can be restricted to twice a week. Skipping, lunging and footwork drills all entail plyometric activity at low intensity. Other means of inducing stretch–shortening cycles may be incorporated in the high-intensity sessions.

2. Depth Jumping:
Drop jumping or depth jumping utilizes the individual’s body weight and gravity to exert force against the ground. The individual steps out from a box, drops to the ground and immediately drives the body upwards as quickly as possible. The eccentric part of the action where the lowering of the body is controlled is known as the amortization phase, before the body is directed vertically. Learning to co-ordinate the whole movement into a smooth performance is essential for this exercise to be fully effective.

  Depth jumping has been adopted with success by high jumpers and triple
jumpers. It is relevant in soccer where the lower-limb muscles generate high
power output in fast ‘explosive’ actions. It was originally prescribed by
Verhoshanski (1969) who recommended a box height of 0.8 m for achieving
maximum speed and 1.1 m for developing maximal dynamic strength. He
recommended no more than 40 jumps in a single work-out. In later studies a box height of 20–40 cm was thought to be sufficient.

3. Squat Jumps:

Here the athlete jumps high into the air from a squat or partial squat with a
loaded barbell supported on the shoulders. Good co-ordination is essential to
prevent overbalancing on landing. Frequently a towel is used underneath the bar to cushion its jarring effect. When ascending it is necessary to pull down hard on the bar to avoid its bouncing against the back of the neck. It is particularly beneficial to athletes such as soccer players who during performances move the body explosively against gravity.

   An alternative exercise is to dispense with any equipment and perform a split squat with a cycling action in mid-air. After jumping upwards, the front leg kicks to the rear position and the back leg comes flexed to the front. The individual lands in the split-squat position to jump again immediately.

4. Pendulum Training:
The pendulum method of training was developed in eastern Europe as a means of inducing stretch-shortening cycle exercises without the accompanying delayed-onset muscle soreness. The individual is located on a set-up as in a child’s swing and pushes towards a wall, absorbs the ‘shock’ and immediately pushes off again. The desired muscle actions are produced whilst body weight is supported on the seat.

   In experimental studies a force platform was built into the wall to record the forces generated when training using the pendulum method. Fowler et al. (1997) established that the system was effective as a training method and reduced both the acute load on the skeleton and the degree of transient muscle damage. Its use is limited to one individual at a time and it has been adopted more for laboratory work rather than as a specific usable training tool.

5. Complex Training:

Different forms of training at high intensities may be combined into a single session. This integration is referred to as complex training. It might include jumping actions with overload in formal arrangements such as 3–4 sets of 6 repetitions. These may be performed alongside repetitive bounding, for example for 30 s, and exercises with loose weights. The regimen includes concentric as well as eccentric actions and exploits the force–velocity characteristic of muscle, most of the exercises being performed towards the faster end of the curve.

Isokinetics describes the form of exercise permitted by machinery with the
facility to adapt resistance to the force exerted. Normally when weights are
lifted through a range of movement the maximum load is limited to that sustainable by the muscles involved at the weakest point in the range. Consequently other points within the range undergo sub-maximal training stimuli. With isokinetic machines this problem is overcome as the speed of
contraction is preset, a speed governor in the apparatus allowing the resistance to adapt to the force applied. In this way, the greater the effort exerted the greater the resistance, and maximal effort can be performed throughout the complete range of movement. Where comparisons have been made, training programmes using isokinetic machines have proved superior to isometric and typical progressive resistance programmes with high speeds producing best results. These results may reflect the fact that the training programmes are mostly evaluated using isokinetic equipment.

   Modern isokinetic equipment permits eccentric as well as concentric actions.
Typically, the top angular velocities available on the equipment tend to be
higher under concentric than in eccentric modes of action. Nevertheless the
angular velocities that are possible are well below the maximal velocities
achieved in playing actions such as kicking a ball.

   Training at high velocities is likely to assist slow movements whereas training using slow movements is likely to be velocity specific in its effects. The velocity specific adaptations are linked with the pattern of motor unit recruitment. Improvements in muscle strength are to be expected from training at slow angular velocities, due to the recruitment of a large population of motor units. Such actions close to maximal efforts can induce muscle hypertrophy, provided repetitions are sufficient and the programmes of training is sustained for some months. In order to avoid muscle hypertrophy whilst at the same time improve strength (due to neuromotor factors), no more than 3 sets of 6–8 repetitions are recommended.

   Isokinetic facilities are expensive and so are not normally available for team
training. Their main benefit is in training muscle strength during rehabilitation.
In this instance it can be allied with physical therapy in comprehensive
progressive programmes.

   A limitation of isokinetic exercise is that it may interfere with the natural
pattern of acceleration employed in competitive actions. Furthermore,
movements are linear and so do not correspond to musculo-skeletal function in the game. Nevertheless isokinetic apparatus is very effective in identifying
deficiencies at individual joints. The appropriate muscle groups can then be
isolated for remedial training.

Multi-Station Equipment:

Use of weights is implemented in circuit-weight training in which a series of
separate exercises is organised for sequential performance in a circle. The individuals rotate in a circle as they make progress through the training session. The circuit should allow variation of muscle groups involved between work stations to avoid cessation of work due to local muscular fatigue. In theory this method is ideal for team training provided the number in the group does not exceed the number of work stations laid out. In practice, group organisation invariably presents some problems as do inter-individual differences. Where weight-training is included in the circuit, a fixed load may not be suitable for all or many of the group while altering the loads slows up the performance and allows untimely recovery. Ideally a homogeneous group, a thoroughly well organised routine and repetition of the circuit or supplementary training are necessary to achieve objectives.

Multi-station exercise machines overcome the organisational problems of
circuit-training and the injury risks of weight training using traditional resistance modes.Muscle groups are rotated from station to station and use of the machine involves abdominal, leg, shoulder, arm and back muscle work. Physiological studies have shown the training stimulus to the circulatory system is significantly greater than conventional circuit-training routines (Reilly and Thomas, 1978). However, as delay in altering loads at any one station is minimal, the circuit of 12 stations can be repeated to perform two or more sets in a training session.

The advantage of stationary equipment for resistance training is that safety is
secured compared to the risk of accidents in using free weights. The arrangement at each station can be changed quickly to accommodate different physiques and capabilities. This type of equipment is available at most fitness centers and sports training complexes.

Technological Aids:

Mechanical devices
Various devices have been developed over the years for facilitating strength
and power training. In some instances the commercial claims have not been
supported by laboratory studies. Some of the most positive constructions are now described.

Pulley systems incorporate both concentric and eccentric actions for back and
thigh muscles. The best devices regulate the magnitude of the eccentric phase by reference to the concentric actions.

Exercise machines with the arc of motion dictated by a cam device have been
in use for some decades. The cam design allows the resistance to be increased at parts of the joint’s range of motion where force is decreased. In this way the resistance accommodates to the force exerted throughout the range of movement, conforming to the force–angle curves for the joint in question.

Vibration plates:
Vibration platforms have been utilized for strength training in some of the top
European soccer clubs. These systems have also been used by professional Rugby Union players as part of their conditioning program. There have been some positive results but it seems that these apply to specific vibration frequencies. Devices like the Galileo Sport machine (Novotec, Germany) have a tilting platform that delivers oscillatory movements to the body at frequencies from 0–30 Hz around a horizontal axle. The individual is largely passive, merely standing on the platform whilst it vibrates at a preset frequency.

For many years ergonomists have recognized the potential adverse effects
of vibrations delivered to the human body by hand-held tools such as pneumatic drills and chain saws. More recently exercise devices that include whole body vibrations have been promoted as training aids. Vibration of soft tissue is a natural phenomenon; the tissues in the lower limbs vibrate at their natural frequencies with the shock impact of landing as the heel strikes the ground on each foot strike. Ideally the frequency of stimulation should be tuned to the resonant frequency of the individual to induce training effects. Results of studies have been mixed, negative results being attributed to amplitudes that were too low, incorrect frequencies or duration that were too long (Cardinale and Wakeling, 2005).

Benefits to performance have generally been observed with sinusoidal
vibrations at frequencies of 26 Hz and amplitudes of 4–6 mm. Acute applications of whole-body vibration for 5 min at 26 Hz and 10 mm amplitude were reported to shift the force–velocity curve of well-trained subjects to the right (Bosco et al., 1999). Other positive effects have been observed on skeletal health and on flexibility (Cardinale and Wakeling, 2005). Acute enhancement of flexibility and muscle power has been attributed to stimulation of the muscle spindles and recruitment of additional motor units via activation of multiple nerve synapses (Cochrane and Stannard, 2005).

Issurin (2005) distinguished whole-body vibration from vibratory stimulation
of local tissues combined with strength-training exercises and static stretching. In these instances vibratory stimulation is superimposed on muscle contraction or stretch. The balance of research evidence indicated positive effects with short-term use but suppressive effects on muscle force when stimulation was prolonged (6–30 min). Greater effects were observed in dynamic than in isometric muscle actions and were most pronounced in fast movements. The main benefit of this method may be in physical therapy for individual muscles.

Functional overload:
Various forms of natural resistance may be provided to overload the active
muscles. These include running uphill, on sand dunes or ankle deep in water.
Traditionally soccer coaches used stadium terrace steps in the pre-season training of their players.

Activities related to the game may also be set up with overload in mind.
Jumping with ankle weights or jackets weighted with lead are examples.
Graham-Smith et al. (2001) examined the use of an additional load of 10% body weight in the form of a Power Vest© during a typical plyometric training session of 5 sets of 10 repetitions of vertical jumping with 3 min recovery between sets. them to conclude that this model was a safe device for resistance training.

Improvement in performance can be pronounced: Bosco (1985) reported an
average increase in vertical jump performance of 10 cm after wearing a vest (11% of body weight) for only 3 weeks.

Running harnesses may be employed to create resistance against the athlete
attempting to accelerate from a standing start. The amount of resistance can
be controlled by a partner or trainer. Alternative means are attachment to a sled or similar load by means of an abdominal belt. For optimal effects the normal vigorous running action should not be unduly modified.
Parachutes have been advocated to increase air resistance when running but
their efficacy has not been seriously addressed.

Functional resistance to locomotion may slow the individual too much for the
resultant training effects to be of benefit to the football player. Murray et al.
(2005) showed that speed was generally reduced over 10–20 m as players towed weights ranging from 0–30% body mass, using a waist harness and a 5-m rope. Loads up to 10% body mass are favored and the procedure seems to promote peak anaerobic power rather than maximal running velocity.
Medicine balls may be utilized for improving sports-specific skills such as
throw-ins. Throwing distance can be increased with a dedicated strength training program for pull-over strength and trunk flexion. These balls can also be used for one-handed exercises in the case of the goalkeeper for whom the skill of throwing the ball long distances is directly relevant.
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