In jumping the player is producing vertical velocity for the center of gravity through plantar flexion in ankle, knee extension, hip extension, trunk, head and arm movements. During takeoff a key factor is to utilize all positive net impulse effects for a high vertical release velocity of the center of gravity of the player. Timing of the segmental movements is a real skill factor in jumping.
Jumping skills are most important for field player in moving header and for goalkeepers in all takeoff actions. The goalkeeper must be a skilled jumper, as many of the saves he will be asked to make will require a maximum jump to reach the ball. The majority of the jumps made by the goalkeeper will be performed using a one legged takeoff, as he often will be moving toward the ball at takeoff and it is faster to use the one legged takeoff. However, since a higher jump can usually be attained using a two legged takeoff, this takeoff is recommended for high shots near the crossbar. As soon as the ball has been released and a high shot is anticipated, the goalkeeper should deepen his crouch, and prepare for the jump by lowering the arms. From this position, the arms accelerate upward in the direction of the jump, and the trunk is also extended upward. The legs then forcefully extend in the direction of the jump, to project the goalkeeper as high as possible in the direction of the ball. The principle of maximizing the impulse at takeoff must be observed in this skill, so the goalkeeper will increase both the force applied at takeoff, and the time over which that force is applied. The range of the arm swing, trunk extension, and depth of crouch will all determine the height attained on the jump. As the goalkeeper lowers his center of gravity prior to takeoff by flexing the legs and flexing the trunk forward, and lowers his arms, he will increase the forces applied to the ground as he accelerates them upwards takeoff.
In the one legged takeoff, the goalkeeper takes a long step onto the takeoff foot, and both the arms and the free leg are accelerated upward just prior to the leg drive. The takeoff leg is then forcefully extended to drive the body upward in the direction of the ball. The goalkeepers and soccer players should be two footed, that is he should be able to takeoff from either foot and still reach maximum height.
Segmental contributions to the rise of the center of gravity in vertical jumps has been highest in knee extension (55 %), then plantar flexion of ankles (25 %). The role of trunk extension and arm movement has been about 10 % from both. For the coach it is important to recognize the priority of different joint and segment movements. If any of these joint actions are not used the output of jumping performance will not be maximal (Luhtanen 1978).
Jumping ability is important for field players in heading and goalkeepers in diving saving. A jump in match conditions can be executed with or without run-up. The take-off without run-up can start from a squatting position , erect position with counter movement or with preparatory movements by arms. The number of jumps in a match is on average 11-20 in a match and headers 9.9 + 5.7. The players jump for headers mainly after run-up. The rise of the center of gravity (C.G.) has not been measured in real football match conditions. The laboratory measurements from standing position indicated that the highest average heights of the C.G. reported in football were in squatting jumps 40.4 cm, counter movement jumps 43.5 cm and in vertical jump with preparatory arm movements 59.8 cm (e.g. Faina et al. 1988, Bosco et al. 1989, White et al. 1988).
The real meaning of jumping ability varies according to the game. In football, the good ability is needed but applied relatively seldom. In volleyball, the excellent jumping ability from physical point of view (explosive strength and jumping endurance) is a key factor.
The real match conditions require that the players can apply their jumping abilities according to the prevailing match situation. Then, the timing in jumping and technical heading performance are important.
A model to evaluate vertical jumping performance can be created in different ways. From point of view of biomechanics the total height reached for a header after take-off in jumping is due to e.g. body height, take-off force and duration of the applied force, vertical release velocity produced according the principle of force impulse and body position and movements for heading in air (Figure 9).
Main attention should put on the mechanics and mechanism of take-off performance.
Figure 9 . A model to evaluate jumping performance
The force impulse production for the release velocity in vertical direction can be evaluated as vertical net impulse of the applied force or indirectly evaluating the amount of total energy observed in different phases, stored elastic energy during impact or eccentric phase, work done during ascent or utilization of the elastic energy and mechanical energy losses due to the inhibitory factors.
Several factors can be identified influencing on the force production during take-off movements in different body segments, joints, muscles and tendons from point of view of neuromuscular mechanism:
From the neuromuscular point of view the stretch-shortening cycle behavior can explain the positive and negative factors for the muscular force production. A part of the imposed energy during stretching may be stored as potential energy and can reappear during the subsequent shortening of the muscle. Thus the concentric work done by the muscle would not be derived entirely from the transformation of chemical energy, but also from stored mechanical energy. The ability to use stored elastic energy is affected three variables: time delay between stretching and shortening of the muscle, magnitude of stretch and velocity of stretch. It has been demonstrated that there should not be time delay between eccentric and concentric contraction; otherwise , some of the stored elastic energy would be lost. The greater is the velocity of stretch, the more elastic energy is stored in the elastic components of the muscle. If the magnitude of the lengthening is too great, a lesser number of cross bridges will remain attached following the stretch, and hence less elastic energy will be stored. It has been speculated that inhibitory influences surpass the facilitatory potentiating effects when the stretch load increases above the optimum. It has been shown that during muscular fatigue the transfer of mechanical energy between the eccentric and concentric phases is drastically reduced and muscle stiffness regulation is altered.
From mechanical point of view for the take-off performance can be applied the principle of energy conservation, energy production from external and internal forces and energy transfer from energy state to energy state. Basically, this means the ability of the player to utilize the energy of run-up (due to the velocity), elastic energy (stored in different tissues, shoe and surface), transfer of the kinetic and elastic energy into the potential energy (rise of C.G.) and transfer of the energy between body segments in an optimal way.
The influencing factors in reaching maximal rise of C.G. jumping performance have been explained in the text.
In developing training process for progression of the jumping ability, it is important to identify the strengths and weaknesses of the players inside of the influencing factors in a priority order. Then the process includes:
In analyzing individual players according to tests, it is essential to evaluate players as individual in their:
The planned individual training programmes have to be individual according to analysis and follow-up in:
In individualizing training programmes, selected scientific principles of the progressive training can be applied combining with strength and speed training:
Player 1 had (Fig. 10) a low rise of C.G. in jumping test and high isometric leg strength level. Player 3 had a high rise of C.G. in jumping test and low isometric leg strength level. Practically these results mean different training programmes for the player 1 and 3.
It can be concluded that the training principles for football players could be clear if only to maximize their jumping abilities through basic strength and explosive leg strength training. Different multi-step jumping exercises and plyometrics training should be included in the programme.
For jumping strength training should be applied progressive principles as follows:
A key factor for players and coaches is that how much time to spend only for jumping training. It has to be integrated at least with speed training. The value of combined speed, jumping, agility, co-ordination and skill training is the highest for the total development of top players.