The release velocity of the ball in the maximal instep kick for skilled soccer players has been reported by several investigators, as follows: 17-28 m/s (e.g. Zernicke & Roberts 1978). Calculations from the 1990 World Cup in Italy on television indicated that the velocities produced by the top professional players could reach the speed of 32-35 m/s. Using typical values for the masses of the foot and the ball, it could be suggested that the ball should travel at about 1.2 times the velocity of the foot. Sub-maximal kicking also appears to follow this general relationship. It has been reported a regression equation between the two variables of foot and ball velocity over a ball speed range of 16 to 27 m/s as follows: V(ball) = 1.23 * V(foot) + 2.72 (Zernicke & Roberts 1978).
Figure 2. Release velocity of the ball in maximal kicking at different age levels (Luhtanen 1984)
Kicking movement in football is a relatively easy series of rotational movements. In this movement, the aim is to produce through the kinematic chain of body segments, high angular velocity to the foot. The length of the body segments or the radius of rotational movements influences the linear velocity of the rotating foot. Thus the body height and lengths of different body segments are an advantageous feature for players, because the linear velocity of rotating levers can be expressed as a product of the radius of rotational movement and angular velocity.
Figure 3. The last step and kicking phase in a maximal kick (Luhtanen 1984)
The role of the arms in kicking is primarily to maintain the balance of the body. The arms are usually extended out to the sides of the body during the forward motion of the kicking leg, to help to keep the center of gravity over the support foot, and to increase the moment of inertia of the trunk and increase resistance to rotation around the spine, or the long axis of the body. As the kicking foot contacts the ball, the opposite arm moves forward and upward across the body to help keep the trunk down and the body in balance.
The momentum of the kicking foot and leg is the product of the mass of the leg and the velocity of the foot at impact, plus the velocity of the body as the player approaches the ball. The greater the mass of the leg, and the greater the velocity of the foot at impact, the greater the resultant velocity of the ball at impact.
From the point of view of biomechanical principles in kicking the ball, the velocity production of the ball can be evaluated according to the conservation of the linear momentum in collision. The action of the ankle can increase the release velocity of the ball a little. Through elastic collision, linear momentum transfers partly to the ball. The bigger is the leg mass the higher the ball velocity. The point of application must be inside the effective hitting area, which depends on the tension in the ankle.
The acceleration of the kicking leg, and the resultant velocity at impact, is determined by the muscle forces being applied by the kicker. It has been reported that the speed of the ball at impact was directly related to the measured strength of his subjects.
The release velocity of the ball with respect to timing had the strongest relationship to the maximal torque produced during the 1. hip flexion 2. knee extension and 3. short ankle stabilizing in the kicking leg. Also the relationship between the maximal resultant forces of the thigh and shank and the release velocity of the ball was strong. The relationships between the release velocity of the ball and age was high but less than with weight or height. Thus the increase of the body mass means increase in the mass of the foot and this automatically increases the release velocity of the ball in the kick. The player can also influence the effective mass of the foot by instantaneously changing muscle tension in the muscles around the ankle. The regulation of the effective mass in the kicking foot might play an important role for getting the high release velocity to the ball (Luhtanen 1988).
Skilled senior soccer players performed maximal kicks. When their foot velocity was on average 20.1 m/s the release velocity of the ball was 27.4 m/s. In these fast kicks the evaluated peak muscle force produced in knee extensors could be about 2000 N (200 kg) (Zernicke & Roberts 1978).
The skillful soccer player produces high ball velocity by maximizing angular velocities of the thigh and shank. The accuracy of kicks depends mainly on the contact area of foot with the ball. The bigger the ball the better the accuracy. Accuracy in kicking have been the highest when the velocity of the ball has been 80 % of the maximal velocity (Asami et al. 1976).
Mechanically the kicker can produce the small extra velocity to the foot and ball by rotational movement of the support leg. The mastering of the biomechanical principles of movement - countermovement, and balance increase still further the release velocity of the ball in kicking. Additionally the utilization of muscular elasticity of hip flexors and knee extensors with correct timing means higher ball velocity.
An important skill in the game of soccer is the ability to kick the ball forcefully and accurately. The instep kick is the kick which is most often used for maximum force and distance, as for a shot on goal or a long pass. The force for the long kick is gained from the run-up into the ball, and from the motions of a maximum number of body parts. These include hip and trunk rotation, and hip flexion, knee extension and ankle plantarflexion to form a rigid surface for impact.
The kick with run-up produces longer and more powerful kicks than the standing kick. This is due to the increased momentum of the kicker at impact. If the kicker is moving forward horizontally at 4 m/s at impact with the ball, this velocity is added to that imported by the kicking leg as it moves about the hip joint (Opavsky 1988). However, some of the horizontal velocity is lost at the time of placement of the support foot, as the center of gravity must be slowed down to allow time for the full leg swing of the long kick.
The support foot remains firmly planted as the kicking foot makes contact with the ball. As the support foot is planted, the kicking leg is left well behind the body, with the hip hyperextended and the knee maximally flexed. The trunk is also rotated backwards and sideways toward the kicking leg, to increase the length of the backswing and to add the force of trunk rotation forward into the kick. The arms are extended out to the sides of the body, to aid in maintaining balance.
Knee linear velocity reaches its peak between 40-70 ms after peak hip velocity is reached (Isokawa & Lees 1988). The angular motion on the thigh segment stops when the knee is approximately in a position over the ball. The thigh is almost stationary at impact, while the leg and foot have reached peak velocity and zero acceleration (Huang et al 1982). The phenomenon of the thigh slowing down or stopping before impact with the ball prior to speeding up in the follow through may not be well known, since the time during which this occurs is so short it is difficult to observe visually. The exchange of angular velocities between the proximal and distal segments would suggest that there may be some transfer of angular momentum between the larger thigh and the smaller leg. Recent findings suggest that the thigh slows down due to the action of the shank as it accelerates toward the ball (Dunn & Putman 1988). It was concluded that in a kicking movement the thigh deceleration is primarily influenced by the motion of the lower leg, and not by the resultant joint moment about the hip.
Following the deceleration of the thigh, the shank then continues to accelerate until impact, so that the peak velocity of the shank and foot segment occurs just before the ball is contacted. It has been reported that the maximal angular velocity of the shank was reached on the average 9 ms before the contact of the foot and ball(Luhtanen 1988). Ankle and toe velocities reached their peak just before impact, and from 40-50 ms after the peak velocity of the knee. Swing time is the time from the landing of the support leg until the contact with the ball. The range of times for this portion of the kick has been reported to be between .13 and .15 second. It has been suggested that there may be two types of kicking pattern within the instep kick: one used a long backswing and had a longer kicking time, the other used a small backswing and moved the lower leg sharply by knee extension which resulted in a shorter kicking time (Isokawa & Lees 1988).
The motion of rotating body segments in kicking can be described in terms of angular position, displacement, velocity or acceleration. The linear velocity of the rotating foot hitting the ball is directly proportional to the sum of both the angular velocity and the radius of rotation of the consecutive segments. The timing of these consecutive rotational movements is important in relation to the impact of the foot with the ball when maximizing the release velocity of the ball in kicking. The linear momentum of the kicking leg transfers to the ball according to the relationship between the force impulse and the change of linear momentum. The angular acceleration of the segmental link system in the kicking leg depends on the external torque of the muscles producing the rotation of the thigh, shank and foot and on the resistance provided by the rotational inertia of these leg segments being moved by the motive moment. The mobility of joints is a precondition for the optimal shooting skill. The inertial resistance of the thigh, shank and foot is determined by the distribution of the mass relative to the axis of rotation.
Many practical benefits can be found when trying to increase the release velocity of the ball. This can be reached by increasing the velocity of the foot mechanically for the contact phase with foot and ball, by leaning the body away from the ball and balancing the body with extended arms during the kicking movement. The moment arm is defined as the perpendicular distance from the axis of rotation (usually through a body joint), to the center of gravity of the resistance, in this case the ball. The greater the distance from the center of the ball, to the center of the active joints in the kick, the longer the lever system acting, the faster the speed of the kick. By fully extending the leg at impact, and leaning away from the ball, the kicker will increase the speed at the end of the foot.
The skilled kicker is seen to lean sideways away from the ball during the swing of the leg and through impact with the ball. Although the kick occurs primarily in the sagittal plane, the trunk is seen to lean markedly toward the non kicking side. By leaning the body away from the ball, this lifts the kicking hip upward relative to the ground, and helps to clear the toes of the kicking foot and prevents them touching the ground. Since the ankle is maximally extended in the instep kick, the hip must be raised to allow the foot to point downwards during the kick. This lean of the body also allows a wider swing of the kicking leg, increasing the length of the moment arm for rotation around the left hip.
An important aspect of the soccer kick is the interplay between the various muscle groups active in the skill. The agonists contract to initiate the movement at each of the joints, but these muscles become the antagonists to slow the rapid angular movements at the joints just prior to or following release of the ball (De Proft et al. 1988). The hip flexor muscles are dominant during the majority of the swing to the ball. They are initially contracting eccentrically, to stop the leg's backswing; then their activity becomes concentric to accelerate the thigh towards the ball (Robertson & Mosher 1985). Just prior to ball contact the hip extensors, for example the hamstrings become dominant, causing the thigh and knee joint to slow down or even stop in some athletes. It can be suggested that this antagonistic activity of the hamstrings has a protective role for the knee joint, as the hamstrings work with the anterior cruciate ligament to keep the tibia in contact with the femoral condyles. The hamstrings pull back on the tibia, to keep it aligned with the femur and maintain the integrity of the knee joint. The follow through is characterized by a flexor concentric contraction, followed and an eccentric contraction. The knee extensors, the quadriceps group, are the dominant muscles during the backswing and downswing. These muscles act eccentrically initially to reduce the rate of knee flexion caused by the leg's backswing and the hip flexor shortening moment. The knee extensors then act briefly to shorten and cause some degree of knee extension. The knee flexors quickly become dominant just prior to ball contact, acting eccentrically to actually reduce the rate of knee extension. This is an interesting finding, as one would expect knee extensor activity through contact. However, it was found no knee extensor activity just prior to ball contact, and in fact the flexors were dominant, eccentrically, causing a reduction in the rate of knee extension. The knee flexors, especially the hamstring group, may be acting to prevent hyperextension and possible damage to the knee. When comparing the muscle activity in the soccer kick between skilled and less skilled soccer players.
The skilled players showed greater muscle relaxation of the antagonistic muscles in the swinging phase (Bollens et al. 1987). There was also greater peak muscle activity in the knee extensors during the swinging phase.