The next phase within the biomechanics of the kick is the swinging or cocking of the kicking limb in preparation for the downward motion towards the ball. During this phase the kicker’s eyes are focused on the ball; the opposite arm to the kicking leg is raised and pointed in the kicking direction to counter-balance the rotating body. As the plant foot strikes the ground adjacent to the ball, the kicking leg is extending and the knee is flexing. The purpose here is to store elastic energy as the swinging limb passively stretches to allow a greater transfer of force to the ball during the downward phase of the kick (Barfield).
Before the end of the swing phase when the hip is nearly fully extended and the knee flexed, the leg is slowed eccentrically by the hip flexors and knee extensors. This is the phase of the kick where there is maximal eccentric activity in the knee extensors. Hip flexion and knee extension The powerful hip flexors initiate this next phase of the kick. The thigh is swung forward and downward with a concomitant forward rotation of the lower leg/foot. As the forward thigh movement slows, the leg/foot begins to accelerate because of the combined effect of the transfer of momentum and release of stored elastic energy in the knee extensors.
The knee extensors then powerfully contract to swing the leg and foot forwards towards the ball. As the knee of the kicking leg passes over the ball, it is forcefully extended while the foot is forcefully plantar-flexed. This exposes the inside top part of the foot (medial dorsum), which is propelled at the ball. There is a linear relationship between foot velocity and resultant ball velocity (Barfield). Foot speed is governed by a combination of hip rotational torque, hip flexor strength and quadriceps strength.
At the end of the swing phase, just prior to ball/foot contact, the hamstrings are maximally active to slow the leg eccentrically. This phenomenon is known as the ‘soccer paradox’, where the knee flexors are maximally active during knee extension and the knee extensors are maximally active during knee flexion. Foot contact with the ball According to various studies, the foot is in contact with the ball for 6-16 milliseconds, depending on how well inflated the ball is. At the point of impact 15% of the kinetic energy of the swinging limb is transferred to the ball.
The rest is dissipated by the eccentric activity of the hamstring muscle group to slow the limb down (Gainor). And because of the large forces involved, this stage in the kicking action is the most likely to produce injury to the hamstrings. At the instant of impact on the kicking leg, the hip and knee are slightly flexed and the foot is moving upwards and forwards. Follow-through The follow-through of the kick serves two purposes: to keep the foot in contact with the ball for longer; and to guard against injury.
As in all ballistic movements, a longer contact time will maximise the transfer of momentum to the ball and thus increase its speed (Barfield). The body protects itself from injury by gradually dissipating the kinetic and elastic forces generated by the swinging, kicking limb after contact. Any sudden slowing of the limb would increase the risk of hamstring strain (Hay). Table 3: Muscular action during follow-through (right-footed kick) Body Part Action Muscles Right hip Eccentric external rotation, eccentric extension and
eccentric abduction Hamstring group, posterior fibres of gluteus med, quadratus femoris, piriformis and gluteus maximus Right knee Eccentric flexion Hamstring group Injury risk from kicking The two main dangers during the kicking action, hamstring strains and posterior ankle impingement, have been flagged up above. Osteitis pubis has been identified as a third risk (Brukner). The main likelihood of osteitis pubis would arise if there were insufficient hip extension or hip internal/external rotation to perform the kick.
The motion would then be transferred elsewhere in the kinetic chain, possibly proximally, producing excessive compensatory motion through the pubic symphysis.
References
Barfield, B. “The biomechanics of kicking in soccer”. Clinics in Sports Medicine. 17(4): 711-728, 1998. Brukner, P. , and Khan, K. Clinical Sports Medicine (2nd ed). Roseville: McGraw-Hill. 2001 Gainor, B. , Pitrowski, G. , and Puhl, J. “The kick. Biomechanics and collision injury”. Am J Sports Med. 6:185-193. 1978. Hay, J. Biomechanics of Sport Techniques. Prentice Hall: New Jersey. 1996.