Biomechanics of Soccer: The soccer-style kick - a slow-motioncommentary on one of the most common sporting actions in the world

Sports all around the world make use of the complex skill of kicking to thrill and entertain us: soccer, rugby league and union, Australian Rules football, grid-iron football and gaelic football. Within each sport a variety of kicking styles has evolved to suit different ball types, game rules and the part that kicking plays in the game.

Here, however, we will examine in detail the kick that is universally used in the game of soccer (football), which is usually called the ‘place kick’, ‘soccer-style kick’ or ‘round the corner kick’. The same style of kick is also used for point-scoring in rugby league, union and grid-iron.

Physiology and development

The soccer-style kick lasts for no longer than five seconds, depending on the length of the approach. The intensity of the kick depends on how far the kicker needs the ball to travel or how fast it has to go. As with any action lasting less than 10 seconds, the kicker uses a purely anaerobic metabolic pathway to produce the necessary energy to kick – in other words, they are relying heavily on the ATP-PC energy system (adenosine triphosphate-phosphocreatinase) for this action⁽¹⁾.

Kicking is a complex motor task which we learn as children. Our kicking skill develops rapidly between the ages of four and six, and by the age of nine the pattern is mature – it does not develop further⁽²⁾.

The most common biomechanical difference between the elite and novice footballer is that elite footballers use a refined and consistent movement pattern where novices use a variable and inconsistent one⁽³⁾. A successful kick is usually defined in the literature either in terms of the velocity of the ball (which needs greater swing limb/ foot speed), or the accuracy of direction of kick, which relies on the position of the ‘plant’ (non-kicking) foot and hip position at impact.

Table 1: Muscular action during kicking preparation (right-footed kick)

Body part	Action	Muscles
Trunk	Stabilisation of rotation to the right	Abdominals, psoas major, erector spinae and spinal postural muscles
Right hip	Extension	Gluteus maximus and hamstring group
Left hip	External rotation and eccentric extension	Gluteus med, gluteus min, hamstring group and adductor magnus
Right knee	Flexion	Hamstring group and popliteus
Left knee	Eccentric extension	Quadricep group
Right ankle	Plantarflexion	Plantarflexors
Left ankle	Eccentric plantarflexion	Plantarflexors
Left shoulder	Abduction	Middle and anterior deltoid and supraspinatus

Components of the kick

For a detailed analysis, we can break the kick action into six stages⁽²⁾:

• the approach
• plant-foot forces
• swing-limb loading
• hip flexion and knee extension
• foot contact
• follow-through.

The approach

Children between two and three years of age generally toddle straight into the ball to try and kick it. As they get older they learn a paced runup and adjust their approach to the ball from front-on to a more diagonal angle. The diagonal approach produces greater swing-limb velocity for greater ball speed. Isokawa and Lees⁽⁴⁾ have shown that a 45-degree angle of approach produces the greatest peak ball velocity, compared to a 15-degree or 30-degree run-up.

Elite athletes tend to take longer strides than novices as they approach the ball⁽⁵⁾.

Table 2: Muscular action during approach and kick (right-footed kick)

Body part	Action	Muscles
Trunk	Stabilisation	Abdominals, psoas major, erector spinae and spinal postural muscles
Right hip	Internal rotation/hip Flexion	Tensor fascia lata, rectus femoris, psoas, iliacus, sartorius and adductor group
Left hip	Extension	Gluteus maximus, hamstring group and adductor magnus
Right knee	Extension	Quadricep group
Left knee	Extension	Quadricep group
Right ankle	Plantarflexion	Plantarflexors
Left shoulder	Horizontal adduction	Anterior deltoid, biceps brachii, pectoralis major

Plant-foot forces

Numerous studies have investigated the relationship between ground reaction force on the plant foot and ball speed, for both novice and elite footballers. These show that skilled players kick faster and produce greater ground reaction forces in all directions than the unskilled.

When kicking, there is a direct relationship between the direction that the plant foot faces and the direction in which the ball travels⁽⁵⁾. The optimal foot plant position for accurate direction is perpendicular to a line drawn through the centre of the ball for a straight kick.

Hay⁽⁶⁾ describes the ideal mediallateral position of the plant foot as 5 to 10 centimetres to the left of the ball (assuming a right-footed kick), although there is no empirical evidence to support this opinion. Barfield⁽²⁾ explains that if the plant foot is greater than 10 centimetres away from the ball, the direction of the kick and the kicker’s balance will both be compromised.

The ideal anterior-posterior (A-P) position of the plant foot when kicking is adjacent to and in line with the ball⁽⁷⁾. The A-P position of the plant foot is what determines the trajectory or flight path of the kicked ball. Novices tend to plant their foot behind the ball, which produces a higher ball flight path⁽⁶⁾, whereas a forward plant foot position results in a low trajectory⁽²⁾.

Swing-limb loading

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⁽⁸⁾. 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⁽²⁾.

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 plantarflexed. 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^(2,5). 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⁽⁹⁾. 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.

EMG studies have shown that elite athletes kick the ball further with less muscle activity and more relaxation during the swing phase, but greater eccentric antagonistic muscle activity than novices⁽¹⁰⁾. This supports the concept that skilled elite kickers have more efficient use of their motor systems and biomechanical control during the kicking action.

Foot contact with the ball

At this point the positions of the kicker’s feet are critical to the success of the kick. According to various studies, the foot is in contact with the ball for 6-16 milliseconds, depending on how well inflated the ball is. But contact times these days are likely to be nearer 16 milliseconds or longer, as the pressure of the ball has been reduced since the most recent analysis of contact times⁽¹⁾.

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⁽¹²⁾. 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. Ben-Sira⁽⁷⁾ has found that among elite soccer players, the contact point is further up the foot, closer to the ankle joint. The same research reports less ‘give’ in the ankle joint. While there is no biomechanical data to support this finding, I agree with the logic of this inference. I would also suggest that the lower (nearer the toes) contact point used by novices may predispose these footballers to posterior ankle impingement, arising from the larger plantarflexion moment arm at impact.

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⁽²⁾. 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⁽⁶⁾.

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⁽¹³⁾. In my view 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. Research would be needed to examine this hypothesis.

References

1. Powers, S, and Howley, E (1997), Exercise Physiology. Theory and Applications in Fitness and Performance. WCB. McGraw-Hill: Boston.
2. Barfield, B (1998), The biomechanics of kicking in soccer. Clinics in Sports Medicine. 17(4): 711-728.
3. Phillips, S (1985), Invariance between segments during a kicking motion. In Matsui, H, and Kobayashi, K (eds), Biomechanics. Human Kinetics: Illinois. pp 688-694.
4. Isokawa, M, and Lees, A (1988), A biomechanical analysis of the in-step kick motion in soccer. In Reilly, T, and Williams, M, (2003), Science and Soccer (2nd ed). Routledge: London. pp. 449-455.
5. Abo-Abdo, H (1981), unpublished doctoral dissertation. In Barfield, B (1998), The biomechanics of kicking in soccer. Clinics in Sports Medicine. 17(4): 711-728.
6. Hay, J (1996), Biomechanics of Sport Techniques. Prentice Hall: New Jersey.
7. Ben-Sira, D (1980), A comparison of the instep kick between novices and elites. In Barfield, B (1998), The biomechanics of kicking in soccer. Clinics in Sports Medicine. 17(4): 711-728.
8. Chysowych, W. (1979), The Official Soccer Book of the United States Soccer Federation. In Barfield, B (1998), The biomechanics of kicking in soccer. Clinics in Sports Medicine. 17(4): 711-728
9. Wahrenburg, H, Lindbeck, J, and Ekholm, J (1978), Knee muscular moment, tendon tension force and EMG during a vigorous movement in man. Scand J RehabMed. 10:99-106.
10. De Proft, E, Cabri, J, and Dufour, W (1988), Strength training and kick performance in soccer players. In Reilly, T, and Williams, M. 2003), Science and Soccer (2nd ed). Routledge: London.
11. Plagenhoff, S. (1971), Patterns of Human Motion. A Cinematographic Analysis. Prentice-Hall: New Jersey.
12. Gainor, B, Pitrowski, G, and Puhl, J (1978), The kick. Biomechanics and collision injury. Am J Sports Med.6:185-193.
13. Brukner, P, and Khan, K (2001), Clinical Sports Medicine(2nd ed). Roseville: McGraw-Hill.