|Know-How of Maximal Instep Kick in Soccer|
1 Department of Kinesiology, University of Lethbridge, Canada
The great attraction of soccer for millions of fans may trace down to the basic idea of the game: the goal – an idea that never ceases to fascinate. Various techniques for gaining goals are sources of excitement, which make soccer the #1 sport world-wide [1, 2]. Among all the techniques, the maximal instep kick may be the most applied technique for defeating goalies (Figure 1). Since the technique is so important, it has been always a focus of soccer training for new comers and young players. After decades of biomechanical studies [3, 4] it is time to update insights obtained from scientific studies for coaches and athletes to get a better understanding of the optimized kicking characteristics of the kicking skill. Such information should help practitioners better develop goal-oriented training programs for improving learning effectiveness.
Figure 1: Exemplars of maximal instep kicking – a skill involving leg swing, trunk rotation/twist and arms movement.
The two basic aspects related to the development of a proper kicking technique
To develop a quality kick, two basic aspects should be kept in mind in skill development training – the kick’s accuracy and power. Accuracy measures the precision when driving the kicking foot toward the ball, as well as the foot contact location with the ball. Power is related to the speed and momentum of the kicking-foot. The highest quality of kick is achieved when the ball is struck accurately with full power. However, accuracy and power are non-autonomous variables. Naturally they interact contrarily . Especially for novice learners, power and accuracy can dramatically work against each other. Typically, training initially concentrates on accuracy and afterward aims to enhance power. The approach, supporting foot placement, the positioning of the kicking foot, tension arc (dynamic posture) and gender differences are features during a soccer kick which will crucially influence the accuracy and the power.
Skilled players performing a maximal instep kick approach a stationary soccer ball from an angle of 23.7° – 43.8° [4, 6] to the direction of ball flight (Figure 2). The main function of using the approach angle is to aid/increase a trunk rotation for gaining more kick power [4, 7, 8]. Usually, skilled athletes take 2-4 steps to carry out the approach .
Figure 2: Definitions of trunk rotation angle (α) and approach (or run-up) angle (ß).
A coach should pay attention on two points when he/she designs a training program: 1) there is a significant difference of the approach angles between skilled males and females; and 2) the length of the last stride/step is important in the maximal instep kick. A study has shown that, on average, the female approach angle is about 20° bigger than male ones . Such a difference is related to the different techniques used by males and females. Please see the section “Gender differences” for details/explanations of the gender-based difference.
The last stride/step length is the second focus of the approach. A study using the same group of professional subjects showed that significant differences existed between stride strategies which resulted in 25 m and 45 m of ball’s flight . 45-meter kicks have greater last step length as well as movement amplitude at the hip than those of 25-meter kicks. Similar results were also confirmed by other studies [9, 11]. These results mean that an increase in the last stride length will increase the kick power, suggesting that the last stride length can be used to evaluate the training progress or training effect in the practice. There are also other advantages of increasing last stride length. Please see “The tension arc” section for details.
The placement of the supporting foot and the positioning of the kicking foot at the kick
The placement of the supporting foot in the last step is just as important as placement of the kicking foot. The supporting foot should be planted parallel with the intended direction of the ball. By placing the foot slightly behind the ball, one will produce a rising ball, instead of a low-to-the-ground ball. The plant distance is between a foot and a foot and a half away from the ball. This will ensure that when one strikes the ball his/her kicking foot will be at a roughly 45° angle to the horizontal plane (Figure 3) . This is extremely important; because such a foot positioning enables one to strike the ball with the laces so that one can 1) generate a most powerful kick and 2) prevent injury such as “footballer’s ankle . Studies have shown that the contact between the kick foot and the ball lasts for less than 1/100 of a seconds – a high-intense impact [9, 14]. If the foot-ball contact happens towards the distal end of the foot (i.e., an improper foot-ball contact) the impact will cause a foot over-extension otherwise known as plantar flexion. The consequences are not only the reduction of ball speed but also an increased risk of injury. The last thing that one needs to keep in mind is that the direction of the supporting foot will decide the flight direction of the ball when proper foot-ball contact is made. The supporting foot should be parallel to the intended ball flight direction.
Figure 3. The posture at foot-ball contact shows the proper placement of the supporting foot and the positioning of the kicking foot.
The tension arc
The kicking step (last step of the kick) is the core of the skill. It requires full body control, the secrets of which cannot be fully revealed by employing a partial body analysis approach. Using 3D motion capture and full body biomechanical modeling, Dr. Shan and his research team are the first to quantitatively establish the key characteristics of a maximal instep kick , which could be extracted as the formation of a tension arc. The fast release of this tension arc, a quasi whip-like leg movement with complimentary upper body movement (Figure 4), characterizes this key aspect of the kick.
Figure 4. Maximal instep kick in soccer – a typical full-body and multi-joint coordination consisting of the formation of a tension arc and its release.
The formation of the dynamic tension arc involves: 1) kick-side hip over-extension and knee flexion, 2) trunk twist towards non-kick-side, and 3) non-kick-side arm pointing to rear-lateral direction (shoulder extension and abduction) (Figure 4, left). The release of the arc consists of: 1) a quasi whip-like control sequence of the kicking leg, 2) upper trunk flexion and twist towards kick-side, and 3) the fast, forward swing toward medial side of the non-kick-side arm (shoulder flexion and adduction during kick) (Figure 4, right). Further, the studies confirm that the tension-arc creates a better condition for generating an explosive muscle contraction through muscle pre-lengthening, as such the tension arc makes the kick more powerful [4, 7, 8]. It should be mentioned that increasing the last step length will amplify the pre-lengthening effects of trunk and leg muscles involved in the kick and in turn will generate more power.
Most often, results obtained using male athletes are transferred to female ones in training without considering the gender differences including anthropometrical structure, body flexibility, muscle force and power . The research results of Shan and his team show that males and females should have different training programs. Basically, there are two major differences: 1) skilled male players use a smaller approach angle than female ones (a difference of 20°, ß in Figure2) and 2) after a powerful kick, males naturally follow through with a jump in order to dissipate residual leg momentum while females avoid this airborne phase; instead they counteract the momentum with upper body flexion (Figure 5). The reasons for this discrepancy are unknown. However, the researchers speculated that vibrations caused by jump take-off and landing could be eliminated by the female technique. Thus it is possible to explain that the gender-based difference could be caused by body structure/anthropometry, e.g. the male body has less chest mass (as well, more solid) than the female body, therefore it is less prone to discomfort caused by vibrations from jumps. Unfortunately, there is still no written record of different training processes based on gender .
The gender differences in anthropometry can also explain the bigger approach angle employed by skilled female players. A bigger angle allows a more curved approach to the ball, which create a greater trunk rotation than demonstrated by males (α in Figure 2) . The increase of the trunk rotation will not only increase the kicking power but also counteract the momentum of the kicking leg after ball contact. Therefore, coaches should pay attention on the approach angle when they design a training program for females.
Figure 5. Gender comparison of maximal instep soccer kick of advanced athletes – posture after ball contact (left –male, right – female)
The quality of the maximal instep kick depends on the kick accuracy and the kick power. The development/training of this skill should begin with the kick accuracy and, afterward, aim to enhance power. A consideration of gender is also in integral to kick quality.
The kick accuracy is influenced by the placement of the supporting foot and the positioning of the kicking foot at ball contact. The supporting foot should be planted in line with the intended ball direction and about a foot from the ball. At the foot-ball contact, the kicking ankle should be positioned at a roughly 45° angle to the horizontal plate for maximizing the kick accuracy as well as power.
The kick power mainly depends on athletes’ dynamic posture. The efficient posture is the so-called tension arc. Proper posture during the maximal instep kick forms the tension arc, and its fast release using a whip-like leg movement generates highest power. During the process, the increase in certain muscles’ pre-lengthening contributes to an explosively powerful kick. Further, it is found that through training, male and female athletes have developed different techniques, characterized by run-up angle, trunk flexion and the dynamic posture after ball contact. Coaches should take these differences into consideration when designing training programs to accommodate the gender differences in order to increase the training efficiency.
Increased kick accuracy and power, as well as the consideration of different gender techniques will improve the players’ maximal instep kick skills, assist in acquiring that ever coveted goal and continue to bring pleasure to players and spectators alike.
7. Shan, G. and P. Westerhoff, Full-body kinematic characteristics of the maximal instep soccer kick by male soccer players and parameters related to kick quality. Sports Biomechanics, 2005. 4(1): p. 59-72.
10. Stoner, L. and D. Ben-Sira, eds. Variation in movement patterns of professional soccer players when executing a long range in-step soccer kick. Biomechanics VII-B, ed. A. Morecki, et al.1981, University Park Press: Baltimore. 337-342.
11. Lees, A. and L. Nolan, eds. Three dimensional kinematic analysis of the instep kick under speed and accuracy conditions. Science and football IV, ed. W. Spinks, T. Reilly, and A. Murphy2002, Routledge: London. 16-21.
12. Shan, G. and X. Zhang, From 2D Leg Kinematics to 3D Full-body Biomechanics – The Past, Present and Future of Scientific Analysis of Maximal Instep Kick in Soccer. Sports Medicine Arthroscopy Rehabilitation Therapy Technology, 2011. 3: p. 23.