In the biomechanical literature, the full-instep kick is the most extensively investigated soccer-specific technique (Lees and Nolan (14)). Barfield (7) identified six components that characterize the kicking motion: approach angle, plant foot forces, swing limb loading, flexion at the hip and extension at the knee, foot contact with the ball, and follow-through. A simpler version comprises the approach, swing, ball contact, and follow-through phases.
Kicking involves the transfer of momentum from the player to the ball. Although this occurs during the ball-contact phase, the momentum of the leg is developed during the approach and swing phases. The duration of the ball-contact phase is in the range of 6-16 milliseconds (7,21). Furthermore, although the deformation of the ball increases during the first half of the contact phase and then decreases during the second half (21), the kicking movement comprises a mixture of impactlike and throwinglike motions (27). This involves the foot-shoe-ball system being displaced by approximately 26 cm while being in contact with the ball. Thereby, muscles of the leg perform work trying to maintain foot velocity.
Soccer shoes influence the kicking performance of athletes (12). A comparison of shod versus barefoot kicking (22) served as our starting point to approach this topic on a broader basis. We separately examined the influence of stance leg traction (23) and shoe upper friction (24). The purpose of this article is to examine additional features of soccer shoes that can influence the maximum ball velocity of full-instep kicks. Based on our work, we put forth the hypothesis that a complex interaction between various soccer shoe features defines the resulting ball velocity.
KICKING TECHNIQUES AND BALL VELOCITY
There are a variety of kicking techniques used in soccer, as dictated by the conditions that occur during the game. Bisanz and Gerisch (9) distinguished between the side-foot kick and three variations of the instep kick. The instep kicks are classified according to the dorsal foot region that predominantly strikes the ball: the inner-instep kick, the outer-instep kick, and the full-instep kick. Whereas the side-foot kick is predominantly used for highly accurate and relatively slow passes or shots on goal over short distances, the instep kicking techniques are mainly used for faster passes and for shots on goal from longer distances. Thus, higher ball velocities achieved with full-instep kicks (15,17, 18) may make the difference between scoring or not.
Among the three types of instep kicks, the full-instep kick produces significantly higher ball velocities compared with the inner- and outer-instep kicks (Table 1). Furthermore, off-center impacts of the foot/shoe on the ball occur mainly when using the inner- or outer-instep kick, and this will impart spin to the ball that compromises its resultant velocity.
FACTORS INFLUENCING BALL VELOCITY IN FULL-INSTEP KICKS
The maximum ball velocity that is achieved with a full-instep kick depends on the level of skill (2,26), the level of maturation (16), and the sex of the player (8). Maximum ball velocity is reduced when kicks are performed with the nondominant leg for all skill levels (6,11), when the player is fatigued (3), and when the movement does not involve an approach phase (19).
Ball velocity also is associated with selected features of the ball-contact phase. These include positive correlations for foot velocity before and after impact and the effective mass of the kicking foot (5,10). The effective mass in kicking depends on the stiffness of the foot's coupling to the lower leg. A negative correlation was shown for ball velocity against the contact time between foot and ball (5). Because a decrease in ball pressure increases ball contact time, it is necessary to control ball pressure during studies on ball velocity. Furthermore, changes of foot angle during the ball-contact phase were negatively correlated with the resultant ball velocity (5), presumably indicating a decrease in the effective striking mass.
SHOES AND BALL VELOCITY IN FULL-INSTEP KICKS
Until recently, the influence of the soccer shoe on ball velocity has been almost neglected in scientific studies. For example, a review article on soccer kicking did not discuss the potential effect of soccer shoes on the kicking movement and ball velocity (7). Hennig and Zulbeck (12) were the first to report that ball velocity during full-instep kicks is influenced by the type of shoe worn by the player. However, the underlying mechanisms were not identified. In a case study, Plagenhoef (20) reported that a shoeless kicker was able to kick a longer distance. However, no scientific study has followed up on this observation.
Amos and Morag (1) found that shoe weight does not influence ball velocity. Rather, the addition of evenly distributed shoe weight decreased foot velocity because of an increase in the mass of the functional unit of foot and shoe, which resulted in an unchanged impulse applied to the ball.
The various features of soccer shoes that could influence the resultant ball velocity are indicated in Figure 1. The significance of several of these factors was examined in a series of studies including the traction characteristics during the final foot strike of the stance leg, kicking with and without a shoe, and the friction properties between ball and shoe upper material (22,23,24). The studies also examined applied aspects of shoe design, such as the influence of different soccer shoe models on ball velocity, shoe weight, outsole stiffness, and toe box height (25). Furthermore, the ability of soccer players to perceive differences in ball velocity across conditions was examined.
Because of the variability in kicking performance between individuals and across kicks, the studies were performed on approximately 20 individuals who each performed 6 maximum kicks per shoe condition. The kicks were performed on artificial turf (DD-Soccer Grass HPF CROWN/DIN 18035 T 7 120 μm, 8800 decitex, density 30,000/m2) toward a target located in a soccer goal. Subjects were required to perform a standardized three-step-approach for all kicks. Standardized resting intervals were required between kicks.
Peak ball velocity was measured with a Stalker Pro radar gun (Applied Concepts Inc, Plano, TX, United States). Ground reaction forces of the stance leg at foot strike were measured with a Kistler 9281 force plate. The duration of the swing phase for the kicking leg was measured from the beginning of foot strike of the stance leg to initial ball movement. Initial ball movement was detected by a photo cell. Subjects were required to rank perceived ball velocity (1 to best). The experimental setup is shown in Figure 2.
Because of different movement requirements for the stance and kicking legs, the studies used a neutral shoe method that required the subjects to wear the same shoe on the kicking leg when examining the influence of different shoes on the stance leg, and vice versa.
The subjects were experienced soccer players with a skill level ranging from ambitious leisure time players to professional players (age, 25.4 ± 3.3 yr; height, 177.6 ± 5.3 cm; mass, 75.1 ± 7.1 kg).
The peak ball velocity achieved by the subjects was influenced by the traction provided to the stance leg, the height of the toe box, the shoe model, and whether the subject wore a shoe. Also, the friction of the upper shoe can influence ball velocity. The largest effect was observed for traction provided to the stance leg, which was responsible for up to 2.46% of the resultant ball velocity across shoe conditions (Table 2). In most cases, subjects were not able to perceive actual differences in ball velocity across conditions as indicated by nonsignificant P values and failure of matching ball velocity measurements (Table 3). The only exception was the condition in which the amount of traction provided to the stance leg was varied.
TRACTION - STANCE LEG
Anjos dos and Adrian (2) found that more skilled soccer players achieved higher ball velocities during full-instep kicking because of greater ground reaction forces during final foot strike. Accordingly, we found that stance leg traction had a substantial influence on both the kicking movement and the resultant ball velocity (23). The peak resultant shear force on the ground during final foot strike was a predictor for the resultant ball velocity across shoe conditions. Furthermore, the increase in resultant ball velocity that resulted from greater peak resultant shear forces was associated with a shorter swing phase (Fig. 3). Traction is partly influenced by stud length, which varies with the ground conditions. This study (23) examined the influence of studs for firm and soft ground conditions on a given surface. Stud length was modified to 0% firm ground stud length (ZFG), 50% firm ground stud length (HFG), 100% firm ground stud length (FFG), and 100% soft ground stud length (FSG). The shoe conditions HFG and FFG produced the best results with the greatest ball velocity, the largest resultant shear force, and the shortest swing duration (Fig. 3).
Poor traction of the stance leg leads to a less powerful swing and a less dynamic ball contact that results in lower ball velocity. In contrast, higher mechanical traction, as measured for the FSG shoe on a two-axis servohydraulic testing machine, does not necessarily provide good functional traction for soccer players in real kicking situations. Therefore, appropriate traction requires a compromise between high traction and functional demands. Functional traction demands are characterized by their suitability for soccer-specific needs with regard to the interaction of the athlete, the shoe, and the surface.
SHOD VERSUS BAREFOOT KICKING, OUTSOLE STIFFNESS, AND TOE BOX HEIGHT
Barefoot (BAR) and sock kicks (SOC) were found to be much more painful compared with shod kicks in subject's own soccer shoe (OSC) and two premium soccer shoe models (brand 1, BBB; brand 2, AAA) when performing full-instep kicks for maximum ball velocity (Fig. 4). However, the greater pain with the unshod kicks did not result in lower ball velocity for those subjects able to perform maximum kicks in a barefoot kicking condition (Fig. 4). In this study, the subjects' perception of ball velocity was related to their perceived pain and not to the actual ball velocity (22).
In a case study, high-speed video images were collected on a soccer player when he kicked barefoot and while wearing a shoe. The analysis revealed that the subject achieved a greater amount of plantarflexion about the ankle and a greater amount of foot angle immediately before ball contact when kicking barefoot (Fig. 5). Therefore, the forced plantarflexion of the foot that occurs during ball contact when kicking with shoes (13) is much less during barefoot kicks. This difference suggests that the foot is more rigid, and its effective mass is greater at initial ball contact when kicking with a barefoot.
The reduction in ankle plantarflexion at ball contact may depend on the degree of outsole stiffness of soccer shoes (12,25). A series of studies, however, did not show a systematic influence of different degrees of outsole stiffness on resultant ball velocity. Therefore, it seems that even a small degree of outsole stiffness is sufficient to resist the active initial plantarflexion of the player. Furthermore, a high outsole stiffness does not increase ball velocity, in contrast to the assertion that stiff outsole materials support the foot and enhance the transfer of momentum.
Effects were observed for the height of the toe box, which determines the spring of the shoe during ball contact. An increase in the toe spring reduced the resultant ball velocity of full-instep kicks (25). This reduced resultant ball velocity may be explained by the same mechanism that forces the ankle into plantarflexion and thereby reduces the transfer of momentum to the ball (13).
FRICTION - SHOE UPPER
When the ball is struck slightly off center, the friction properties of shoe upper materials may enhance the creation of spin that would reduce the energy provided for ball translation (4). However, the use of different friction properties of shoe uppers only showed a trend toward higher ball velocities for those friction properties within the range of currently used materials. They were compared with higher and lower friction properties (24). Nonetheless, it would be of interest to investigate the influence of new innovative materials that offer specific rebound properties on ball velocity.
SHOE MODELS AND SHOE WEIGHT
The influence of different commercially available soccer shoes on ball velocity was first shown by Hennig and Zulbeck (12) and confirmed for current premium soccer shoes by Sterzing (25). Commercially available shoes incorporate multiple components. Therefore, it is difficult to link the performance characteristics of different shoe models to single components because these interact with one another.
Shoe weight was found to have no effect on ball velocity in accordance to the finding of Amos and Morag (1).
Soccer shoes alter the resultant ball velocity of soccer players when performing full-instep kicks. The shoe must provide adequate traction for the stance leg, which involves a compromise between high mechanical traction and functional biomechanical demands. The shoe must protect the dorsal foot region and enhance the rigidity of the foot but should not include such interfering structures as the heel counter or inappropriate outsole constructions or features that reduce the transfer of momentum to the ball (Fig. 6). In contrast, neither shoe weight nor outsole stiffness had any effect on resultant ball velocity. Generally, different shoe models alter the resultant ball velocity, whereas the influence of shoe upper friction and general shoe comfort cannot be finally proven.
Various soccer shoe features were shown to influence kicking velocity. However, the understanding of the complex interaction of these features is the key to the role of soccer shoes in full-instep kicking.
Studies (22,23,24,25) were supported by Nike USA, Inc.
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Keywords:©2008 The American College of Sports Medicine
full-instep kicking; ball velocity; shoe features; stance leg; kicking leg