Kicking is the most fundamental skill in soccer. Although various types of kicking are performed during a soccer game, the instep kick is of particular interest because it is the primary technique that is used for shots on goal and to clear the ball by defenders. Understanding the biomechanical factors that influence instep kicking performance is therefore important from a coaching and training standpoint. Given as such, the biomechanics of instep kicking has been examined extensively (1,11,16,17,22).
The most common metric of performance used in biomechanical studies of instep kicking has been ball speed (3,14), which is closely related to foot velocity before impact (11,17,22). Previous investigators have shown that thigh and shank angular velocities of the swing limb are the primary kinematic factors contributing to foot velocity in adult males (1,2,11,16,22). Although several studies have examined kinetic factors from the swing limb that function to maximize foot velocity, there are conflicting reports regarding the relative influence of specific joints. Generally, the swing limb hip flexor and knee extensor moments are the primary kinetic factors acting to advance the kicking limb (1,11,16,22). Some studies have suggested that the knee extensor moment is the most important variable influencing kicking performance (16,22), whereas other investigators have reported that the hip moment is the most important factor (11,20).
Although swing limb mechanics during kicking have been well described, the influence of the stance limb on kicking performance has received little attention. Because of the interaction between the stance limb and swing limb dynamics, biomechanical evaluation of the stance limb in relation to kick performance seems warranted. To date, studies that have analyzed stance limb biomechanics during kicking have focused on ground reaction forces, limb alignment, and muscle activation in relation to different kicking approach angles (2,9,15) and kick type (7). How the stance limb joint kinetics (i.e., net joint moments) may affect instep kicking performance has not been investigated.
Another limitation of the current understanding of instep kicking biomechanics is that findings from previous studies were mostly derived from adult male soccer players. Despite the growth in female soccer participation, only a few studies have examined kicking biomechanics in adult female soccer athletes (4,9,24). Apart from the limited research involving female soccer players, only two studies have examined the biomechanics of instep kicking in young soccer athletes (both of which used male players) (13,18). Thus, it is not known whether the current understanding of kicking biomechanics can also be applied to young female soccer athletes. To our knowledge, no study has determined whether kinematic and kinetic kicking patterns in young females are similar to that reported for adults. Qualitatively, the kicking pattern has been suggested to be adult-like by 8-11 yr (5,8). However, it has been found that development of the mature kicking pattern may be slower in females (8). A detailed kinematic and kinetic analysis of instep kicking at different stages of physical maturation in females is needed to address this question. Such an analysis would provide insight regarding when a mature kicking pattern is developed in female soccer athletes and an opportunity to identify biomechanical factors that contribute to improved kicking performance across maturation.
The purpose of the current study was to compare swing and stance limb kinematics and kinetics between pre- and postpubertal female soccer players while performing an angled two-step approach kick. A secondary purpose of the study was to identify biomechanical predictors of peak foot velocity across maturation.
Ten prepubertal (age = 9.9 ± 0.3 yr, height = 142 ± 3.6 cm, mass = 36.1 ± 5.1 kg) and 10 postpubertal (age = 14.6 ± 0.5 yr, height = 162.5 ± 6.5 cm, mass = 57.3 ± 6.7 kg) female soccer players participated in this study. The classification of subjects as prepubertal or postpubertal was based on scores obtained from self-reported Tanner staging (25,28) and the modified Pubertal Maturation Observational Scale (PMOS) (10). To improve the accuracy of classification, parents assisted participants in identifying Tanner stage and completing the PMOS. The Tanner scale has five ordinal stages of physical maturation based on pubic hair and breast development from figured drawings (28). The PMOS categorization is based on the identification of eight secondary sex characteristics (i.e., muscle development, increased perspiration with physical activity, acne, facial or body hair, deepening of the voice, menarche, and breast development) in addition to parent report of less obvious characteristics such as a growth spurt (i.e., increase in height of 3-4 inches in the past year) (10). Prepubertal status was established when participants had both a Tanner stage of 1 and reported one item or less on the PMOS questionnaire. Postpubertal status was established when participants had both a Tanner stage of 5 and reported six or more items on the PMOS and had completed a growth spurt.
At the time of this study, all subjects were participating in organized soccer at the club level. On average, the years of soccer experience were 4.5 ± 2 and 8.5 ± 1.4 for the pre- and postpubertal groups, respectively. Typical training schedules required athletes to participate in practice and/or competition at least 3-4 d·wk−1 for ≥6 months·yr−1.
To be considered for the study, participants had to be free of lower extremity pain or injury. Participants were excluded from the study if they reported any of the following: 1) history of previous anterior cruciate ligament injury or repair; 2) previous injury that resulted in ligamentous laxity at the ankle, hip, or knee; or 3) presence of a medical or neurologic condition that would impair their ability to perform the soccer kick. Before participation, all procedures were explained to each participant and parental/guardian informed consent was obtained as approved by the University of Southern California Institutional Review Board.
Prior to data collection, participants were fitted with the same style of athletic shoe (New Balance X700; Boston, MA), and their height and mass were recorded. Twenty-one reflective markers (14-mm spheres) were affixed to the following anatomical landmarks bilaterally: distal second toe, first and fifth metatarsal heads, medial and lateral malleoli, medial and lateral femoral epicondyles, greater trochanters, iliac crests, anterior superior iliac spine, and L5-S1. In addition, noncollinear tracking marker clusters were placed on the shoe heel counters, lateral shanks, and lateral thighs. A standing calibration trial was then obtained to establish the local segmental coordinate system (Fig. 1). After the calibration trial, the anatomical markers were removed. The tracking marker clusters and L5-S1 and iliac crest markers remained on the participant during the kicking trials.
Three-dimensional kinematics of the lower extremities were obtained using an eight-camera motion analysis system (Vicon, Oxford Metrics, Ltd., Oxford, England, UK) at a sampling frequency of 250 Hz. Ground reaction forces were obtained from a force platform (Advanced Mechanical Technologies, Inc., Newton, MA) at a sampling frequency of 1500 Hz.
Participants performed kicking trials with their preferred leg using a FIFA-approved soccer ball (size 4 for the prepubertal group and size 5 for the postpubertal group). The soccer ball was positioned 3 m from a 2.4 × 1.8-m soccer net. Participants were instructed to kick the ball into the net using an angled (45°-60°) two-step approach as if they were kicking a penalty shot. Four kicking trials were obtained for each participant. A trial was acceptable if 1) the stance foot landed on the force plate and 2) the ball was kicked into the net.
Visual3D (C-motion, Inc., Rockville, MD) was used to compute segmental kinematics and kinetics for the swing and stance limb. The raw trajectory data were filtered using a fourth-order zero-phase lag Butterworth low-pass filter (12 Hz). The cutoff frequency was selected on the basis of a residual analysis (29). Net joint moments (internal) were derived using inverse dynamics equations and were normalized by dividing the values by body mass and height. This was done to account for the maturation-related influences of body size on the biomechanical measures. All data were normalized to 100% of the kick period. The kick period was operationally defined as the time from initial contact of the stance foot to peak foot velocity.
The primary dependent variable evaluated in this study was peak swing limb foot velocity, which has been shown to be related to ball velocity after foot contact (1,11,16,17,22). Additional kinematic variables of interest included the angles of peak swing limb hip extension and knee flexion, as well as peak stance limb hip and knee flexion. Kinetic variables of interest included peak swing limb hip flexor and knee extensor net joint moments and peak stance limb hip and knee extensor net joint moments. All variables represented the mean of the four kicking trials, and this value was used for statistical analyses.
Independent-samples t-tests were used to determine whether there were group differences for each of the kinematic and kinetic variables of interest. Pearson correlation coefficients were used to examine the relationship between peak foot velocity and each of the kinetic and kinematic variables of interest (data from both groups were combined). Stepwise multiple regression analysis was used to identify which variables were predictive of peak foot velocity. Only the variables that were significantly correlated with peak foot velocity (P ≤ 0.05) were included in the stepwise regression analysis. Entry to the stepwise regression model required that a variable exhibit a P value ≤0.05. Likewise, a P value of >0.10 was used as the criteria for removal. Assumptions of normality and distribution of the residuals were verified for the derived regression model. Statistical analyses were performed using SPSS statistical software (version 15; Chicago, IL). Significance was set at P ≤ 0.05.
On average, peak foot velocity was significantly greater in the postpubertal group when compared with the prepubertal group (13.4 ± 1.4 vs 11.6 ± 0.8 m·s−1, P = 0.003; Table 1). Peak stance limb hip and knee flexion were significantly greater in the prepubertal group when compared with the postpubertal group (P < 0.05; Table 1, Fig. 2). No differences in peak swing limb hip extension and knee flexion were found between groups (Table 1, Fig. 3).
The peak swing limb hip flexor moment was significantly greater in the postpubertal group when compared with the prepubertal group (P = 0.03; Table 1, Fig. 4A). No differences were observed for the peak swing limb knee extensor moment or the stance limb hip and knee moments (P > 0.05; Table 1, Figs. 4B and 5).
Three kinematic variables were significantly correlated with peak foot velocity: 1) peak swing limb hip extension (r = 0.62, P = 0.004); 2) peak stance limb hip flexion (r = −0.60, P = 0.006); and 3) peak stance limb knee flexion (r = −0.53, P = 0.02). In addition, two kinetic variables were found to be significantly correlated with peak foot velocity: 1) peak swing limb hip flexor moment (r = 0.73, P < 0.001) and 2) peak swing limb knee extensor moment (r = 0.56, P = 0.01).
Using the stepwise procedure, two variables were included in the final regression model. Peak swing limb hip flexor moment entered the model first (ΔR2 = 0.53), followed by peak swing limb hip extension (ΔR2 = 0.12). Combined, these variables explained 65% of the variance in peak foot velocity (F2,17 = 15.85, P < 0.001).
This study examined lower extremity kick biomechanics among young female soccer players at two distinct maturational stages. Consistent with previous literature, peak foot velocity increased with maturation (18). On average, peak foot velocity in the postpubertal group was 16% greater than that in the prepubertal group. Our results are in agreement with Luhtanen (18), who reported that ball velocity for pubertal males was 23% greater than that for prepubertal males. The peak foot velocity achieved by the female athletes in this study (prepubertal = 11.6 m·s−1 and postpubertal = 13.4 m·s−1) is similar to the values recently reported by Katis and Kellis (13) for a group of pubertal males (12.2 m·s−1) with a minimum of 5 yr of experience. Our data indicate that skilled young female soccer athletes can achieve similar foot velocity when compared with young males.
As illustrated by the time series data, the pre- and postpubertal groups in the current study demonstrated similar swing limb kinematic patterns (Fig. 3). The mature swing limb kinematic kicking pattern has been suggested to develop by 8-11 yr (5,8) and is consistent with the results of the current study. Indeed, peak swing limb hip extension and knee flexion for the female athletes did not differ between groups and are within the range reported for adult male collegiate soccer players (7,17).
There is limited information regarding stance limb kinematics during soccer kicking. The patterns of the stance limb hip and knee sagittal plane motions in the current study are similar to one previous report (15). In contrast to the similar swing limb kinematics among different maturation groups, peak stance limb hip and knee flexion were significantly greater in the prepubertal group when compared with the postpubertal group. In addition, the prepubertal group exhibited greater hip and knee flexion throughout the kicking period when compared with the postpubertal group (Fig. 2). The negative correlation between foot velocity and peak stance limb hip and knee flexion angles suggests that greater stance limb flexion may impair active advancement of the swing limb. Why the prepubertal group demonstrated greater stance limb hip and knee flexion is unknown; the stance limb pattern may reflect a strategy needed to increase stability (i.e., lower center of mass) during this dynamic single-leg stance task (8).
The influence of stance limb joint kinetics on kick performance has not been previously reported. In general, the stance limb net joint moments were substantially larger than the swing limb. The stance limb kinetics were characterized by a hip extensor moment throughout the kick period, whereas a brief knee flexor moment was observed during the first 20% of the kick period followed by a knee extensor moment through the rest of the kick period (Fig. 5). Collectively, the kinetic data suggest that the hip and knee extensors function to support the flexed hip and knee posture throughout the kick period. Despite the large net joint moments at the hip and knee, stance limb kinetics neither differed between groups nor entered the regression model as a predictor for the peak foot velocity. Whether stance limb kinetics may prove more important when using different approach angles or in players that are more skilled would require further investigation.
Previous investigations have shown that the magnitude of the hip flexor and knee extensor moments of the swing limb are important for increased foot velocity in adults (16,22) and young males (18). In the current study, the normalized peak swing limb hip flexor moment was significantly greater in the postpubertal group compared with the prepubertal group, whereas no group differences were observed in the peak swing limb knee extensor moment or stance limb kinetic variables. Moreover, the stepwise regression analysis indicated that the peak hip flexor moment of the swing limb was the strongest predictor of peak foot velocity, suggesting that the ability to generate a greater hip flexor moment of the swing limb is critical to achieve increased foot velocity during kicking.
The importance of the swing limb hip flexor moment on foot velocity observed in the current study is in agreement with previous reports in adult male soccer athletes (1,11,20). Dörge et al. (11) reported that the faster foot velocity of the preferred limb when compared with the nonpreferred limb was the result of greater work on the shank from the angular velocity of the thigh. Recently, detailed analysis of the muscle and motion-dependent moments acting on the knee were examined in collegiate males during the instep soccer kick (20). Findings from Naito et al. (20) indicated that the kicking leg centrifugal force-dependent moment at the knee due to hip flexion angular velocity was the primary contributor to rapid knee extension and thus foot velocity. Because greater hip angular velocity is achieved primarily from a greater hip net joint moment, our results support these previous studies (11,20). Specifically, our results suggest that across maturation, young female soccer players rely primarily on the swing limb hip flexor moment to increase foot velocity. These findings imply that training to maximize instep kick performance should focus on explosive hip flexor strengthening.
In addition to the peak hip flexor moment, increased peak swing limb hip extension during the kick was associated with greater foot velocity in the current study. One explanation for this finding is that greater hip extension may function to enhance hip flexor contractility through the stretch-shortening cycle (6,12,27), thereby enhancing the ability of the hip flexor muscles to generate a greater hip flexor moment. It is also possible that a larger peak hip extension would allow for a larger hip flexion excursion, thus providing more time to generate a larger angular impulse to increase the angular velocity of the thigh segment.
The stance limb kinetic and kinematic variables were not included in the final regression model for predicting peak foot velocity. It should be noted that in a post hoc analysis, peak stance limb hip flexion was significantly correlated with peak swing limb hip extension (r = −0.56, P = 0.01). Because swing limb hip extension had a stronger correlation with peak foot velocity when compared with peak stance limb hip flexion (r = 0.62 and −0.60, respectively), peak stance limb hip flexion was not selected in the regression model because its contribution had been mostly explained by the peak swing limb hip extension. The negative correlation between peak stance limb hip flexion and peak foot velocity indicates that greater stance limb flexion may have limited swing limb extension and subsequent rapid forward limb advancement. This is supported by the finding that the postpubertal group demonstrated less stance limb hip flexion and greater (not statistically significant) swing limb hip extension when compared with the prepubertal group. Therefore, although swing limb factors primarily influenced foot velocity in this study, stance limb kinematics may also influence swing limb dynamics.
A limitation of this study is the potential influence of the filtering method on the kinematic and kinetic variables near ball impact. It has been reported that traditional filtering methods cannot handle impact situations properly, thereby creating errors (i.e., false braking forces) near ball impact (17,23). However, the potential errors near ball impact associated with the filtering technique used in the current study would have no influence on our swing limb variables because the peak values for the hip and knee occurred well before ball contact (i.e., within the first 70% of the kick period). It is possible that peak foot velocity could be impacted by our filtering approach. The potential error resulting from our filtering method would be a slight slowing of the peak foot velocity (23). However, it should be noted that the peak foot velocity of the postpubertal female athletes recorded in this study is similar to the peak foot velocity reported for pubertal (13) and adult male athletes (15-17 m·s−1) (16,19,22). In addition, it is reasonable to assume that any error introduced by filtering would be a systematic error for all participants (17) and thus would not affect the overall conclusion of our study.
The hip flexor moment and hip extension motion combined explained 65% of the variance in peak foot velocity. Other factors not included in the current study would likely account for the unexplained variance. For example, frontal- and transverse-plane kinematics and kinetics were not analyzed in this study. Although it has been reported that angular rotations and moments in the frontal and transverse planes primarily guide the swing limb for proper orientation rather than advance the swing limb during the instep kick (17,21), it is possible that frontal- and transverse-plane factors also may have influenced foot velocity magnitude. In addition, recent studies have suggested that the trunk may influence kicking performance and should also be considered in future work (20,26).
In conclusion, this study examined the stance and swing limb kinematics and kinetics during the instep soccer kick in young females at two distinct phases of physical maturation. On average, postpubertal female soccer players demonstrated greater swing limb foot velocities during the kick compared with prepubertal athletes. The swing limb kinematic kicking pattern observed in the prepubertal female soccer athletes is similar to that previously reported in adult males. The more mature female athletes (i.e., postpubertal) achieved greater foot velocity through a combination of greater swing limb hip flexor moments and hip extension. In particular, the ability to generate a greater hip flexor moment seems most important for increasing foot velocity across maturation in this sample of young female soccer players. The findings from this study indicate that training strategies aiming to improve foot velocity during the instep soccer kick should focus on enhancing the hip flexor muscle contribution to forward progression of the swing limb.
This study was funded by the National Institutes of Health (R01AR053073-02).
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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