Kinetic Analyses of Maximal Effort Soccer Kicks in Elite Female Athletes

1. Kinetic Analyses of Maximal Effort Soccer Kicks in Elite Female Athletes Sarah E. Clagg and James S. Thomas Department of Physical Therapy at Ohio University, Athens, OH Introduction Female soccer players often tear the ACL of their plant leg during kicking maneuvers, however the mechanics of maximal effort kicking tasks have not been adequately characterized. To better understand risk of injury, an important first step is to characterize the kinetics of the plant leg in healthy female soccer players. Thus, we examined the effect of plant leg and approach condition on the peak joint torques of the hip, knee, and ankle in elite female soccer players performing maximal effort kicking tasks. Methods Nine healthy female collegiate soccer players performed a series of kicking tasks from three different approach angles (60 degrees to the left of the ball, directly behind the ball, and 60 degrees to the right of the ball) (Figure 1). An ipsilateral approach condition occurred when the plant leg and approach angle were the same, while a contralateral or off-axis approach condition occurred when the plant leg and approach angle were opposite. Figure 5 is a photograph of a participant performing a maximal effort kick from an ipsilateral approach condition. A center approach condition occurred when the subject approached the ball from directly behind. An off-axis kicking approach condition was used in this study to mimic the many variable and unpredictable kicking approaches that often occur throughout high-level competition. Kinematic data of the hip, knee, and ankle were recorded using the Motion Monitor System and three-dimensional joint torques of the plant leg were determined using a linked segment inverse dynamics model. For each trial, the kick was defined as the time from force plate contact of the plant leg to maximum hip flexion of the kicking leg. The peak-to-peak joint torques of the hip, knee, and ankle were extracted for each trial from the time series data using custom software written in MatLab. Data Analysis Within subject ANOVA’s were used to analyze the effects of plant leg and approach condition on the peak joint torques of the hip, knee, and ankle. All interactions were followed by repeated measures with simple effects analyses. Results Peak flexor and extensor torque of the hip, knee, and ankle of the left and right plant leg for the three approach conditions are illustrated in Figures 2A-F. There was an interaction of plant leg and approach condition on peak flexor torque of the hip and ankle (F= 14.26, p<.05; F= 43.80, p<.05). For the hip, peak flexor torque was greater for the left plant leg compared to the right, but only for the contralateral (F= 13.63, p<.05) approach condition. However for the ankle, peak flexor torque was greater for the left plant leg compared to the right for both the contralateral (F= 20.55 p<.05) and center (F= 10.95 p<.05) approach conditions. No significant differences for peak flexor torque were observed between the left and right plant legs in elite athletes during the ipsilateral approach condition. Peak flexor torque of the knee and ankle was significantly different between the left and right plant legs (F= 6.54, p<.05; F= 7.93, p<.05). Peak flexor torques of the hip, knee, and ankle of left plant leg were greater than the right plant leg during the contralateral and center approach conditions. These differences between the left and right plant legs during the contralateral approach demonstrate that the subjects produced greater peak flexor torques when kicking with their dominant limb. Peak extensor torque was greater for the right plant leg compared to the left for all three approach conditions. These differences in peak extensor torque were significant for the hip, knee, and ankle (F= 10.05, p<.05; F= 27.03, p<.05; F= 12.15, p<.05). Subjects produced greater peak extensor torques when kicking with their non-dominant limb at each approach condition. Peak extensor torque of the hip during the contralateral approach was greater than the ipsilateral approach condition for both the left and right plant legs. There was an interaction effect of plant leg and approach condition on peak external rotation torque of the hip (F= 7.42, p<.05). A A A D D D Force Plate B B B E E E Figure 1. An illustration of the approach conditions during the soccer kick from 60 to the left of the ball, 60 to the right of the ball, and from straight behind the ball. C C C F F F Figure 5. A participate performing a maximal effort kick from an ipsilateral approach condition. Figure 2A-F. Peak flexor and extensor torque data of the left and right plant leg from the ipsilateral, center, and contralateral approach conditions. Data are presented in means ± standard error. Figure 3A-F. Peak internal rotation and external rotation torque data of the left and right plant leg from the ipsilateral, center, and contralateral approach conditions. Data are presented in means ± standard error. Figure 4A-F. Peak abduction and adduction torque data of the left and right plant leg from the ipsilateral, center, and contralateral approach conditions. Data are presented in means ± standard error. Results Cont. Peak external and internal rotation torque of the hip, knee, and ankle of the left and right plant leg for the three approach conditions are illustrated in Figures 3A-F. Peak external rotation torque of the hip was greater for the right plant leg compared to the left for the center (F= 25.05, p<.05) and contralateral (F= 10.25, p<.05) approach conditions. Peak external rotation torque of the hip and knee of the right plant leg was greater compared to the left for all three approach conditions (F= 13.75, p<.05; F= 84.06, p<.05). There were significant main effects of approach condition on peak internal rotation torque of the hip, knee, and ankle (F= 5.69, p<.06; F= 6.71, p<.05; F= 9.15, p<.05). A significant difference was found between the center and contralateral approach conditions. Peak internal rotation torque of the hip was smallest during the ipsilateral approach and greatest during the contralateral approach condition. There were also significant main effects of plant leg on peak internal rotation torque of the knee and ankle (F= 17.02, p<.05; F= 7.77, p<.05). Subjects produced greater peak internal rotation torque of the left plant leg compared to the right for all three approach conditions. Peak abduction and adduction torque of the hip, knee, and ankle of the left and right plant leg for the three approach conditions are illustrated in Figures 4A-F. There was a main effect of approach angle on peak abduction torque of the hip (F= 9.36, p<.05). Peak abduction torque of the hip was greater for both the left and right plant legs during the ipsilateral approach compared to the center and contralateral approach conditions. Peak abduction torque of the hip, knee, and ankle was greater for the left plant leg compared to right. In contrast, peak adduction torque of the hip, knee, and ankle was greater for the right plant leg compared to the left for all three approach conditions. There were significant main effects of plant leg on peak abduction and adduction torques of the hip, knee, and ankle. Conclusion It is known that female athletes suffer non-contact ACL injuries of the plant leg. The findings from this study provide insight into the mechanics of the plant leg during maximal effort kicking tasks. Overall the results show that there are minimal differences in peak flexor and extensor torque between the left and right plant legs in elite female athletes when kicking from a ipsilateral approach condition; however, even at an elite level differences in peak flexor and extensor joint torque emerged between the left and right plant legs when subjects kicked from an off-axis approach. This suggests that the participants had to change their kicking strategy when kicking with their non-dominant versus dominant limb and when kicking from different approach conditions. Furthermore, it needs to be determined if differences in the mechanics of off-axis kicks emerge between healthy participants and those who have suffered an ACL injury. This research was supported by The National Institutes of Health Grant R01-HD045512 to J.S. Thomas. Figure 6. Stick figures illustrate a soccer participant performing a maximal effort kick from a straight approach condition.