Cross-sectional change of ball impact in instep kicks from junior to professional footballers
Along with the forward swing of the kicking leg, ball impact is one of the most important factors for fast ball kicking in soccer. Ball impact is a phase during which the players transfer the momentum of their kicking limb to the ball. Through daily training, the players acquire ways to impart the leg and foot momentum into the ball effectively. In most instep kicking, rapid passive motion of the kicking foot occurs due to large ball reaction force during ball contact (Asai et al., 2002; Nunome et al., 2006; Shinkai et al., 2009). To enhance the ball impact efficiency, it has been considered that the magnitude of the foot deformation needs to be minimized by ankle rigidity at ball contact (Asami and Nolte, 1983; Rodano and Tavana, 1993; Lees and Nolan, 1998). To date, there have been several studies to describe the maturity process of ball kicking technique in soccer. With regard to the forward swing of the kicking leg, it has been reported that the maximum angular velocity of the shank or the values at ball contact increased with age (Bloomfield et al., 1979; Luhtanen, 1988). On the other hand, the kicking foot motion during ball impact phase and those changes from childhood to adolescent soccer players have never been systematically investigated. The aim of this study was to investigate the change of ball impact characteristics in instep kicking with physical growth of the soccer players by the cross-sectional method.