Analysis of time-varying impact points in soccer kicking using curved foot geometry

The impact phase in soccer kicking, while extremely brief, determines both ball velocity and rotation. Quantifying the moment governing this rotation requires precise, time-resolved identification of the impact point; however, a robust methodology has not yet been established. This study developed a more accurate approach to determine the impact point by incorporating the curved geometry of the foot's impact area, enabling comprehensive calculation of mechanical variables of ball motion. To capture foot surface geometry under static conditions, 105 reflective markers were applied at high density to the shoe's impact area and recorded using an optical motion capture system at 200 Hz. Following the removal of these markers, kicking trials-straight, curve, knuckle, and slice-were recorded using the same motion capture system at 2000 Hz. The foot surface geometry during impact was then reconstructed using the previously acquired marker coordinates. The time-varying impact points were defined frame-by-frame as the surface locations closest to the ball center and compared with a fixed point defined at impact onset. Mechanical variables, including ball velocity, rotation, deformation, impact force, moment, and angular momentum, were computed using both definitions. The agreement between the angular impulse of the moment and the resultant change in angular momentum was evaluated using scatter plots and Bland-Altman analysis. The time-varying definition showed better agreement, with a smaller mean bias and narrower 95% limits of agreement, validating its utility for detailed mechanical analysis. This method also enables quantification of the time-varying moment across diverse kick types.