Purpose: The purpose of this study was to reveal the foot-ball interaction during ball impact phase of soccer instep kicking.
Methods: Eleven soccer players performed maximal instep kicks. The behavior of kicking foot and ball during ball impact was captured using two ultrahigh-speed cameras at 5000 Hz. Foot motion was described three dimensionally, and the motion of the center of gravity of the ball (CGB) was estimated by the spherical shell model in which the ball deformation was taken into account. The peak ball reaction force acting on the foot was estimated from Newton's equation of motion in which the peak CGB acceleration in sagittal plane was calculated from its velocity slope near the peak ball deformation.
Results: During ball impact (9.0 +/- 0.4 ms), the foot was passively abducted and everted. Moreover, an unknown feature--slight dorsal flexion before distinctive plantarflexion--was quantified in most trials. The CGB velocity exceeded that of the foot when the ball was maximally deformed (6.2 +/- 0.6 cm). The magnitude of peak ball reaction force reached 2926 +/- 509 N, which corresponds to approximately twice as that of the mean force (1403 +/- 129 N). From the changes of the foot velocity, the CGB velocity, and the ball deformation, the ball impact phase can be divided into four phases.
Conclusions: The ultrahigh-speed video and methodology in this study documented complex three-dimensional foot motions to impact in soccer instep kicks, dynamic foot-ball interaction, and larger peak ball reaction force on the foot that previously estimated. It can be considered that effectual duration to accelerate the ball is roughly three fourths of visually determined ball contact time.