Three-dimensional kinetic analysis of side-foot and instep soccer kicks

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Abstract
The purpose of this study was to identify the kinetic aspects of side-foot and instep soccer kicks to understand the different mechanics underlying the two kicks. The motions of both kicks were captured using a three-dimensional cinematographic technique. The kicking leg was modeled as a three-link kinetic chain composed of thigh, shank, and foot, from which joint torques and angular velocities were computed. The ball velocity of the side-foot kick (23.4 ± 1.7 m·s−1) was significantly slower than that of the instep kick (28.0 ± 2.1 m·s−1). Significant differences were also observed between the two kicks for the magnitude of hip external rotation torque (56 ± 12 N·m in the side-foot kick; 33 ± 8 N·m in the instep kick) and hip external rotation angular velocity (11.1 ± 2.4 rad·s−1 in the side-foot kick; 6.0 ± 2.0 rad·s−1 in the instep kick). These results indicated that to hit the ball with the medial side of the foot, a complicated series of rotational motions are required for the side-foot kick. The hip external rotation torque dominantly exhibited in the side-foot kick caused the clockwise rotation of the thigh-shank plane at the later stage of kicking. This may allow the hip external rotation motion to increase directly the forward velocity of the side foot, with which players can squarely impact the ball.