The flight trajectory of a non-spinning or low-spinning soccer ball may fluctuate in unpredictable ways, with anomalous horizontal shaking or rapid falling. This "knuckling shot" is famously exploited by Cristiano Ronaldo in his free kicks. However, the technical aspects of kicking a knuckling shot are still unclear, and considerable analysis is required. In this study, we used high-speed video cameras to compare the knuckling shot with curve and straight shots. The results indicate that the angle of attack in the knuckling shot (3.3 ± 0.9°) was smaller than those in the straight and curve shots (19.3 ± 2.4°, 32.6 ± 3.9°, respectively). In sum, we demonstrated that in the knuckling shot, reduced angles of attack between the face and swing vectors at the impact surface and the primarily translational motions at ball impact are fundamental mechanisms.
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