Effects of visual occlusion by the wall on goalkeeper performance during free kicks in football

22 Free kicks are an important goal scoring opportunity in football. It is an unwritten rule that the 23 goalkeeper places a wall of defending players with the aim of making this harder for the attacking 24 team. However, the defensive wall can occlude the movements of the kicker, as well as the initial part 25 of the ball trajectory. A ball coming into view later will likely delay movement initiation and possibly 26 affect performance. Here, we used virtual reality to investigate the effect of the visual occlusion of the 27 initial ball trajectory by the wall on the performance of expert and novice goalkeepers. We showed 28 that movements were initiated significantly later when the wall was present, but not by the same 29 amount as the duration of occlusion (~200ms, versus a movement delay of ~70-90ms); movements 30 were thus initiated sooner after the ball came into view, based on less accumulated information. For 31 both novice and expert goalkeepers this delayed initiation significantly affected performance (i.e., 1.532 3.6cm larger spatial hand error, 7-8% fewer interceptions). These performance reductions were 33 significantly larger for shorter flight times, reaching increased spatial errors of 2.8-4.5cm and 11-13% 34 fewer interceptions. Further analyses showed that the wall-induced performance reduction did not 35 differ significantly between non-curved and curved free kicks. The wall induced early movement 36 biases, but only for ball trajectories with curve in the same direction as the required movement; the 37 effect of these biases was negative, away from the final ball position. In two additional experiments 38 (with variations in initial forward and sideward ball position, respectively) largely consistent results 39 were found (i.e., the wall-induced performance reduction was smaller and failed to reach significance 40 with variations in initial sideward ball position). These results may suggest that, certainly for expert 41 free kick takers who are very good at shooting hard free kicks that avoid the wall (reducing its function 42 as a block), goalkeepers could consider omitting the wall. 43 44 45