Do fielders know where to go to catch the ball or only how to get there?

Skilled fielders were filmed as they ran backward or forward to catch balls projected toward them from a bowling machine 45 m away. They ran at a speed that kept the acceleration of the tangent of the angle of elevation of gaze to the ball at 0. This algorithm does not tell fielders where or when the ball will land, but it ensures that they run through the place where the ball drops to catch height at the precise moment that the ball arrives there. The algorithm leads to interception of the ball irrespective of the effect of wind resistance on the trajectory of the ball. The everyday nature of the act of running to catch a ball can obscure the remarkable predictive ability that it requires. Figure 1 shows the trajectories of three balls projected at 45 ° and approximately 22, 24, and 26 m/s toward a stationary fielder 45 m away. They will land 5 m in front of, at, or 5 m behind the fielder, respectively. The solid line shows the trajectory of each ball in the first 840 ms; the dashed line shows the rest of the flight. Within 840 ms, most competent fielders would have started running forward for the ball on the lower trajectory and backward for the ball on the higher trajectory. 1 Yet, the only difference between these two flights at this time is the difference between the longest and shortest solid lines. How is the fielder able to work out where to go from so little information? Precise calculation of the trajectory is not possible because the essential ball flight parameters of projection angle, velocity, and wind resistance are available to the fielder only as, at best, crude estimates. Nor, given the infinite variation of trajectory, does it seem possible that learning to catch involves learning individual trajectories. An alternative is that an algorithm exists that links the visual information obtained from watching the balis flight to a running speed that will bring the fielders to the correct place, irrespective of their starting position or the balis trajectory. Learning to catch would involve the discovery of this algorithm.