A ball is travelling towards you. How do you know when it’s going to arrive, so you can get your hands in the right position to catch it? Muscles in the hand must be prepared so that the hand is widest open before the ball hits just above the palm (ideally), the wrist forearm and elbow must be prepared to absorb the kinetic energy of the ball, and the finger joints must also be prepared to close around the ball after impact. Muscle responses need to start about 100 milliseconds before the ball arrives.
From physics, the time taken for the ball to arrive will be its distance away from you divided by the speed at which it’s travelling. The brain can deduce information about the ball’s position from the size of its image on the retina; the smaller the image, the further away it is. The rate at which the image is getting bigger tells the brain how fast the ball is coming towards you (i.e. its speed).
The brain doesn’t actually calculate the ratio between them, but it knows, through practice, that when this ratio reaches a critical level, it’s time to send the necessary messages to the arm and hand muscles.
Savelsbergh et al conducted an interesting experiment in 1991 using a balloon that deflated as it approached the catcher. Because the image size on the retina did not increase as normal, the catchers overestimated the time to contact, showing that they were using image size for their timing rather than just the speed of the ball. The effect was most noticeable when one eye was covered. It was still there when both eyes were open, but less so, suggesting that stereoscopic information from vision can also be used.
This is fine for a ball that’s travelling at a steady speed, but what about a ball that’s accelerating? An outfielder under a skied catch has to deal with a ball that’s dropping at a rate of 32 ft/sec², and they may have to run to get in line with it as well.
If the fielder is directly under a skier, the brain can use something known as the “zeta angle”. This is the apparent size of a falling object relative to its apparent size at the moment the object started to fall. If it can make an allowance for gravity, the brain can initiate the catching actions when the zeta angle reaches a certain level, as long as the point when the ball started to drop was observed.
Experiments on astronauts on board Space Shuttle Columbia have shown that we do indeed make an adjustment for the effect of gravity. In space, the astronauts were too early in their attempts to catch the ball, because their brains were programmed to take into account the effect of a non-existent gravity.
If the ball is coming straight to the fielder, then his angle of gaze – the angle between the eye and the ball – will remain constant. If it’s not coming straight at him, then the angle of gaze will vary as the ball travels. The brain can use this information by trying to keep the angle constant or within a certain range. This information will get enable the catch to be made (assuming the fielder can run fast enough). The brain doesn’t calculate where the ball will land: it just uses the visual information to get the fielder to where the ball will land.
Finally, the fielder needs to be in a position where the ball appears to be travelling at a constant speed. If it’s speeding up, you need to move back, or move forward if it appears to be slowing down.
The unsurprising conclusion from all of this is that the more experience of catching from a young age that one gets, the better.
For more information please visit http://www.donneroptometrists.co.uk/sports-vision.htm