Friday, 9 August 2013

The A - Z of Sports Vision - Occlusion

Andy Murray is generally reckoned to be the best service returner in the world. He achieves this because he is the best at anticipating the opponent’s serve. How does he do that? One of the best ways of finding out is by occluding the vision of experts so that they can only see part of the opponent’s service action. These experiments have shown that experts can predict the direction of a serve before the racket makes contact with the ball. They are able to use cues such as the motion of the arm holding the racket that occur early in the opponent’s service action (Abernethy & Russell, 1987). The next step has been to see if lesser players can be trained to anticipate better by having their vision occluded at crucial points. Farrow & Abernethy (2002) put this to the test with 32 intermediate-level schoolboy tennis players. They were tested before and after training, with a retention test 32 days later. The tests consisted of facing actual tennis serves on a court whilst wearing occlusion goggles that cut off their sight at various points from 900ms before the server’s racket hit the ball, to a time after contact when the ball was approaching the net. They anticipation was judged according to the direction of the first movement they made as they attempted to return the serve (even though they couldn't actually see it at that point). During training, they watched video clips of serves that were occluded at various points. They had to say immediately where they thought the serve was directed. They would then watch it again when there was no occlusion, so they could assess how accurate they had been. One group was given specific information about the anticipatory cues used by experts, such as the location of the ball toss and the racket head angle just before contact (explicit knowledge), whilst a second group was not given this information but had to estimate the speed of each occluded serve (implicit knowledge). A placebo group did not have video training, but watched videos of tennis matches for an equivalent time. There was also a control group that didn't watch any videos. All four groups also had physical tennis training as well during the 4 weeks. The only group that showed a significant improvement in their anticipation was the implicit training group. They showed an improvement when their vision was occluded after the ball had been tossed, but before contact was made. This period has been shown to be crucial for service prediction (Farrow, Abernethy & Jackson, 2005). The placebo and control groups showed no improvement. Although the explicit group had high scores on average, they did not show an improvement in anticipation. This could be because they had too much information, whereas the implicit group concentrated on the racket movement in order to estimate the speed of the serve, and this was also the best indicator of the serve’s direction. Similar improvements from occlusion training have been found for baseball hitters (Fadde, 2006). Other sports will no doubt catch up in time: for example, there is no reason why it shouldn't be used to help goalkeepers trying to save penalties. David Donner