Wednesday, 2 June 2010

Sports Vision - Fielding in Cricket

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.

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Sports Vision - Bowling in Cricket

After 5 good deliveries, the bowler bowls a poor ball that gets hit for four. A batsman has been tied down by a spell of good bowling, and ten plays a wild shot, throwing his wicket away. A bowler has bowled several good overs, then a poor ball, “the worst ball I’ve bowled all day”, takes a wicket.

These things happen all the time in matches, but how often do they happen in the nets? Rarely, if ever. Most of the time, bowlers are used as fodder to give the batsmen time at the crease. Are they really practising their trade effectively?

In a match, the bowler can put pressure on a batsman by bowling a succession of testing deliveries. So why not get bowlers to ball 6 balls at a time in the nets? Markers on the nets can indicate fielders’ positions, and if you have an umpire in place, judgements can be made on appeals and runs scored.

Bowling is an aiming activity, with the point where the ball pitches being the target. This point will vary according to the type of delivery the bowler wants to bowl, as well as the style and stature of the striker. These can be marked on the floor of the net, and adjusted if necessary, for instance if the striker starts batting out of his crease. These marks may also help the striker pick up the length of deliveries, so they help both bowlers and batsmen.

If no balls are a problem, the popping crease can be highlighted, but it’s essential that he bowler uses their peripheral vision for the popping crease, maintaining fixation on the point where they want the ball to pitch. The no ball problem can often be cured by simply moving he bowler’s mark forward (Note: not back).

Sports Vision - Batting in Cricket

“The head must be kept upright and turned towards the bowler with the eyes as level as possible” (The MCC Cricket Coaching Book).

But why should the eyes be as level as possible? According to the MCC, only in this position can “the batsman focus both eyes together”. But if you look at an object and twist your head slightly, you’ll notice that things don’t go blurred or double. The reason is that the balance organs in the ears register this head tilt and, via the vestibulo-ocular reflex, produce a counter-rotation of the eyes to maintain clear vision.

Another reason that’s commonly given is that keeping the eyes level helps speed up your reaction time. I have been unable to find much evidence to support this claim, however. I did find one experiment that showed a link between head tilt and lower reaction times, but there are one or two problems with applying it to batting. Firstly, subjects had to react to a sound. They were also sitting down………in the dark.

Even if there were a link, how much of batting is to do with reaction times, anyway? Sir Donald Bradman was found to have slower reactions than the average University student. Yet, as Jim Laker said “Bradman always seemed to know where the ball was going to pitch, what stroke he was going to play, and how many runs he was going to score”. It seems that batting has more to do with anticipation than reaction times.

This is not to say that one shouldn’t have ones eyes level. For some batsmen it might help them focus on the bowler’s action and release of the ball, and for others it might be necessary to ensure a consistency of eye dominance between watching the bowler and playing the shot. But it suggests that there isn’t one perfect technique that should be employed by everyone, but it’s more a question of the individual’s brain working out (ideally subconsciously) what works best for them. And is it possible that the twisting that’s’ required in order to get the eyes level from a sideways stance contributes to the back injuries suffered by many batsmen?

Bradman did not have a classic technique. He was entirely self-taught, and didn’t play on a grass pitch until he was 18, having been brought up on concrete pitches covered in matting. He had an unusual grip, with his right hand nearly facing straight down the pitch, and the “V” of his left hand in line with the splice. His bat was closed and locked between his feet, and he picked it up at an angle of 45 degrees to the flight of the ball. None of this could be found in any coaching manual, and led to much criticism early in his career. He was, however, the master of seeing the ball early, and playing it late.

He also fits into the classic profile of an elite sportsman in having had essentially unlimited access to practice in his developing years whilst pursuing a range of sports. In Bradman’s case, he spent many hours playing imaginary Test matches, throwing a golf ball against the brick base of a water tank and hitting it with a stump. He was a highly proficient billiards player, played off a scratch handicap in golf for many years, and excelled in tennis, which he played before taking up cricket.

There are three key visual stages in batting: watching the bowler’s action and release of the ball; watching the pitch of the ball; and watching the contact between bat and ball.

By observing the bowler’s action, Test match batsmen are able to determine what type of ball a swing bowler is delivering (outswinger, inswinger, short ball) by the time the front foot has landed. They can also determine a leg spinner’s delivery (leg spinner or googly) by the same time from observation of the bowler’s wrist. Accurate determination of the length of the delivery seems to require some early tracking of the ball after release.

Land & McLeod (2000) monitored the eye movements of expert batsmen when facing a bowling machine. They found that even elite batsmen weren’t able to track the ball throughout its flight. They would track it for a while and then jump ahead to the point where they expected the ball to pitch. If the ball was short-pitched, they would make that jump in fixation earlier. If it was over-pitched, they could track almost the whole way.

A very quick delivery may pitch less than a quarter of a second after the bowler releases it. Because novices aren’t so god at anticipating the length of the ball, they make their jump to the bounce point too late. Either they’re still trying vainly to follow the path of the ball, or they are making their jump as the ball pitches. Because vision is suppressed when one makes this jumping (“saccadic”) eye movement, they can lose sight of the ball completely.

If the batsman knows the where the ball has pitched, and the speed at which it’s travelling, he can work out when it will arrive. The brain doesn’t actually work out these calculations in a mathematical way. Rather it develops rules of thumb based on long periods of practice. But it still needs to pick up information about the speed of the ball and where it’s pitched in order to make the correct shot decisions.

A sense of the speed of the ball is obtained from the time it took to leave the bowler’s hand to reach the bounce point. An adjustment then needs to be made for loss of speed after pitching. This is why batsmen generally need to play themselves in: so they can make the necessary adjustments according to whether the pitch is fast or slow.

According to Land & McLeod, the position that the ball pitched can be determined by the formula B/tanФ, where B is the height of the batsman’s eye and Ф is the angle that the batsman’s eyes have had to look down. So the more the batsman has to look down, the closer to him the ball has pitched. It may be, however, that the brain establishes the pitch point from more general principles of how far away things are, for instance by using the different images that form on the two retinas from near objects.

However the brain does it, one clear message should be that it’s more useful to tell a batsman to concentrate on where the ball has pitched, than a general “watch the ball” instruction. If you know accurately where the ball has pitched, you’re most of the way to knowing what shot to play and when. It can be useful to ask batsmen in the nets where they think the ball pitched, so they can feedback on their perception.

The final key visual stage is the contact between bat and ball. In other interception sports, such as tennis, focusing on the contact point has been shown to be highly important for elite players. Some imagine a hitting area into which the ball arrives, and they maintain focus on that area even after the ball has been hit. Batting tees can be useful aids in concentrating on the contact point.

Differences in batting style have been observed when facing a bowling machine compared with facing a bowler (Pinder, Ross & Davids 2006), such as shorter and later backswing and shorter stride length. As an umpire, I’ve seen young batsmen display excellent stroke-play, but with no idea of how to build an innings, no shot placement, and often pre-meditating the shot. They look like they’re playing in a net, rather than in a match. So there’s clearly a danger in over-relying on net practice, and bowling machines in particular, although they clearly have their place.

It is therefore useful to try and make the nets more like a match, for instance by having two batsmen alternating the strike, and by marking fielder’s positions on the nets. Ideally, an umpire could stand and make decisions as well as giving a notional score for each shot.

When the batsman surveys the field, the obvious thing is just to note where the fielders are. However, this can often lead to the ball being struck precisely to where those fielders are. More useful is to focus on the gaps between the fielders, ideally to a specific point between them. This becomes much more powerful if the striker also visualises hitting the ball to that specific point.