# Running to catch

Elite Swing Mechanics

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In the previous section, we considered how people move through a stationary visual environment. More complex issues are raised when the crucial part of the visual environment is also moving. The example we will consider is that of a fielder at rounders, cricket, or baseball who has to run several metres at high speed to catch a ball. This ability is more surprising than you might imagine. The fielder only has information about the trajectory or flight path of the ball as seen from his or her perspective (this is the optical trajectory), and that trajectory is influenced by various factors such as wind resistance.

At a general level, Oudejans et al. (1996) found that fielders obtain very valuable information as they run towards the ball. They used a machine that shot tennis balls from behind a screen. The participants were only allowed to see the ball moving for one second, during which they either ran towards the ball or remained stationary. Those who ran towards the ball perceived the catchability of the ball much more accurately than did the stationary participants.

What information is used by fielders in motion towards a ball? McLeod and Dienes (1996) filmed expert fielders as they ran forwards or backwards to catch balls projected from a machine. They found that the fielders "ran at a speed that kept the acceleration of the tangent of the angle of elevation to the ball at 0" (McLeod & Dienes, 1996, p. 531). The tangent of the angle of elevation corresponds to the ratio of the height of the ball above ground to the horizontal distance of the fielder from it. Running so as to keep the rate of change of the tangent constant involves shortening one's horizontal distance from the ball in proportion to the rate at which it is dropping out of the sky, so as to intercept it at ground level. Use of this measure does not allow fielders to know in advance where or when the ball will land, but ensures that they arrive at the right place at the right time.

The McLeod-Dienes approach only relates to balls moving in the direction of the fielder, and does not cover the more common cases in which the ball is struck to the left or the right of the fielder. There are also other limitations and problems. As McLeod and Dienes (1996, p. 542) drily remarked:

Our data do not indicate how the computational problem of keeping d2 (tan alpha) dt2 at zero is solved.

Scepticism about the conclusion might stem from the feeling that d2 (tan alpha)/dt2 does not seem a particularly likely quantity for the nervous system to represent.

McBeath, Shaffer, and Kaiser (1995) produced a more general solution to the problem of how fielders catch balls. According to them, fielders run along a curved path designed to keep the optical trajectory (flight path as perceived by an observer) as straight as possible. Fielders following this strategy would arrive at the right place just in time to catch the ball, but would not know ahead of time where the ball would drop. Fielders using this strategy do not allow the ball to curve optically towards the ground, and they achieve this by continuously moving more directly under the ball.

McBeath et al. (1995) obtained support for their theory from two students who used shoulder-mounted cameras, and who were filmed trying to catch balls. According to McBeath et al.'s analysis, fielders should generally catch balls on the run rather than arriving in the catching area ahead of time. Less information about curvature of flight is available to fielders when the ball is coming straight at them, and thus it should paradoxically be harder to catch the ball in such circumstances. Both of these predictions were confirmed.

### Evaluation

The theoretical approaches of McLeod and Dienes (1996) and of McBeath et al. (1995) have definite strengths. They show that fielders can make use of some invariant feature of the information potentially available to them to run into the optimal position to catch a ball. However, there are some unresolved issues. First, the research evidence is consistent with the theoretical approaches, but strong support is lacking. For example, it would be valuable to show that experimental manipulations of the key theoretical variables in artificial situations led fielders to make systematic errors. Second, the internal processes allowing individuals to calculate the measures allegedly involved remain unspecified. It is likely to prove difficult to show how anyone manages to calculate d2 (tan alpha)/dt2 in the stressful circumstances of a competitive cricket match or baseball game. Third, more research is needed to resolve the differences between the two theoretical approaches we have discussed.