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An ecological model of motor behavior presented by Davis and Burton (12) suggests that the qualitative and quantitative aspects of motor behavior for all persons emerge from three sets of constraints: performer, environmental, and task. The involvement and performance of movement activities by children with physical impairments may be optimized by carefully manipulating one or more of these three types of constraints, and by recognizing and accepting that the optimal movement patterns used by these children with unique performer constraints may differ from those exhibited by other children.

Ten males and 10 females in each of four grade/age groups threw styrofoam balls of six different diameters as hard as possible at a wall 6.7 m away. Each ball size was thrown four times. The first hypothesis, that the levels of the five components of the one-hand overhand throw would be quite stable for individuals for throws of a particular ball size, was supported. Ball sizes at which the component levels were unstable marked the beginning of a transition to a new component level 70.6% of the time. The second hypothesis, that five components would change from higher to lower levels for most of the subjects as ball size was scaled up, was supported only for the backswing and forearm components. These components were more likely to be affected by increasing ball size because the higher level components required a firm, one-hand grip on the ball, which became more difficult as ball diameters exceeded the subjects’ hand widths. The results indicate that practitioners need to recognize that different ball sizes may elicit different throwing patterns, and specifically that a critical ball diameter may be reached when it is equal to hand width.

This study examined the effect of ball size on the movement patterns used by children and adults to grasp a ball and then to throw it as hard as possible. A total of 104 kindergarten, second-grade, fourth-grade, eighth-grade, and young adult males and females were asked to pick up six styrofoam balls of different diameters (from 4.8 to 29.5 cm) four times each as they were presented in random order, and then throw them as hard as possible at a wall 6.7 m away. Transitions from one- to two-hand grasps were made as ball diameters increased, with older subjects switching at significantly larger diameters than younger subjects (p<.0001); however, when ball size was scaled to hand size, older subjects switched at significantly smaller relative diameters than younger subjects (p<.Ol), indicating that hand size may be a critical factor in determining grasp form. Transitions from one- to two-hand throws were made by less than 25% of the subjects (mostly kindergartners and females), demonstrating a strong preference by older children and adults for throwing with one hand, even with ball diameters larger than a subject’s hand size.