Body-scaled ratio as a control parameter for prehension in 5- to 9-year-old children.

The purpose of the experiment was threefold: (a) To find evidence that grasping is body-scaled and thus remains invariant during development; (b) to seek evidence that the body-scaled ratio of cube and hand size serves as a control parameter for the phase transition from one-handed to two-handed grasping by identifying the presence of sudden jump, enhanced variance, multistability, and hysteresis; and (c) to examine whether the stability properties of the observed grasping patterns increase with age. Thirty-three children aged 5, 7, and 9 years old were required to grasp and lift 14 cardboard cubes of different sizes (2.2, 3.2, 4.2, etc. to 16.2 cm diameter). Three conditions were used: (a) an increasing condition with sizes ordered from the smallest size to the largest; (b) a decreasing condition, with the sizes ordered from the largest to the smallest; and (c) twice in a different random order. Video recordings were analyzed and scored for the percentage of one-handed grasps. The results showed that the shift from one-handed to two-handed grasping occurred at the same body-scale ratio between cube size and finger span for all three age groups. Evidence was found for the presence of a sudden jump, enhanced variance, multistability, and hysteresis, indicating that the body-scaled ratio of cube and hand size serves as a control parameter. No change with age for the stability properties of the grasping patterns were observed.