A biomechanical invariant for gait perception.

Viewers can determine the gender of a walker from sagitally projected, dynamic displays of point-lights attached to prominent joints. This article explores three interrelated approaches in search of a biomechanical invariant that viewers might use. The first, an index of torso structure, accounts for the data handsomely but seems inappropriate because it is not directly revealed in the dynamic stimuli. The second, a dynamic index of visible torsion in the trunk of a walker, also fits the data well but seems to have a logical problem and a difficulty in accounting for performance in certain conditions of several previous studies. The third has the strengths of the first two indices, and it can account for some other data as well. It is the center of moment and is a "deeper", more general description of the invariant. This center is a point around which all movement occurs. It can be thought of as one specification of the gestalt law of common fate and may be helpful for the study of movement perception in general.