Regulation of pendulum length as a control mechanism in performing the backward giant circle in gymnastics.

Seven female elite gymnasts performed backward giant circles on the high bar under different conditions of loading. The magnitude (2 or 4 kg) and location (shoulders, waist, and ankles) of load systematically influenced the overall swing duration as well as the relative timing of movements at the joints. An analysis of the mechanical constraints operating suggested that the gymnast should be considered as a pendulum of variable length. Increasing and decreasing pendulum length at appropriate phases of the swing effectively allows energy to be injected into the system, thereby compensating the energy lost to friction. A sharp negative peak in the relative rate of change of pendulum length, characteristic of the upward swing phase of all gymnasts, was found to invariably occur at a particular value of the first-order time-to-closure of the body orientation gap with respect to the vertical. The presence of this invariant suggested that the gymnasts organize their behavior on the basis of such a first-order temporal relation.