Dynamical origins of stereotypy: relation of postural movements during sitting to stereotyped movements during body-rocking.

The relation between the movement dynamic properties of sitting still and of seated body-rocking in adults with stereotyped movement disorder and mental retardation and a contrast group of typically developing age-matched adults was examined. Continuous measurement of sequential displacements in center-of-pressure was made using a force platform while subjects were engaged in seated body-rocking and quiet sitting. Properties of movement were compared across conditions (rocking, sitting) and groups (stereotyped movement disorder, contrast). The contrast group had the same modal frequency for both movement properties. The intrinsic dynamics of the stereotyped movement disorder group were similar to those of the contrast group for body-rocking but very different for quiet sitting. Findings support the suggestion that body-rocking in stereotyped movement disorder originates partly as an adaptation to an inability to control posture in a seated position.