Variability of stereotypic body-rocking in adults with mental retardation.

The variability of stereotypic body-rocking motions of adults with severe and profound mental retardation not on medication was examined through a kinematic analysis. A matched nonretarded group of adults was also examined in the production of preferred rates of body-rocking. The inter- and intra-individual variability of the body-rocking motions was, on average, higher for the individuals with mental retardation, although a few participants showed as low a variability as the least variable participants in the nonretarded group. These findings provide further evidence that the kinematic variability of stereotypies is not lower than that displayed by others engaged in similar movement activities and support the proposition that low variability of discrete kinematic variables may not be a defining feature of stereotypy.