More information on the nature of stereotyped body-rocking.

Body-rocking was exposed to kinematic analysis in two studies. In the first study, amplitude was larger in the natural body-rocking of individuals with mental retardation than in the natural body-rocking of college students. Variability did not differ. In the second study, natural body-rocking of people with mental retardation was compared with their artificial body-rocking. Amplitude and variability were larger in the artificial condition. However, this result was not clear. We suggest that group and condition differences might have been a function of practice.