Intra-subject inconsistencies in quantitative assessments of body sway

Many reports have appeared in which the postural stability of different groups of individuals has been compared by means of body sway measurements. A common quantitative index of body sway is obtained by observing the subject standing still on a force plate and measuring the length of the trajectory of the centre of pressure over the surface of the plate during a specified period of time. We have measured body sway on normal healthy adult subjects. It appears that repeated measurements upon the same individual show a very large spread of values. Many of the subjects did not show consistent results when means of 10 trials each of 1 min duration were measured on consecutive days. We conclude that repeated tests upon a particular individual would have limited value in diagnosis or assessing the progress of a neurological disorder or the efficacy of medication, or therapy, in its treatment. Further, we would be unable to use an individualʹs value of the body sway to determine reliably whether he falls within a particular group.