Two different balance testing methods were compared: (1) measurement of spontaneous postural sway during quiet standing, and (2) measurement of induced postural sway in response to an applied postural perturbation. Eyes‐open tests were performed in 64 healthy young and elderly adults and in five elderly subjects with a history of falling. In both balance tests, the sway was defined in terms of the displacement of the center of pressure on the feet. Spontaneous sway was quantified using a number of different amplitude‐ and frequency‐based parameters. Induced sway was measured in response to anterior‐posterior acceleration of a platform on which the subject stood. The induced‐sway test was specially designed to be safe and nonthreatening for elderly subjects; thus, the platform perturbation was confined to small accelerations and a gentle pseudorandom motion was used. To derive a measure of postural stability, the data from this test were fitted with a model that was then used to predict the response to sudden (transient) perturbations, thereby simulating the response in actual falls. Although both induced‐ and spontaneous‐sway measures demonstrated significant aging‐related decreases in stability, the differences were more pronounced for the induced‐sway data. Conversely, some of the spontaneous‐sway measures were much more successful in distinguishing the fallers from the nonfallers. There was a significant correlation between induced‐sway and certain spontaneous‐sway measures in the normal young adults; however, in the elderly normals and fallers, the data from the two types of balance tests either showed no correlation or, for certain spontaneous‐sway measures, tended to show an inverse relationship. © 1990 The American Geriatrics Society
