Assessing disability in studies on physical activity.

Despite the increased focus on disability as an important research outcome, there remains a need to improve disability instrumentation. This article discusses the need for improved operational differentiation in instruments used to assess disability outcomes. It details the challenge of clearly distinguishing between the concepts of function and disability in outcome instruments used in physical activity studies. The second issue in existing instruments is the limited precision to detect important changes in disability in response to physical activity interventions. Computer adaptive testing is discussed as a promising avenue for resolving the methodologic limitations inherent in existing outcome instruments. Priorities for future research that move the field forward in both of these areas are discussed.