• Susan L Murphy and Dylan M Smith.
• Department of Physical Medicine and Rehabilitation, University of Michigan, 300 North Ingalls Street, Ann Arbor, MI 48109-2007, USA. sumurphy@umich.edu
• J. Gerontol. A Biol. Sci. Med. Sci. 2010 Feb 1; 65 (2): 184-9.

BackgroundFatigue is associated with loss of independence in older adults; however, little is known about optimal treatment or how fatigue manifests in daily life activities. "Fatigability" was recently proposed to clarify the fatigue-activity relationship. The purpose of this study was to present a new measurement method of fatigability and begin to test its validity.MethodsOur sample included 40 adults with knee or hip osteoarthritis (OA) and 20 healthy controls. Fatigue was measured by ecological momentary assessment several times a day along with continuous measurement of physical activity using a wrist-worn accelerometer. Fatigability was measured as the fatigue increase after a period of high activity.ResultsCompared with controls, participants with OA were approximately four times more likely to have an increase in fatigue after a high activity interval (37.0% vs 9.8%). Among people with OA, average fatigue and fatigability were not highly related (r = .13). Fatigue was most strongly associated with reported physical function, pain, and vitality, whereas fatigability was most strongly associated with body mass index, OA severity, and knee strength.ConclusionsAlthough fatigue among people with OA was more associated with subjective reports of physical function and symptoms, pairing fatigue reports with physical activity tapped objective factors that may be related to the biomechanical demands of daily life activities. Thus, fatigability measurement may help discern how symptoms relate to daily life function and help to refine treatment approaches in OA.

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