Fatigue and fatigability in older adults.

Fatigue is believed to be a common complaint among older adults; however, multiple studies of self-reported fatigue across the lifespan have found this may not be the case. To explain this paradox, this article considers "fatigability"-a phenotype characterized by the relationship between an individual's perceived fatigue and the activity level with which the fatigue is associated. Fatigability may be measured by combining self-report measures of fatigue with performance of physical or cognitive activities, provided that the work of the activity is known or can be standardized. Doing so prevents self-pacing and allows meaningful comparisons across subjects and between studies. Increased fatigability with aging may arise from a variety of factors including age-related changes in energy production or utilization, and inflammatory mechanisms. A few published intervention studies have targeted fatigue in older adults, though none have examined fatigability specifically. Because fatigue may represent a physiologic warning system, future clinical studies may benefit from a focus on fatigability, where both symptoms and function are considered.