Measuring and Understanding the Health Impact of Greater Fatigability in Older Adults: A Call to Action and Opportunities.

Different from fatigue, an instantaneous state of tiredness, weakness and lack of energy, fatigability is a trait that contextualizes whole-body fatigue to the level of activity (i.e., intensity and duration) with which the fatigue is associated. Fatigability can be perceived or performance-related. Measuring fatigability improves upon traditional fatigue measures by accounting for self-pacing as older adults likely slow down or limit their daily activity to maintain fatigue in a tolerable range. Anchoring fatigue to activities/tasks improves sensitivity and allows for meaningful comparisons across individuals/between studies, as well as evaluating change over time and treatment effects. Two well-validated approaches are utilized to measure perceived fatigability: 1) a 5-minute slow-paced (1.5 mph/0.67 m/s, 0% grade) treadmill walk immediately followed by Borg rating of perceived exertion; and 2) a self-administered 10-item questionnaire, Pittsburgh Fatigability Scale, with both physical and mental subscales. Many walking-based performance fatigability measures are based on certain lap time or distance, while the Pittsburgh Performance Fatigability Index uses raw accelerometry data to quantify percent of cadence decline over the entire long distance walking tasks. Perceived fatigability prevalence ranges from 20-90% in older adults varying by assessment tool, and is higher with advancing age and in women compared to men. Fatigability is associated with physical and cognitive function, fall risk, mobility decline, and mortality. Unfortunately, the available research lacks representativeness in terms of racial and ethnic diversity. The time is now to incorporate our established sensitive and validated fatigability measures into global research and clinical practice to better understand mechanistic underpinnings and reveal intervention effects to reduce the burden and lessen the consequences of greater fatigability worldwide.