A common definition of fatigue is a “subjective lack of physical and/or mental energy perceived to interfere with usual or desired activities.” Fatigue is a typical response to physical or mental exertion such as exercise, challenging physical tasks, or a long day’s work or dealing with emotionally taxing situations. Yet, everyday fatigue is a common symptom among older adults, which may signal the onset or progression of a pathological condition, often serious. To this end, it is no surprise that a large body of evidence links fatigue with higher mortality risk.
When someone perceives fatigue, their initial response is to adjust their activity to avoid the feeling. In terms of physical fatigue, this adjustment could mean walking slower, carrying less weight during day-to-day activities, or simply not engaging in higher-intensity activities—all of which are also associated with higher mortality risk. This phenomenon is referred to as “self-pacing” or setting a new normal (and slower) pace of activity. As self-pacing occurs, measurement of fatigue becomes complicated as effort is reduced to avoid feelings of fatigue. This issue compounds as perceptions of fatigue become more common and subsequent declines in activity accumulate, often unnoticed by the individual or clinical assessment tools.
Fatigability circumvents the self-pacing issue associated with measuring self-reported fatigue. Though fatigability is primarily associated with muscle contractile capacity and exhaustion, we examined perceived fatigability conceptualized as a whole-body measure after performing a standardized physical task. This facilitates comparing the perception of fatigue across individuals who perform a standardized physical task. Further, when the task is lower intensity (i.e., walking 1.5 mph for five minutes, as in our paper), greater perception of fatigue could signal impending physical decline, even among those who appear healthy and well-functioning. As such, the link between perceived fatigability and mobility decline among well-functioning older adults has been previously demonstrated.
In our prospective observational study of 1,076 men and women, we compared whether measures of fatigue and fatigability were differentially associated with mortality. Among the entire sample, fatigue and fatigability were similarly associated with mortality over an average of 10 years. Yet, when we accounted for the presence of medical conditions, we found that fatigability was only associated with mortality risk among those with little to no morbidity. In contrast, the association between fatigue and mortality appeared to be only present in those living with chronic disease (i.e., diabetes, chronic obstructive pulmonary disease). Our results suggest that perceived fatigability can identify elevated mortality risk in seemingly healthy middle- to older-aged people.
To date, perceived fatigability is rarely used outside of research. Yet, we show that fatigability is an important and early marker of accelerated aging and disease processes. Fatigability has the potential to capture the accumulation of physical function deficits undetectable to current testing. We believe the assessment of perceived fatigability can be highly useful in settings of prevention and rehabilitation (e.g., exercise program tailoring) and may be targeted to improve quality of life and health for older adults.
Amal A. Wanigatunga, PhD, MPH, FACSM, is an Assistant Professor of Epidemiology at Johns Hopkins School of Public Health and Core Faculty of the Johns Hopkins Center on Aging and Health (COAH). He is a member of ACSM’s Strategic Health Initiative – Aging committee and is the chair-elect for ACSM’s Aging Interest Group.
Jennifer A. Schrack, PhD, MS, is a Professor of Epidemiology and Medicine at Johns Hopkins School of Public Health and School of Medicine, director of the Johns Hopkins COAH, and the co-director of the National Health and Aging Trends Study (NHATS). She is a member of the ACSM Aging Interest Group.
For over seven years, they have been working together studying fatigability as a key identifier of declines in physical activity and functional performance. Much of their work utilizes cutting-edge technology and methodology to objectively assess the inter-relationships between physical activity, function, and fatigability.
