Fatigability is not a new concept in the exercise physiology realm. Researchers have long paired physical activity measures with ratings of perceived exertion or contractions of isolated muscle groups during tasks to quantify self-reported or muscle fatigability. Of late, the term “fatigability” has evolved and is widely used in aging research to represent a whole-body trait of an individual’s vulnerability to fatigue anchored to standardized physical task(s) of specific duration and intensity. This represents a sensitive prognostic marker of deleterious aging. More severe fatigability is associated with lower physical activity levels, higher chronic inflammation, greater cardiovascular burden and brain atrophy. Importantly, older adults manifesting more severe fatigability are at greater risk for functional limitations, mobility decline, frailty and even death.
Fatigability is highly prevalent, with more than one in four older adults ≥ 60 years of age reporting feeling more severe physical fatigability in their everyday life. Sensitive and validated tools exist to measure perceived fatigability (i.e., what an individual thinks they can do). A common method is to rate one’s perceived effort following a standardized physical task or via self-administered questionnaire (e.g., Pittsburgh Fatigability Scale) that includes physical activities ranging across type, intensity and duration. Another construct of fatigability, performance fatigability (i.e., what an individual does do), is quantified as decline in velocity or decrement in performance during a physical task (primarily walking-based). However, the assessment of performance fatigability lacks validated objective measurements. With advances in technology and statistical methods, we can now detect detailed features of walking patterns with wearable devices. Thus, we developed the Pittsburgh Performance Fatigability Index (PPFI) to objectively quantify performance deterioration during in-lab walking tasks using accelerometry.
Our study, published in the October 2022 issue of Medicine & Science in Sports & Exercise®, described the derivation of the PPFI using wrist-worn tri-axial raw accelerometer data. Conceptually, PPFI quantifies the percentage of cadence decline during a walking task from participants’ own maximal cadence. We applied PPFI in a study of 63 older adults (mean age 78 years, 56% women) and calculated PPFI scores during fast-paced and usual-paced 400-meter walks. The PPFI scores from both types of walks were associated with physical function, gait speed, chair-stands speed, physical fitness and mobility. Collectively, these findings revealed that PPFI is a valid and sensitive objective measure of performance fatigability for older adults.
The novelties of the PPFI include the objectiveness of quantifying granular-level slowing down and the ability to compare scores across various in-lab walking tasks. Using accelerometry makes it easier to measure performance fatigability in large population-level studies, and more importantly it enables an early detection of minimal performance decrement to inform clinical decisions. Additionally, utilizing accelerometry opens up the potential to continuously monitor fatigability in real-world settings as older adults tend to over-perform in the lab. Objectively measuring performance fatigability “in the wild” may better represent what one can actually do. Including PPFI in future studies and clinical practice can deepen our understanding of causes and potential therapeutic targets to ameliorate the adverse effects of fatigability in older adults to prevent disablement, mitigate disease burden and promote healthy aging.
Nancy W. Glynn, PhD, is an associate professor of epidemiology and director of master’s degree programs at the University of Pittsburgh School of Public Health. She is a physical activity epidemiologist with advanced training in exercise physiology. Dr. Glynn’s work focuses on novel methods of measuring fatigability and physical activity in older adults to understand their role in the disablement pathway. She designed and validated the novel Pittsburgh Fatigability Scale, a widely used tool to measure perceived physical and mental fatigability in older adults that is currently available in 16 languages. Dr. Glynn is a member of ACSM and serves as chair of the Aging Interest Group.
Yujia (Susanna) Qiao, ScM, is a doctoral student in the University of Pittsburgh Department of Epidemiology. She is trained as an epidemiologist and specializes in accelerometry, physical activity, physical function and aging. Her research interests center on wearable technology for human health and performance monitoring. Her dissertation work focuses on utilizing accelerometer-derived gait patterns to understand the disablement pathway. She plans to graduate in 2023 and is excited to apply her current research to real-world big data.
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