Table 2.

Overview of fatigability studies.

Study and subjects	Design	Study aim	Measures used to assess fatigability	Major findings
Barbosa et al., 2015 N = 48 Elderly women	Cross-sectional	To determine the relationship between perceived fatigability and oxygen consumption (VO₂), carbon dioxide production, respiratory exchange ratio, and the energy cost of walking in older women.	• Fatigability • Severity of perceived fatigability • Severity of performance fatigability	Severity of perceived fatigability was correlated with greater VO₂, low physical activity, low walking distance, and severity of performance fatigability.
Buchowski et al., 2013 N = 17 Elderly individuals	Cross-sectional	To measure changes in performance and fatigue across a series of standardized physical tasks that required different levels of energy expenditure and to explore the relationships among these measures.	• Changes in perceived fatigue • Changes in performance • Performance fatigability severity • Perceived fatigability severity	Performance and perceived fatigability severity scores were correlated, and both perceived and performance fatigability severity scores were associated with physical activity-related energy expenditure.
Glynn et al., 2015 N = 1,013; 483 (for validation) Elderly individuals (aged ≥60)	Cross-sectional	To describe the development of the Pittsburgh Fatigability Scale and to establish its reliability and concurrent and convergent validity against performance measures.	• The Pittsburgh Fatigability Scale instrument	Physical fatigability scores of the instrument, adjusted for age, gender, and race, were greater for those with high performance fatigability, slow gait speed, worse physical function, and lower fitness.
Gonzales et al., 2015 N = 45 Elderly individuals (aged 60–78)	Cross- sectional	To test the hypothesis that central (carotid artery) and peripheral (superficial femoral artery, SFA) arterial stiffness would be associated with perceived fatigability during walking in older adults	• Perceived fatigability	The SFA β-index was identified as an independent predictor of perceived fatigability.
Keyser et al., 2015 N = 13 Physically inactive patients with interstitial lung disease	Prospective (one-group pretest-posttest design)	To examine the hypothesis that variability in cardiorespiratory function could exert a modulating influence on fatigability that is independent of variability in the maximum capacity of the system.	• Performance fatigability	Performance fatigability was reduced 11% in patients with interstitial lung disease after aerobic exercise training.
Lin, Roiland, Heffner et al., 2014 N = 49 Elderly individuals (aged 75+) with vascular risk	Cross-sectional	To test a new way of measuring objective mental fatigability by examining its association with perceived mental fatigability and to identify associated psychological, physiological, and situational predictors.	• Objective mental fatigability • Perceived mental fatigability	Objective mental fatigability and perceived mental fatigability were correlated, and they were not associated with the same factor.
Lin, Roiland, Polesskaya et al., 2014 N = 55 Elderly individuals	Retrospective	To investigate the effect of fatigability on cognitive processes and inflammatory response (IL-6) following an acute cognitive stress task in older adults	• Perceived fatigability	The high fatigability cluster had higher levels of IL-6 response than the low fatigability cluster. After controlling for multiple covariates, fatigability was found to moderate the relationship between processing speed and IL-6 reactivity.
Manty et al., 2012 N = 1,181 Elderly individuals (nonagen-arians)	Cross-sectional	To evaluate the prevalence and associated health factors of indoor mobility- related fatigability among nonagenarians	• Two items of the Avlund Mobility-Tiredness Scale	Fatigability was associated with cardiovascular diseases, musculoskeletal pain, medications, walking speed, and depressive symptoms.
Murphy & Smith, 2010 N = 60 (40 osteoarthritis patients and 20 controls)	Cross-sectional	To present a new measurement method for fatigability and begin to test its validity.	• The increase in fatigue severity after a period of high activity (1 standard deviation above mean activity) measured using accelerometers	Subjects with osteoarthritis (OA) were about four times more likely to have an increase in fatigue after a high-activity interval than controls. Among subjects with OA, average fatigue and fatigability were not highly related. Fatigability was most strongly associated with body mass index, OA severity, and knee strength, but fatigue was most strongly associated with reported physical function, pain, and vitality.
Richardson et al., 2014 N = 36 Elderly individuals	Cross-sectional	To examine the hypotheses that (a) slower preferred gait speed is associated with higher energetic requirements during walking and higher levels of fatigability and (b) preferred gait speed is not associated with fatigue.	• Situational Fatigue Scale • Rating of perceived exertion (RPE) at the end of a 400-meter walk • RPE after walking at standard speed (0.72 m/s) on a treadmill • RPE after walking at subject-preferred speed on a treadmill	Slow walkers reported higher RPE after walking and greater overall fatigability on the scale but no differences in fatigue.
Santanasto et al., 2014 N = 30 Elderly individuals	Cross-sectional	To examine the effects of higher and lower fatigability on lower skeletal muscle oxidative capacity.	• RPE using the Borg scale after a 5-min treadmill walk	Subjects with high fatigability required a higher proportion of VO₂ peak to walk at 0.72 m/s compared to those with low fatigability.
Schnelle et al., 2012 N = 43 Elderly individuals	Descriptive, cross-sectional	To document the stability, concurrent validity, and clinical correlates of two fatigability severity measures recommended by the American Geriatrics Society.	• Perceived fatigability severity • Performance fatigability severity	Perceived fatigability severity was related to performance fatigability severity, “tired today,” gait speed, activity counts, fatigue severity, frailty, and gender. Performance fatigability severity was related to perceived fatigability severity, “tired today,” gait speed, activity counts, fatigue severity, and frailty.
Simonsick et al., 2014 N = 605 Elderly individuals	Retrospective (cross-sectional analysis of data from the Baltimore Longitudinal Study of Aging)	To evaluate the criterion validity of two measures of fatigability defined as (a) perceived effort to perform a standardized task and (b) performance deterioration.	• Perceived exertion • Performance deterioration	High fatigability in perceived exertion was associated with tiredness, weakness, and reported and observed mobility deficits. High fatigability in performance deterioration was strongly associated with self-reported fatigue and walking ability but weakly associated with performance-based mobility measures.
Simonsick et al., 2016 N = 602 Elderly individuals	Retrospective (cross-sectional analysis of data from the Baltimore Longitudinal Study of Aging)	To extend previous work by examining (a) free thyroxine (FT-4) levels in men and women spanning a 30-year age range from 68 to 97 and (b) their association with functional mobility, fitness, and fatigue.	• RPE using the Borg scale after a slow-paced 5-min walk (0.67 m/s)	Adjusting for gender, age, race, height, weight, exercise, and smoking, researchers found that reported walking ability, usual and rapid gait speed, 400-meter time, fatigability, and reported energy level were less favorable with increasing FT4 levels.