Dual Roles of Fitness and Fatigability in the Life Space Mobility of Older Adults

Abstract

Background: Cardiorespiratory fitness and fatigability are interrelated components of physical capacity that may jointly facilitate movement within one’s living environment (life-space mobility). We examined whether fitness and perceived fatigability, and the interaction between them, were associated with life-space mobility in a well-characterized cohort of older adults. Methods: Participants were from the preliminary data release of the Study of Muscle, Mobility, and Aging (SOMMA) baseline cohort (N=387, Mage=76.4±5.0, 57% women). Life Space Assessment scores (range: 0-120) incorporated level, frequency, and assistance used for life-space mobility (Mean=82.7±18.8). Fitness was measured as VO2peak (Mean=19.5±4.2 mL/kg/min) from symptom-limited treadmill testing. Fatigability cut-points included: 1) Borg Rating of Perceived Exertion (RPE) ≥10 after a steady-state treadmill test, 2) the Pittsburgh Fatigability Scale (PFS) Physical ≥15, and 3) PFS Mental ≥13. Linear regressions were adjusted for demographic, lifestyle, and health confounders. Results: The relationship between fitness and life-space mobility was nonlinear, where those within the lower range of fitness scores (VO2peak≤18) had 2.2-point greater life-space scores per 1-mL/kg/min greater VO2peak (95% CI: 0.68, 3.67, p=.005). The association was not significant for the upper range of fitness scores (VO2peak>18). Participants with higher fatigability on all measures (RPE≥10, PFS Physical≥15, PFS Mental≥13) had a 7.6-point lower mean life-space score (95% CI: -13.84, -1.34) after adjusting for demographics, but this was not significant after further adjusting for lifestyle and health factors. There was potential moderation of the fitness-life-space relationship by fatigability, where associations within the lower fitness range (VO2peak≤18) were only significant for those with RPE≥10 (B=3.3, 95% CI: 1.06, 5.53, p-interaction=.078). Conclusion: Fitness may primarily limit life-space mobility if it falls below a critical threshold, where older individuals may need to operate closer to their maximum aerobic capacity to traverse their daily environments. Higher fatigability may moderate this relationship, as those with both low fitness and high fatigability had the lowest life-space scores. Public health interventions that target this high-risk group may mitigate further functional declines resulting from life-space constriction