Role of Walking Energetics and Perceived Fatigability on Mobility Differ by Walking Speed: The Study of Muscle, Mobility and Aging (SOMMA)

Background Slow gait speed is a risk factor for poor health outcomes among older adults and may be driven by decreased energy availability and increased fatigability. Objective Examine walking energetics and perceived physical fatigability with gait speed among slower and faster walkers and understand whether fatigability statistically mediates the association between energetics and gait speed. Methods Perceived physical fatigability was assessed using the Pittsburgh Fatigability Scale (PFS) Physical score (range 0-50, higher=greater). A three-phase cardiopulmonary exercise treadmill test collected peak oxygen consumption (VO 2 peak mL/kg/min), energetic cost of walking per distance travelled (EC W , mL/kg/meter), and cost-capacity ratios (VO 2 /VO 2 peak, %). Gait speed was determined by 4m walk; slower (<1.01m/s) vs faster (≥1.01m/s) walkers were classified using median 4m gait speed. Linear regressions and statistical mediation analyses were conducted. Results Slower walkers had lower VO 2 peak, higher EC W at preferred walking speed (PWS), and greater PFS Physical score compared to faster walkers (all p<0.05) (N=849). One standard-deviation higher increment of VO 2 peak, EC W at PWS, cost-capacity ratios at PWS and slow walking speed (SWS), and PFS Physical score were associated with 0.1m/s faster (VO 2 peak only) or 0.02-0.09m/s slower gait speed. PFS Physical score was a significant statistical mediator in the associations between VO 2 peak (15.2%), cost-capacity ratio (15.9%), and EC W at PWS (10.7%) with gait speed, and stronger among slower walkers. Conclusions Fitness and fatigability are associated with slower gait speed yet contributions may differ among slower and faster walkers. Future interventions may consider targeting fatigability among slower walkers and fitness among faster walkers.