Energy Cost Of Walking, Oxygen Consumption Kinetics, Fuel Selection, And Fatigability In Older Adults

Although older adults may have a greater energy cost of walking (CoW) than young adults, the associations between increased CoW and metabolic factors such as fuel selection or oxygen consumption (VO2) kinetics have not been fully established during prolonged walking, nor has fatigability (∆perceived exertion) been examined in this context. PURPOSE: Test the hypotheses that, in older adults, CoW is related to 1) the O2 deficit, 2) VO2 off-kinetics, 3) carbohydrate (CHO) use, and 4) fatigability during a 30-min treadmill walk (30MTW). METHODS: Aerobic capacity (VO2peak) was determined in 12 healthy adults (median 70 yr, range 65-80; 4 F). On a separate day, VO2 and carbon dioxide production (VCO2) were measured during a 30MTW at 1.3 m·s-1. Relative exercise intensity was expressed as %VO2peak. Energy expenditure was normalized to body mass (EE; J·s-1·kg-1). Net CoW (J·kg-1·m-1) was calculated from net EE (steady state - quiet standing)/1.3 m·s-1. The O2 deficit (ml·kg-1) was calculated over the first 7 min, and VO2 off-kinetics (time constant, s) using a monoexponential fit of VO2 from the final min of walking to 2 hr of seated recovery. CHO oxidation was calculated from VO2 and VCO2 (kJ·kg-1). Changes from quiet standing to end-walk in ratings of perceived exertion (RPE, 0-10) quantified fatigability. Statistics: Spearman correlations. RESULTS: CoW (2.5 J·kg-1·m-1, 1.8-3.1) was associated with O2 deficit (16 ml·kg-1, 6-28; rs = 0.80, p = 0.003) and CHO oxidation (4 kJ·kg-1, 3-6, rs = 0.74, p = 0.008), but not with VO2 off-kinetics (115 s, 98-153; rs = 0.02, p = 0.968) or fatigability (3, 1-8, rs = 0.46, p = 0.131). Relative exercise intensity (50%, 31-76) was associated with fatigability (rs = 0.66, p = 0.039). CONCLUSIONS: Support of Hypotheses 1 and 3 indicates greater nonoxidative energy production at the onset of walking and a shift to greater CHO use over 30 min in older adults with more costly locomotion, while the lack of support for Hypothesis 2 indicates that the rate of metabolic (VO2) recovery may be independent of the net CoW. Our final Hypothesis was not supported, suggesting that CoW alone does not explain fatigability during a 30-min walking task. Notably, the relationship between relative exercise intensity and fatigability suggests that the combination of a greater CoW and lower VO2peak may lead to fatigability in old age.