Preserved skeletal muscle oxidative capacity in older adults despite decreased cardiorespiratory fitness with ageing

Key points: Healthy older adults exhibit lower cardiorespiratory fitness ((Formula presented.)) than young in the absence of any age-related difference in skeletal muscle mitochondrial capacity, suggesting central haemodynamics plays a larger role in age-related declines in (Formula presented.). Total physical activity did not differ by age, but moderate-to-vigorous physical activity was lower in older compared to young adults. Moderate-to-vigorous physical activity is associated with (Formula presented.) and muscle oxidative capacity, but physical inactivity cannot entirely explain the age-related reduction in (Formula presented.). Abstract: Declining fitness ((Formula presented.)) is a hallmark of ageing and believed to arise from decreased oxygen delivery and reduced muscle oxidative capacity. Physical activity is a modifiable lifestyle factor that is critical when evaluating the effects of age on parameters of fitness and energy metabolism. The objective was to evaluate the effects of age and sex on (Formula presented.), muscle mitochondrial physiology, and physical activity in young and older adults. An additional objective was to assess the contribution of skeletal muscle oxidative capacity to age-related reductions in (Formula presented.) and determine if age-related variation in (Formula presented.) and muscle oxidative capacity could be explained on the basis of physical activity levels. In 23 young and 52 older men and women measurements were made of (Formula presented.), mitochondrial physiology in permeabilized muscle fibres, and free-living physical activity by accelerometry. Regression analyses were used to evaluate associations between age and (Formula presented.), mitochondrial function, and physical activity. Significant age-related reductions were observed for (Formula presented.) (P < 0.001), but not muscle mitochondrial capacity. Total daily step counts did not decrease with age, but older adults showed lower moderate-to-vigorous physical activity, which was associated with (Formula presented.) (R² = 0.323, P < 0.001) and muscle oxidative capacity (R² = 0.086, P = 0.011). After adjusting for sex and physical activity, age was negatively associated with (Formula presented.) but not muscle oxidative capacity. Healthy older adults exhibit lower (Formula presented.) but preserved mitochondrial capacity compared to young. Physical activity, particularly moderate-to-vigorous, is a key factor in observed age-related changes in fitness and muscle oxidative capacity, but cannot entirely explain the age-related reduction in (Formula presented.).