| Sumario: | Background In younger individuals, fitness is mostly influenced by muscle mitochondrial oxidative phosphorylation (OxPhos) and cardiac output. However, compared with younger individuals, various impairments may also negatively affect fitness in older adults. Objective To investigate the relationship of OxPhos with cardiorespiratory fitness, the energetic cost of walking and aerobic resilience with respect to age. Design Cross-sectional. Setting Population. Subjects Six hundred and forty-nine Baltimore longitudinal study of ageing participants (mean age 64.5 years, 56.9% females). Methods Muscle mitochondrial OxPhos was measured as phosphocreatine recovery rate (k) through P magnetic resonance spectroscopy. Based on age- and sex-specific k z-scores, we classified individuals with low (≤ −0.5 standard deviations [SD]), average (−0.5 to 0.5SD) and high (>0.5SD) OxPhos. Cardiorespiratory fitness was measured as peak oxygen consumption (MVO peak) during a treadmill testing. The energetic cost of usual pace walking was expressed as the average oxygen consumption per 100 metres. Aerobic resilience was the ratio between MVO peak and average VO during usual pace walking. Results Participants with higher k had 4.07 (95%CI: 2.88, 5.26) ml/kg/min higher MVO peak and 0.19 (95%CI: 0.06, 0.32) higher aerobic resilience than those with lower k. The energetic cost of walking was greater by 0.84 (95% CI: 0.21, 1.47) ml/kg/100 m in those with high than low k. A multiplicative interaction between age and k was identified in the regressions predicting MVO peak and aerobic resilience (p = 0.01), with differences between OxPhos groups attenuating after age 70. Conclusion Muscle mitochondrial OxPhos contributes to interindividual variability in cardiorespiratory fitness, especially in young and middle adulthood.
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