Physiological Determinants of VO₂max Increase with Endurance Training in a Group Including Older and Young Adults

Purpose To examine central and peripheral physiological adaptations contributing to increases in maximal oxygen uptake (VO2max) following a 12-week of vigorous endurance exercise program in healthy, older and young adults. Methods Fourteen participants (7 older: 68 ± 7 years and 7 young: 26 ± 7 years; 9 males and 5 females) engaged in a cycling training program 3 times/week for 45 min at ∼70%VO2max. Changes in VO2max, cardiac function, O2 extraction, muscle capillarization, and mitochondrial content from pre- to post-training were examined. Results The increase in VO2max from pre- to post-training for all participants (20 ± 8%) was accompanied by increases in maximal cardiac output (Qmax: 15 ± 11%), maximal stroke volume (SVmax: 14 ± 12%), muscle fiber cross-sectional area (CSA: 17 ± 21%) and perimeter (P: 7 ± 9%), citrate synthase activity (CS: 144 ± 175%), individual capillary-to-fiber ratio (C:Fi: 32 ± 17%), capillary-to-fiber perimeter exchange index (CPFE: 30 ± 15%), capillary contact (CC: 38 ± 20%), capillary density (CD: 22 ± 17%), and arteriovenous O2 difference (a-vO2diff: 4 ± 6%) (p < 0.05 for all). No training-related variation existed for maximal heart rate (HRmax: -1 ± 4%; p = 0.448). Changes in VO2max were positively correlated with Qmax (r = 0.830: p < 0.001), SVmax (r = 0.655; p = 0.011), CD (r = 0.546; p = 0.043), and CS activity (r = 0.630; p = 0.021). No significant correlations were found for changes in VO2max and changes in HRmax, a-vO2diff, muscle fiber CSA and P, CC, C:Fi and CPFE (p > 0.05 for all). Conclusions Cardiorespiratory fitness improvements were primarily determined by enhancement in central (i.e., cardiac function) and peripheral (i.e., vascularization) adaptations within the O2 transport system as well as the upregulation of mitochondrial aerobic enzymatic activity (i.e., CS activity) at the intracellular level.