Physiological Determinants of V̇O2max Increase with Endurance Training in a Group Including Older and Young Adults.

Abstract

Purpose: To examine central and peripheral physiological adaptations contributing to increases in maximal oxygen uptake (V̇O2max) 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%V̇O2max. Changes in V̇O2max, cardiac function, O2 extraction, muscle capillarization, and mitochondrial content from pre- to post-training were examined.

Results: The increase in V̇O2max from pre- to post-training for all participants (20 ± 8%) was accompanied by increases in maximal cardiac output (Q̇max: 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 V̇O2max were positively correlated with Q̇max (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 V̇O2max 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.