Physiological Determinants of V̇O 2max Increase with Endurance Training in a Group Including Older and Young Adults

This study aimed to examine central and peripheral physiological adaptations contributing to increases in maximal oxygen uptake (V̇O 2max ) following a 12-wk vigorous endurance exercise program in healthy, older and young adults.

Fourteen participants (7 older: 68 ± 7 yr and 7 young: 26 ± 7 yr; 9 males and 5 females) engaged in a cycling training program three times a week for 45 min at ~70%V̇O 2max . Changes in V̇O 2max , cardiac function, V̇O 2 extraction, muscle capillarization, and mitochondrial content from pre- to post-training were examined.

The increase in V̇O 2max from pre- to post-training for all participants (20% ± 8%) was accompanied by increases in maximal cardiac output (Q˙ max ; 15% ± 11%), maximal stroke volume (SVmax; 14% ± 12%), muscle fiber cross-sectional area (CSA; 17% ± 21%) and perimeter (P; 7% ± 9%), citrate synthase (CS) activity (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 O 2 difference (a-vO 2 diff; 4% ± 6%) ( P < 0.05 for all). No training-related variation existed for maximal heart rate (HRmax; -1% ± 4%; P = 0.448). Changes in V̇O 2max were positively correlated with Q˙ max ( 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 V̇O 2max and changes in HRmax, a-vO 2 diff, muscle fiber CSA and P, CC, C:Fi, and CPFE ( P > 0.05 for all).

Cardiorespiratory fitness improvements were primarily determined by enhancement in central (i.e., cardiac function) and peripheral (i.e., vascularization) adaptations within the V̇O 2 transport system as well as the upregulation of mitochondrial aerobic enzymatic activity (i.e., CS activity) at the intracellular level.