Time course and mechanisms of adaptations in cardiorespiratory fitness with endurance training in older and young men

Abstract

The time-course and mechanisms of adaptation of cardiorespiratory fitness were examined in 8 older (O) (68 ± 7 yr old) and 8 young (Y) (23 ± 5 yr old) men pretraining and at 3, 6, 9, and 12 wk of training. Training was performed on a cycle ergometer three times per week for 45 min at ∼70% of maximal oxygen uptake (V̇o2 max). V̇o2 max increased within 3 wk with further increases observed posttraining in both O (+31%) and Y (+18%), ( P < 0.05). Maximal cardiac output (Q̇max, open-circuit acetylene) and stroke volume were higher in O and Y after 3 wk with further increases after 9 wk of training ( P < 0.05). Maximal arterial-venous oxygen difference (a-vO2diff) was higher at weeks 3 and 6 and posttraining compared with pretraining in O and Y ( P < 0.05). In O, ∼69% of the increase in V̇o2 max from pre- to posttraining was explained by an increased Q̇max with the remaining ∼31% explained by a widened a-vO2diff. This proportion of Q̇ and a-vO2diff contributions to the increase in V̇o2 max was consistent throughout testing in O. In Y, 56% of the pre- to posttraining increase in V̇o2 max was attributed to a greater Q̇max and 44% to a widened a-vO2diff. Early adaptations (first 3 wk) mainly relied on a widened maximal a-vO2diff (∼66%) whereas further increases in V̇o2 max were exclusively explained by a greater Q̇max. In conclusion, with short-term training O and Y significantly increased their V̇o2 max; however, the proportion of V̇o2 max increase explained by Q̇max and maximal a-vO2diff throughout training showed a different pattern by age group.