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

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 (Vo_{2 max}). VO_{2 max} increased within 3 wk with further increases observed posttraining in both O (+31%) and Y (+18%), (P < 0.05). Maximal cardiac output (Qmax, 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-vO_2diff) 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 Vo₂ max from preto posttraining was explained by an increased Q_max with the remaining ∼31% explained by a widened a-vO₂diff. This proportion of Q and a-vO_2diff. contributions to the increase in Vo_{2 max} was consistent throughout testing in O. In Y, 56% of the pre- to posttraining increase in Vo_{2 max} was attributed to a greater Q_max and 44% to a widened a-vO_2diff. Early adaptations (first 3 wk) mainly relied on a widened maximal a-vO _2diff (∼66%) whereas further increases in Vo_{2 max} were exclusively explained by a greater Qmax. In conclusion, with shortterm training O and Y significantly increased their Vo_{2 max} however, the proportion of Vo_{2 max}., increase explained by Q_max and maximal a-vO_2diff throughout training showed a different pattern by age group.