Mechanisms for increases in V⊙O_2max with endurance training in older and young women

Purpose: To examine the time course and mechanisms of cardiorespiratory fitness adaptation to training in older (O) and young (Y) women. Methods: A total of six O (69 ± 7 yr) and eight Y (25 ± 5 yr) women were examined before training and after 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 V⊙O_2max. Results: V⊙O_2max (mL⊙kg ^-1⊙min^-1) increased within 3 wk, with further changes observed at weeks 6 and 9 in both O (17% ± 14%) and Y and also posttraining (12 wk) in Y (22% ± 6%, P<0.05). Maximal cardiac output (Q⊙_max, open-circuit acetylene) and stroke volume increased only in Y after 9 wk of training (P<0.05). Age × testing time interactions in maximal arterial-venous O2 difference (a-vO_2diff) after 6 wk of training revealed a greater dependence on a-vO_2diff in O compared with Y (P<0.05); ∼65% of the change in V⊙O_2max from pretraining to posttraining was explained by a widened maximal a-vO _2diff in O compared with almost equal increases in Q⊙ max and maximal a-vO_2diff in Y. The early adaptations (first 3 wk) in O relied exclusively on a nonsignificant increase in Q⊙ max, whereas Y depended on a widened maximal a-vO_2diff. Later changes in V⊙O_2max were explained exclusively by an improved maximal a-vO_2diff in O and a larger Q⊙ max in Y. Conclusions: O and Y women displayed a different time course of training adaptation in V⊙O _2max, with Y (after an initial improvement in maximal a-vO _2diff) depending more on changes in Q⊙_max and O mostly relying on a widened maximal a-vO_2diff.