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

Murias JM, Kowalchuk JM, Paterson DH. Time course and mechanisms of adaptations in cardiorespiratory fitness with endurance training in older and young men. J Appl Physiol 108: 621-627, 2010. First published January 7, 2010; doi:10.1152/japplphysiol.01152.2009.-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 ((V) over dot(O2) max). (V) over dot(O2) max increased within 3 wk with further increases observed posttraining in both O (+ 31%) and Y (+ 18%), (P < 0.05). Maximal cardiac output (Q. max, 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, similar to 69% of the increase in (V) over dot(O2) max from preto posttraining was explained by an increased Q. max with the remaining similar to 31% explained by a widened a-vO2diff. This proportion of Q. and a-vO2diff contributions to the increase in (V) over dot(O2) max was consistent throughout testing in O. In Y, 56% of the pre-to posttraining increase in (V) over dot(O2) max was attributed to a greater Q. max and 44% to a widened a-vO2diff. Early adaptations (first 3 wk) mainly relied on a widened maximal a-vO(2diff) (+ 66%) whereas further increases in (V) over dot(O2) max were exclusively explained by a greater Q. max. In conclusion, with short-term training O and Y significantly increased their (V) over dot(O2) max; however, the proportion of (V) over dot(O2) max increase explained by Q. max and maximal a-vO2diff throughout training showed a different pattern by age group.