Effects of aging, sex, and physical training on cardiovascular responses to exercise

Background. The relative contributions of decreases in maximal heart rate, stroke volume, and oxygen extraction and of changes in body weight and composition to the age-related decline in maximal oxygen uptake (V̇o₂max) are unclear and may be influenced by sex and level of physical activity. Methods and Results. To investigate mechanisms by which aging, sex, and physical activity influence V̇o₂max, we quantified V̇o₂, cardiac output, and heart rate during submaximal and maximal treadmill exercise and assessed weight and fat-free mass in healthy younger and older sedentary and endurance exercise-trained men and women. For results expressed in milliliters per kilogram per minute, a three-to-four-decade greater age was associated with a 40-41% lower V̇o₂max in sedentary subjects and a 25-32% lower V̇o₂max in trained individuals (p<0.001). A smaller stroke volume accounted for nearly 50% of these age-related differences, and the remainder was explained by a lower maximal heart rate and reduced oxygen extraction (all p<0.001). Age-related effects on maximal heart rate and oxygen extraction were attenuated in trained subjects (p<0.05). After normalization of V̇o₂max and maximal cardiac output to fat-free mass, age- and training-related differences were reduced by 24-47% but remained significant (p<0.05). For trained but not sedentary subjects, maximal cardiac output and stroke volume normalized to fat-free mass were greater in men than in women (p<0.05) Conclusions. A lower stroke volume, heart rate, and arteriovenous oxygen difference at maximal exercise all contribute to the age-related decline in V̇o₂max. Effects of age and training on V̇o₂max, maximal cardiac output, and stroke volume cannot be fully explained by differences in body composition. In sedentary subjects, however, the sex difference in maximal cardiac output and stroke volume can be accounted for by the greater percentage of body fat in women than in men.