Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, and University of Texas Southwestern Medical Center at Dallas (Fu, Levine)
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There are physiological and morphological gender differences in humans. It is likely that certain gender-specific factors such as differences in some hormonal levels, menstrual cycle variability, and physical characteristics (primarily cardiac size and function) may influence the cardiovascular response to exercise in women.The most consistent gender difference in cardiovascular responses during submaximal exercise and at peak effort is the lower stroke volume (SV) of women and the smaller increase in SV from rest to exercise. It has been proposed that the smaller SV response during exercise in women is due mainly to a smaller cardiac size, particularly left ventricular (LV) volume and mass (5,11). Additionally, the difference in cardiac size has also been assumed to account for a majority of the difference in maximal oxygen uptake (V̇O2max) during exercise in genders, which is usually reported to be higher in men than in women, whether it is expressed in absolute value or relative to body mass (11). V̇O2max is the best objective measure of fitness and is a widely used index of the integrity of cardiovascular function (21).Previous studies demonstrated that in normal men during upright exercise, SV was augmented through both an increase in LV end-diastolic volume and a decrease in LV end-systolic volume, resulting in an increase in LV ejection fraction (8,19,22). However, it has also been suggested that LV ejection fraction did not increase and even decreased from rest to peak exercise in women (1,6,7). These results may indicate that gender is an important factor determining the central hemodynamic response to exercise in normal humans. This notion was supported by the findings that during treadmill exercise, stroke index (SV normalized by body surface area) was lower in women than in men and did not increase from rest to peak exercise in women (9,10). In contrast, Sullivan et al. (23) observed no differences in cardiac index, stroke index, and LV end-diastolic and end-systolic volume indexes in the time course or magnitude of changes with respect to oxygen uptake (V̇O2), expressed as percentage of V̇O2max in men and women.Our unpublished data on HR responses to exercise in untrained and highly trained young individuals is consistent with the findings of Sullivan et al. (23). In accordance with all previous studies, we found that HR increased progressively during submaximal exercise and reached a maximal value (HRmax) at peak exercise effort in all subjects. The increase in HR was greatest in untrained women and smallest in highly trained men at the same absolute work rate, which was expressed as V̇O2 (mL·kg−1·min−1), whereas highly trained women had a smaller increase in HR than untrained men. However, HRmax was not different between genders. Interestingly, when we compared the HR responses at the same relative work rate, namely, the percentage of V̇O2max (%), we found no difference among groups (Fig. 1). Results from our study and that of Sullivan et al. may suggest that cardiovascular control during dynamic exercise is similar in men and women, no matter whether they are trained or untrained.One of the key principles in exercise physiology in humans is the remarkably constant relationship between the increase in V̇O2 and the corresponding increase in cardiac output (14). In general, about 6 L of cardiac output are required for every liter of V̇O2 above rest, regardless of age, gender, or fitness level (20).