RPB during exercise
During constant work rate cycling exercise (60 W in women and 90 W in men), there were no differences in RPB between women (2.06 ± 1.33) and men (2.60 ± 1.17) (Fig. 2). There was no significant relationship between the oxygen cost of breathing and RPB during constant work rate exercise at 60 W (women) or 90 W (men). RPB during constant work rate exercise was not correlated with any cardiorespiratory measurement, with the exception of relative oxygen uptake (i.e., V˙O2 as a percent of predicted peak V˙O2; r = 0.76, P = 0.018) in women.
Cardiorespiratory responses during submaximal exercise
Cardiorespiratory measures during constant work rate exercise at 60 (women) or 90 W (men) are shown in Table 4. Based on V˙E (V˙E/PMVV ratio), V˙O2 (% predicted peak V˙O2), and HR (%predicted peak HR), the ventilatory demand was low, and the relative intensity of exercise was similar in nonobese women and men. In addition, there were no sex differences in the EELV.
There are several important findings from the present research study. First, the oxygen cost of breathing was not different between the nonobese women and men we studied over the ventilatory ranges we tested. Second, the oxygen cost of breathing was not associated with RPB during constant work rate exercise. Finally, RPB during constant work rate exercise was similar between nonobese women and men.
We hypothesized an increased oxygen cost of breathing in the nonobese women compared with the men as suggested by others (18,19,34). However, the results from the present study suggest that the oxygen cost of breathing is not higher in nonobese women compared with nonobese men. A higher work of breathing in women at absolute ventilations and for a given exercise work rate relative to body mass ratio has been reported to be because of a higher resistive work of breathing during inspiration and expiration (18). It has also been suggested that total mechanical work of breathing is higher in endurance-trained women compared with men during progressive exercise (19). Another study reported a higher oxygen cost of breathing (i.e., hyperventilation induced by added external dead space) in women compared with men, but the magnitude of the added external dead space and method of analysis limit the interpretation of the data (34). Although it is plausible that sex differences in respiratory capacity could influence the work of breathing in women (i.e., higher airway resistance, smaller airway size, lower maximal expiratory flow rates, and increased prevalence of expiratory flow limitation in women compared with men), the comparisons between studies are difficult to interpret because of differences in breathing pattern, breathing mechanics, measurement techniques, and ventilatory ranges, which increases the variability of the results, especially at high ventilatory levels. In a very careful and controlled study, the work of breathing measured during voluntary hyperpnea was not different in the single woman in comparison with seven men (1), which is similar to the findings in our study. Given that we used very similar techniques and ventilatory ranges in the women and men in our study, it is probably safe to suggest that any differences in respiratory function between the women and men were not large enough to make the oxygen cost of breathing different between sexes. The results could be different if measured at higher ventilatory demands but that was not the purpose of this study. Although the sample size of this study is not large enough to generalize our results as reference values, they provide a baseline in which to compare our recent data in obese women and men. These data also suggest that sex differences in the oxygen cost of breathing are negligible between women and men at moderate ventilatory levels despite sex-related differences in ventilatory capacities.
In general, the measured oxygen cost of breathing observed in this study was somewhat lower than those reported by some investigators (3,11,14), although we had similar values compared with others (1,12,13). The ventilatory ranges studied in the present investigation are considerably lower than in publications that reported higher oxygen cost of breathing values. For instance, Bradley and Leith (11) reported an oxygen cost of breathing of approximately 4.5 mL·L−1 at an average V˙E of 161 L·min−1. Another study reported an oxygen cost of breathing of ∼2.8 mL·L−1 during prolonged (4.5 min) maximal hyperventilation (120–240 L·min−1). On the other hand, Aaron et al. (1) estimated an oxygen cost of breathing of 1.8 mL·L−1 at a V˙E of 73 L·min−1, which is the closest ventilatory range to that of our study. Their study included data on one woman. In addition, higher levels of V˙E are associated with increased variability in the relationship between oxygen cost of breathing and pulmonary ventilation (1,3,9,11). In summary, it is apparent that the studies in which the reported oxygen cost of breathing is substantially higher than our measured values used ventilatory rates that were also much elevated compared with the present investigation. As suggested by Bartlett et al. (9), the measurement of the oxygen cost of breathing at very high V˙E (i.e., at or near-maximal efforts) results in increased variability, and much less scatter is observed when the effort is submaximal. Therefore, a better approach to use the relationship between V˙O2 and V˙E (for setting norms and for comparison purposes) would be to perform the maneuvers at lower levels of V˙E.
We specifically measured the oxygen cost of breathing during submaximal ventilatory ranges (i.e., rather than using maximal V˙E) for several reasons. First, we were interested in measuring the oxygen cost of breathing without additional ventilatory constraints (i.e., expiratory flow limitation) like those observed at high levels of V˙E, which would influence the work of breathing (8). Second, we wanted to use similar ventilatory ranges as the ventilations adopted during the 6-min constant work rate exercise so we could investigate potential relationships between the oxygen cost of breathing and breathlessness during moderate exercise. Finally, we wanted these measurements to be comparable with our previous published work in obese adults (7,15). Therefore, we chose to use lower absolute ventilatory ranges in women (40–60 L·min−1) than in men (60–90 L·min−1) so we could make comparisons between our groups at the same relative ventilatory intensity (i.e., similar V˙E/PMVV ratios between sexes; Table 3).
Also, we took the values from the current study and compared them with our prior data on obese individuals (7,15), as the protocols and relative intensities used were very similar. Except for the lack of obesity in the subjects from the current study, this group had similar characteristics as the healthy obese individuals used in earlier projects (7,15). We expected that the oxygen cost of breathing in obese adults would be higher than in the nonobese subjects from this study, but we wanted to see if the values in the obese were statistically larger. Therefore, we retrospectively compared the oxygen cost of breathing data from the subjects in this investigation with our obese individuals with exertional dyspnea (7,15). Independent t-test revealed that the oxygen cost of breathing was significantly higher in obese women and men compared with nonobese individuals with exertional dyspnea (3.04 ± 1.08 vs 1.17 ± 0.26 mL·L−1 in women and 2.01 ± 0.75 vs 1.21 ± 0.42 mL·L−1 in men, respectively; P < 0.01). This large increase in the work of breathing in obesity is a very important concern, especially during exercise, which is a major component in the prevention and treatment of obesity.
The intensity of respiratory sensations (RPB) was also similar between nonobese women and men. Of the nonobese subjects recruited for this study, only one woman (11%) and two men (20%) had an RPB ≥ 4 during constant work rate exercise (indicating increased breathlessness with exertion), which is a lower proportion than the reported in obese adults (7,15,30,32,35,36). It is important to note that the relative intensity of exercise and the ventilatory demand were similar in nonobese women and men (Table 4). In contrast to our earlier findings in obese women (7), there was no significant relationship between the oxygen cost of breathing and the RPB during constant work rate exercise. However, these observations agree with our findings in obese men (15); because the oxygen cost of breathing is not increased in nonobese women compared with men, there is no reason for RPB to be higher in women (given the exercise intensities were similar).
In summary, this study reports that there is no difference in the oxygen cost of breathing in younger healthy nonobese women compared with men over the ventilatory ranges studied and using the same technique. Therefore, there are no gender differences in the work of breathing in young healthy adults at moderate ventilatory demands. In addition, these values are lower than those previously reported in obese individuals (7,12,15,23). Finally, the RPB during 6 min of constant work rate cycling is similar in nonobese women and men at the same relative exercise intensity, and the oxygen cost of breathing does not seem to be an important factor in determining RPB in young healthy nonobese women and men.
This work was supported by the King Charitable Foundation Trust, the American Lung Association, the American Heart Association, The Research and Education Institute at Texas Health Resources, Cain Foundation, the National Institutes of Health (HL096782), and the Texas Health Presbyterian Hospital Dallas.
The authors appreciate the considerable time and effort of the subjects who participated in this project. The authors also wish to express their appreciation for the assistance of Raksa Moran, Todd Bassett, and Sarah Haller on this project.
The authors declare no conflicts of interest.
The results from the present study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:©2012The American College of Sports Medicine
WORK OF BREATHING; SHORTNESS OF BREATH DURING EXERCISE; BREATHLESSNESS DURING EXERTION; RESPIRATORY MUSCLE WORK