INTRODUCTION

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CARDIAC OUTPUT (c) is a major determinant of systemic O₂ transport in humans. With aging, maximal c during exercise is reduced, and this reduction explains a significant portion of the age-related decline in maximal O₂ uptake (O_{2 max}; Refs. 9, 12, 17, 21, 24, and 25). In healthy older individuals at rest, c is usually lower compared with younger control subjects (26, 29). At a given submaximal exercise intensity [O₂ uptake (O₂)], c may (4, 18, 29, 30) or may not (2, 19, 25) decline with aging. However, the slope of the c-O₂ relationship during graded dynamic exercise is generally considered to be well maintained with advancing age (3, 4, 14, 25, 26, 29).

Younger women reportedly achieve a higher absolute level of c at a given submaximal O₂ than do younger men (2, 5, 15, 20). This gender-related difference has not been reported among sedentary older populations (2, 18). Moreover, recent studies by Spina and colleagues (27, 28) show that c at a given O₂ is reduced in older women after endurance exercise training but is unchanged in older men or in younger groups with training. On the basis of these results, it is unclear how gender, aging, and physical activity status interact as determinants of this important physiological relationship. The equivocal nature of the data on this issue could be due, in part, to variation in c measurement techniques, exercise modes, and/or subject fitness levels among studies.

With this information as background, the present study was designed to evaluate whether the c-O₂ relationships during graded leg cycle ergometry are different in chronically endurance-trained male and female subjects from two discrete age groups. Our general hypothesis was that the slope and/or absolute level of c at a given submaximal O₂ would be reduced in endurance-trained older women compared with the other groups. Stroke volume responses were also studied because of recent evidence that stroke volume may not plateau during graded exercise in some highly endurance-trained younger (7) and older (24) men.

	METHODS

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Subjects

In general, these four groups of subjects were successful in regional running/cycling competitions, but only one older female runner (72 yr) was an elite-caliber athlete. The treadmill O_{2 max} and physical characteristics of the four groups given in Table 1 reflect a highly trained, but nonelite, sample. All subjects gave written informed consent before the study, according to Mayo Clinic Institutional Review Board guidelines.

Rationale for Using Leg Cycle Ergometry

Measurement of O₂

Measurement of c

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Systolic and diastolic brachial arterial blood pressures were estimated by using a semiautomated cuff inflation system (model PE 3000, Narco Biosystems) and an amplified stethoscope. Mean arterial pressure was estimated, in the conventional manner, as pulse pressure/3 + diastolic pressure.

Body Composition

Test Protocols

Sessions 1 and 2: Peak exercise testing. In session 1, subjects underwent a dual-energy X-ray absorptiometry scan followed by a treadmill O_{2 max} test (23). During session 2, subjects were oriented to the cycle ergometer, and a O_{2 peak} test (described in Measurement of O₂) was conducted.

Session 3: Submaximal cycle ergometer testing. During session 3, subjects completed submaximal bouts (5-6 min) of cycling at 40 and 70% (±2%) of O_{2 peak}. During minute 4 of these submaximal bouts, HR and a rating of perceived exertion (RPE) were collected. During the final minute, subjects practiced the acetylene rebreathing procedure. These procedures allowed us to accurately define the steady-state O₂ (and power outputs) at these intensities and also provided estimates of the rebreathing bag volumes that would be used for the test in session 4 (see below). O_{2 peak} tests were also repeated during session 3.

Session 4: c testing. During the final session, c measurements were made during seated rest and submaximal (40 and 70% of O_{2 peak}) and near-maximal (~90% of O_{2 peak}) exercise. Resting trials were conducted three to four times, with the two closest values used for averaging (19). During the 40 and 70% bouts, power output, HR, and RPE were closely monitored for 3 min (to achieve steady-state values obtained during session 3), followed by blood pressure and c measurements. After a 1- to 2-min recovery, this sequence was repeated to provide two measurements of c, blood pressure, and HR for averaging at each exercise intensity. If the two values for c differed by >10% for a given work intensity, the bout was repeated a third time. After at least a 15-min recovery from these submaximal bouts, subjects performed two graded cycling bouts (4-5 min each) that concluded with c and HR measurements when 85% of peak power output was reached. Because O₂ does not reach a steady state at this high exercise intensity (33), O₂ was estimated by using the power output-O₂ relationship obtained during prior testing (session 3) for each subject. These O₂ estimates averaged ~90% of O_{2 peak}. The HR reached during these bouts averaged 90-95% of peak HR (overall range 85-100%). Therefore, these bouts were characterized as "near maximal."

Statistics

	RESULTS

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Subjects (Table 1)

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Peak Cycle Ergometry (Table 2)

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Hemodynamic Responses

View larger version (14K): [in this window] [in a new window]   Fig. 1.   Cardiac output (c)-O2 uptake (O2) relationship during graded leg cycle ergometry in endurance-trained younger and older men and women. Data represent 8 subjects/group except for younger women (n = 6). Regression lines represent mean responses for a given group derived from individual subject regression coefficients. Regression lines were derived from resting and 40 and 70% of peak O2 (O2 peak) responses. Slope analysis of submaximal c-O2 relationship indicated no significant differences (P = 0.73) among groups.

Stroke volume (Fig. 2) increased from baseline to the first level of exercise (40% of O_{2 peak}) in all subjects, with larger increases occurring in the men than in the women (P = 0.01). Between 40 and 70% of O_{2 peak}, the stroke volume continued to increase in the men but not in the women (gender effect; P = 0.02). As exercise intensity increased to a near-maximal level (90% of O_{2 peak}), there was a significant age-related difference (P < 0.01) in the maintenance of stroke volume: in the younger men, stroke volume continued to increase slightly, whereas the older men as a group maintained it. The younger women also maintained stroke volume at this near-maximal intensity, whereas the older women did not. Within the older groups (men and women combined), there was a trend (P = 0.05) for a decline in stroke volume between these two highest work intensities (70-90% of O_{2 peak}). This response appeared to be related to age (r = 0.50), with the oldest subjects having the largest decrease.

View larger version (18K): [in this window] [in a new window]   Fig. 2.   Stroke volume (SV) responses of 4 subgroups during submaximal (40 and 70% of O2 peak) and near-maximal (~90% of O2 peak) intensities of leg cycling. Values are means ± SE. SVs were higher in men than in women at all intensities. Additionally, SV increased during submaximal intensities (40-70%) of cycling in men, but not in women. However, older men and women showed an impaired ability, relative to their younger counterparts, to augment or maintain SV at 70-90% of O2 peak.

Seated resting levels of systolic, diastolic, and mean arterial pressure were similar among groups (all P > 0.58; data not shown). Increases in mean arterial pressure from rest to 70% of O_{2 peak} (blood pressure not measured at 90% work intensity) were proportional to exercise intensity and did not differ among groups. TPR (mean arterial pressure/c) was higher at rest and during exercise in the older groups and in women vs. men, but the exercise-induced reductions in TPR (%change from baseline) were similar (P > 0.05) among groups. The only consistent hemodynamic difference among groups was the lower absolute systolic blood pressure response of the younger women.

	DISCUSSION

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The primary new finding of this study is that the slope of the c-O₂ relationship, as well as the c associated with a given O₂, does not significantly differ between younger and older chronically endurance-trained women. The c-O₂ relationships in these women were similar to those seen in younger and older endurance-trained men in this and in previous studies by using graded leg cycle ergometry (1, 4, 10, 16, 17, 19, 25, 29). These findings suggest that neither aging nor gender per se significantly modifies the c response to submaximal dynamic exercise among chronically endurance-trained individuals. An additional new finding is that chronically endurance-trained older women showed a reduced ability, relative to their younger counterparts and to their male cohorts, to maintain stroke volume at near-maximal intensities of leg cycling. Because we obtained these measurements in chronically endurance-trained younger and older men and women in a single investigation, these findings are unlikely to be confounded by variations in c measurement techniques, protocols, physical activity, or subject motivational factors.

Slope of c-O₂ Relationship

The slope of the c-O₂ relationship for each group (Fig. 1) was evaluated by using seated resting and submaximal levels of steady-state exercise (40 and 70% of O_{2 peak}). We also studied our subjects at ~90% of O_{2 peak}, a non-steady-state workload even for endurance-trained subjects (33). When the c values obtained at 90% of O_{2 peak} were included in the regressions, the overall slopes were reduced slightly in each group (i.e., <5%) but remained similar among groups and ranged from 5.0 to 5.5 l/l. This indicates that our use of three data points (baseline and 40 and 70% of O_{2 peak}) for the computation of the slope of the c-O₂ relationship was equally effective in defining the rise in c during graded exercise for each of the four subject groups.

Absolute Values of c at a Given O₂

It has been reported that young women have a higher absolute level of c at a given submaximal O₂ than do young men, possibly due to the lower hemoglobin concentration in women (2, 5, 20). However, Zwiren et al. (34) found no gender-related difference in submaximal c during leg cycling when they compared young men and women carefully equated in training background and with similar O_{2 max} values normalized to lean body mass. Our results are consistent with those of Zwiren et al. and extend this observation to older endurance-trained groups. Collectively, these findings suggest that when differences in physical activity and cardiopulmonary performance capacity (i.e., O_{2 max}/kg lean body mass) are controlled, gender differences in the absolute c responses to submaximal exercise are abolished.

A primary hypothesis of this study was that the c responses of endurance-trained older women would be lower at a given O₂ compared with other endurance-trained subject groups. This was based on recent studies by Spina and colleagues (27, 28), who reported endurance-training-induced reductions in c at a given O₂ in older women but not in other groups. Our data do not support this hypothesis. These apparently conflicting findings may be explained by the fact that the older women we studied had been training for ~20 yr (i.e., during the period before and after menopause), whereas the studies of Spina and colleagues were conducted in women who had trained for much shorter periods (1 yr), after a decline in circulating estrogen had presumably already occurred.

Stroke Volume (Absolute Responses)

Stroke Volume (Graded Responses)

The stroke-volume response of the older women deserves special mention. At near-maximal intensities of cycling (90% of O_{2 peak}), the older women showed an impaired ability, relative to the younger women, to maintain their stroke volume. This impaired response absolute difference in stroke volume (delta) between 70 and 90% of O_{2 peak}] was less evident in the four women who were 51-59 yr old and receiving estrogen replacement than in the oldest women (61-72 yr, n = 4) who were not. As a result of recent attention focused on the possible effects of estrogen on exercise stroke volume in older women (27, 28), this observation deserves further attention in future studies. Several of the oldest men also showed a modest decline in stroke volume at 90% of O_{2 peak}. When the stroke volume responses of the older women and men were evaluated together, there was a tendency for stroke volume to fall in the oldest subjects at workloads between 70 and 90% of O_{2 peak} (r = 0.50, P = 0.05).

In summary, the findings of this study demonstrate that the slope of the c-O₂ relationship and the absolute c associated with a given O₂ during submaximal leg cycling are well maintained with age in chronically endurance-trained older women and men. Because hemoglobin concentrations were similar among age groups, these results suggest that aging does not alter the linkage between systemic O₂ transport and utilization during dynamic exercise in chronically trained older athletes of either gender. However, there was evidence of an impaired ability of the older subjects, relative to their younger counterparts, to maintain stroke volume at high absolute O₂ levels (i.e., 70-90% of O_{2 peak}). This effect was most prominent in the oldest women. The influence of estrogen replacement on cardiac function in the older female athlete is deserving of further investigation.