Maximal oxygen consumption (O2max) is the accepted criterion for assessing the capacity of the respiratory and circulatory systems for metabolic work (5). Key features of O2max are that it improves with training (16) and declines with age (10). However, measurement of maximal oxygen uptake is problematic when applied to older or habitually sedentary people because of the heavy physical demands placed on the subject (17,20). It is a difficult measure to use in large population samples because of the equipment and staff required. Therefore, a variety of submaximal tests of aerobic power have been developed; among the most widely used are physical work capacity at 170 beats·min−1 HR (pwc170) (18,24) and the Åstrand-Ryhming test (6).
Estimated O2max (est.O2max) of an individual from a submaximal test has a number of limitations that have been documented extensively (14,19). The main problems include the variability in predicting maximum HR, which has a coefficient of variation of ∼10% (23); the asymptotic relationship between HR and oxygen consumption near maximum effort (26); and that mechanical efficiency varies by 5-10% between individuals for cycle ergometry, stepping, and treadmill exercise (19). Numerous studies have validated these and other submaximal tests against the criterion of directly measured O2max in an attempt to predict maximum aerobic power. The results are generally superior for younger subjects (13), although others have included persons up to 70 yr (21). Typically the errors of predicted O2max for an individual are ∼15% (12,27).
Physical work capacity (pwc) is a useful measure in many types of research, including attempts to characterize the health benefits of physical activity as a function of aerobic power (8). While aerobic power is usually estimated with extrapolated O2max, interpolated measures of pwc may provide the same information (7) with superior face validity. The aim of this study was to assess the face validity of both extrapolated and interpolated measures of submaximal work capacity (Fig. 1) in men and women in six different age groups between 18-78 yr who were participants in a population survey. Four different indices of aerobic power were assessed to determine whether they showed the key characteristics of measured O2max, that is, a decrease with age and an increase with participation in moderate to vigorous physical activity. While three of these measures are well known (pwc150, pwc170, and est.O2max), one measure (physical work capacity at 75% of age predicted maximum HR: pwc75%) has only recently been reported (9).
Participants. As part of the 1990 Pilot Survey of the Fitness of Australians, members of a randomly-selected sample of households in metropolitan Adelaide, South Australia, were approached to participate in a physical activity survey that included a comprehensive interview and physical health assessment. Details of the sampling procedure, response rate, the method and content of the interview, and the physical health assessment have been reported elsewhere (11). All 2298 questionnaire respondents were invited to attend the physical health assessment. Of the 1198 respondents who attended, 151 were excluded or failed to satisfactorily complete the test of physical work capacity. Respondents were excluded using the American College of Sports Medicine (4) contraindications for exercise or if they were taking medication that affected their HR or blood pressure. Respondents whose estimated aerobic power exceeded 5.0 W·kg−1 (N = 4) were also excluded, leaving 1043 cases available for analysis. All subjects provided written informed consent.
Exercise measures. The physical work capacity of each participant was measured on a cycle ergometer (Monark Model 868, Monark-Crescent AB, Varberg, Sweden) pedaled in time to a metronome at a cadence of 60 rpm. The total number of pedal revolutions was counted electronically and the ergometers were calibrated dynamically before, at the midpoint, and immediately after the 7-month study using a torque meter (25). HR was monitored with a Polar Electro PE3000 electronic receiver and electrode belt (Polar Electro, Kempele, Finland) for those aged less than 45 yr, and with a Medtel HS5 cardiac monitor and a BR2 chart recorder (Medical Telectronics, Sydney, Australia) for those aged 45 yr or older.
The test was preceded by a 2-min warm-up which was used to establish the correct cadence and to provide an estimate of the first workload. The protocol comprised three submaximal workloads, each 3-6 min duration, separated by 1-min rest periods. The workloads were selected to produce steady-state HR of approximately 55, 65, and 75% of the predicted maximum (estimated as 220 minus age in years (15), where a steady state was defined as a HR change of 3 beats·min−1 or less over successive min. A conservative approach to establishing workloads was observed, and in some instances a fourth workload was needed to reach the target of 75% of predicted maximum HR.
The steady-state HR, workload (W), total pedal revolutions and duration (in minutes) of effort were recorded at the conclusion of each workload. HR was regressed against workload and the resultant regression equation was used to calculate physical work capacity (W) at HR of 150 beats·min−1 (pwc150) and 170 beats·min−1 (pwc170), as well as at 75 and 100% of age-predicted maximum HR (pwc75% and pwc100%, respectively) (Fig. 1). The pwc100% was used to predict O2max by regression of workload against oxygen consumption using the standard values provided by Åstrand and Rodahl (5). The values of physical work capacity were finally divided by the respondent's body mass to provide measures of physical work capacity and est.O2max per kilogram of body mass.
Self-reported activity. A questionnaire completed 3 wk before the physical health assessment was used to estimate the respondents' leisure time physical activity energy expenditure (EE). Respondents were asked to report the activities in which they had participated over the previous 2 wk and the frequency, duration, and intensity for each activity (9). Standard tables (1), which provide the rate of EE of specific activities, were used to calculate the total EE as the sum of all leisure time activities. Each respondent was then classified into one of four EE categories:
- Vigorous activity: participation in an aerobic activity at least six times over the last 2 wk for a minimum of 20 min per session and a total EE value of greater than or equal to 3.8 kcal·d−1·kg−1;
- Moderate activity: total EE value greater than or equal to 1.8 kcal·d−1·kg−1 and not included in Vigorous category;
- Low activity: total EE value 0.12-1.79 kcal·d−1·kg−1, inclusive; and
- Sedentary: total EE value less than 0.12 kcal·d−1·kg−1.
To simplify the analysis, the Low active and Sedentary EE categories were combined into the Inactive group and the Moderate and Vigorous EE categories were combined into the Active group.
Data analysis. Pearson correlation coefficients were calculated between age and each of pwc150, pwc170, pwc75%, and est.O2max. Separate analyses were undertaken for men and women. Multiple linear regression was then used to estimate the difference in mean pwc150, pwc170, pwc75%, and est.O2max between inactive and active men and women after adjusting for age. An alpha level of ≤ 0.05 was used for all analyses.
Although pwc150 and pwc170 showed some decline with increasing age, the decline was small and inconsistent (Table 1). This was reflected in the small correlation coefficients, none of which were statistically significant. In contrast, both pwc75% and est.O2max declined consistently with age (although est.O2max reached a plateau among older people) for both men and women, and when correlated with age yielded moderately high coefficients. Pwc75% declined by 8.1% per decade for men and 9.8% per decade for women.
Overall, the observed differences between inactive and active respondents were very similar to the age-adjusted differences (Table 2). As indicated by the confidence intervals, the differences in each measure of aerobic power between inactive and active respondents were statistically significant. In each case, the mean for the active group was 10-15% higher than the mean for the inactive group.
The results of this study suggest that for population studies pwc75% has good face validity as a method to evaluate physical work capacity in both men and women aged 18-78 yr, since it both declines with age and increases with higher levels of physical activity. Pwc75% showed a linear decline with age at a rate (∼9% per decade) consistent with that reported for directly measured O2max(10). On the other hand, pwc150 and pwc170 did not show the expected decline with age, and est.O2max reached a spurious plateau among older men and women. Although the findings on the relationship between self-reported physical activity and the measures of aerobic power did not assist in identification of a more valid measure of aerobic power, all four measures were 10-15% higher among active than inactive men and women. The magnitude of this difference is compatible with the literature which indicates that physical training can increase O2max by 5-30% (2,3).
Measured pwc75% addresses two of the main problems of submaximal tests used to assess aerobic power in epidemiological and population-based studies: 1) the inter-individual variation in maximal HR and 2) the asymptotic relationship between HR and oxygen consumption near maximum. Even though the HR achieved will not usually be the true 75% of maximum (maximum HR is highly variable between subjects (22,23)), pwc75% does represent a measured, or interpolated, rather than an extrapolated score. The pwc75% test is also superior to the pwc150 and pwc170 tests because, in large scale studies with persons aged 70 yr or older, even pwc150 most likely represents a supra-maximal rather than steady-state HR as was the intention of Sjostrand (18) and Wahlund (24).
Despite the limitations of submaximal tests, their relatively lower cost, greater safety, and greater acceptability to a wider age range makes them a preferred test compared with direct O2max tests for large scale, population-based surveys and other forms of research. The pwc75% test described and used successfully on over 1000 subjects, both men and women aged up to 78 yr, varied consistently with the key characteristics of O2max. That is, it demonstrated both an age-dependent decline and activity-dependent increase. Unlike estimates of O2max for specific population samples (13,21), pwc75% has utility in any setting. The pwc75% test may therefore be suitable for other epidemiological studies of aerobic power and studies of the relationship between fitness and morbidity/mortality as well as in the validation of physical activity self-report questionnaires. However, criterion validation of pwc75% is still required by comparison with directly measured O2max.
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