V˙O2 did not change between phases 1 and 2 despite a significant increase in workload (METs, calculated from the ACSM equations for walking or running; Table 3). RER was higher during phase 1, and lactate concentration was higher in phase 2 (Table 3). On the basis of the verification test (phase 2), 52 individuals who did not attain a plateau in V˙O2 in phase 1 could be classified as having reached a plateau in phase 2 on the basis of an increase of <50% of that expected on the basis of the change in workload from phase 1 to phase 2. When both phases 1 and 2 are considered, 85% of participants attained a plateau in V˙O2.
Plateau versus no plateau.
As shown in Table 4, neither were there differences in the age, BMI, and body weight of participants who did or did not attain a plateau in phase 1 nor were there differences in maximum HR, RER, blood lactate concentration, RPE, and V˙O2peak expressed in both absolute and relative terms. The number of other criteria attained was not different between those who did and did not attain a plateau (3.0 ± 1.2 vs 3.2 ± 0.9 criteria, P = 0.10). Furthermore, those individuals who did not attain a plateau in phase 1 of the testing had no greater increase in V˙O2peak in phase 2 than those who did plateau (0.10 vs 0.34 mL·min·kg−1, P = 0.57). The peak data for phases 1 and 2 (booster test) are shown in Table 3.
The primary aim of this study was to determine the frequency of a plateau in V˙O2 at the termination of a continuous incremental exercise test in overweight and obese individuals and to investigate whether attainment of other criteria was contingent on achieving a plateau in V˙O2. The main findings of this study were that: 1) 46% of participants achieved a plateau in V˙O2 at the point of terminating a continuous incremental exercise test (phase 1); 2) a large proportion of participants met the criteria for each of the other markers; 3) there were no differences in age, body weight, BMI, the maximal exercise responses, or attainment of the other criteria between those who did and did not achieve a plateau; and 4) the verification test (phase 2) revealed that 52 individuals (39%) who did not exhibit a plateau in V˙O2 in phase 1 had no further increase in V˙O2 in phase 2 despite an increase in workload. Therefore, when both phases 1 and 2 are considered, 85% of participants attained a plateau in V˙O2.
Attainment of a plateau in V˙O2 in overweight and obese individuals.
Of interest in the present study is the question of whether excessive body fat impedes the ability of an individual to attain a plateau in V˙O2; that is, are overweight and obese individuals less likely to attain a plateau in V˙O2 than leaner individuals? This question is of clinical interest in evaluating the effect of a weight loss intervention on cardiorespiratory function and functional capacity, where it is important that the comparison before and after weight loss is made on the basis of the same termination criteria. There seems to be no physiological reason why individuals with excessive body fat should be less likely than lean individuals to cease a maximal test because of an oxygen limitation and therefore exhibit a plateau in V˙O2. Astorino et al. (2) reported that body composition was not predictive of a plateau in V˙O2. However, all participants were very lean, and their %BF fell within a very narrow range (11%-18%). Although it is not surprising that body composition was unrelated to the likelihood of attaining a plateau in this group, similar findings have previously been reported in individuals with a wider range of %BF (up to 34% 8). In this early work, Buskirk and Taylor (8) reported no differences in V˙O2max expressed per kilogram of FFM between individuals grouped on the basis of their %BF (0%-10%, 10%-25%, and >25%), indicating that the presence of body fat, at least in this range, did not hinder the capacity of the cardiorespiratory system to deliver oxygen during maximal exercise. However, many obese individuals have levels of body fat far greater than the maximum of 34% reported by Buskirk and Taylor (8); in the present study, %BF ranged from 22% to 47% in males and from 26% to 57% in females.
The authors are aware of only two studies conducted with the specific purpose of evaluating the attainment of V˙O2max in overweight and obese individuals. These studies, from Donnelly et al. (12) and Misquita et al. (25), reported that as few as 15% and 18% of obese individuals attained a plateau in V˙O2, respectively; this is considerably lower than the 49% of overweight and obese individuals who attained a plateau in the present study. There are several postulates for the markedly higher occurrence of a plateau in the present study. Donnelly et al. (12) and Misquita et al. (25) studied only obese females, and it is therefore possible that the larger proportion of individuals attaining a plateau in the present study may be attributable to the greater heterogeneity of our sample, specifically the inclusion of both males and females, and those classified as overweight in addition to only obese individuals. However, similar proportions of overweight and obese males and females achieved a plateau in oxygen consumption. Furthermore, although not significant, the plateau criterion was attained with a greater frequency in obese females (63%) than in the other subgroups (overweight and obese males and overweight females). It therefore seems unlikely that the attainment of a plateau can be explained by the inclusion of a more heterogeneous population.
It seems more likely that the low incidence of a plateau in the studies of Donnelly et al. (12) and Misquita et al. (25) is attributable to the premature termination of the exercise test and thus failure to reach a true physiological maximum, rather than being indicative of the fact that individuals in this population were less able to attain a plateau. If we consider only the obese females in the present study for ease of comparison, although the women in our study were younger than those in the studies by Donnelly et al. (12) and Misquita et al. (25), they were similar in BMI and %BF. Unfortunately, Donnelly et al. (12) did not report the actual V˙O2peak values, but the obese females in the present study achieved higher V˙O2peak values (expressed in absolute terms as well as relative to body weight) than the women in the Misquita et al. study (25) (27 vs 19 mL·min·kg−1). Although the postmenopausal women in the cohort of Misquita et al. (25) were older than the women in the present study, this factor alone does not account for the lower V˙O2peak, given that it remains lower when adjusted for age (10th vs 30th percentile for gender-specific, age-adjusted maximal aerobic power) (1). That the low incidence of a plateau may be attributed to the premature termination of the exercise test seems even more plausible given that only 57% of the women in the study by Misquita et al. (25) reached an RER of ≥ 1.10 and only 60% of those in the study by Donnelly et al. (12) met the criterion for RER that was only set at >0.95. In contrast, 74% of the obese females in the present study achieved an RER ≥ 1.15. Furthermore, only 54% of the participants in the study by Donnelly et al. (12) attained a peak HR within 10 bpm of age-predicted maximum HR compared with 83% of participants in the present study, and only 44% of participants in the study by Donnelly et al. (12) had an RPE of at least 19, whereas 74% of those in the present study met this criterion.
It is also important to consider the results of the present study in the context of previously reported data in nonobese individuals. It has been suggested that motivation, high pain tolerance, muscle strength, high anaerobic capacity, and greater ability to buffer hydrogen ions during maximal work could collectively, or independently, differentiate between those individuals who do, and those who do not, exhibit a plateau in V˙O2 (2,20,29). According to Wagner (29), only those able or willing to tolerate the pain associated with a maximal effort may exhibit a plateau in oxygen consumption, which implies that attaining a plateau is dependent largely on motivation. This opinion is supported more recently by Shephard (28), who noted regarding the testing of athletes that, "the power of the observer is important to the reaching of an oxygen 'plateau,' and laboratories that have difficulty in demonstrating this phenomenon probably need to upgrade their motivational skills."
Whereas certain populations, for example, children, the elderly, and low-fit individuals (19), may have greater difficulty achieving a plateau, the frequency with which even young, healthy, active, individuals exhibit a plateau has been reported to be as low as 17% (26). This is the case even among the most highly motivated individuals and those presumably well accustomed to the pain and discomfort associated with producing and sustaining a maximal effort. Indeed, it has been reported that only 25%-39% of elite male and female middle- and long-distance runners (11) and 47% of elite male road cyclists (20) exhibited a plateau in oxygen consumption at the completion of continuous incremental tests. When considered in this context, the occurrence of a plateau is unlikely to be explained by motivation alone because the individuals in the present study, possibly one of the least motivated groups (on the basis of long-term sedentary lifestyles), were no less likely, and in fact were more likely, to attain a plateau in V˙O2 than arguably some of the most highly motivated individuals (i.e., athletes). This is not to undervalue the importance of creating a highly motivating testing environment but rather to note that motivation is unlikely to be the primary determinant of a plateau in V˙O2.
If motivation was the main factor in determining whether an individual attained a plateau in V˙O2, we would have expected to find differences in the maximal responses and a higher incidence of the other criteria in those who exhibited a plateau compared with those who did not. However, in the present study, there were no differences in age, body weight, BMI, V˙O2peak, and maximal exercise responses between those individuals who did and did not exhibit a plateau (Table 4). As noted by Bassett and Howley (4), if a subject fatigues just as V˙O2max is reached, there may be too few data points for a plateau to be evident. To have enough data points to provide evidence of a plateau, a participant must be able or be prepared to continue exercising for a sufficient duration at an increasing level of anaerobic energy metabolism (i.e., once V˙O2max has been achieved). The blood lactate responses in the present study, and those reported by Lucia et al. (20) in elite cyclists, support this notion. In the present study, the peak blood lactate tended to be lower (not statistically significant) in the group that did not attain a plateau (Table 4). This is consistent with the findings in elite cyclists (20) where, although peak blood lactate concentration was lower and pH was higher in cyclists who did not plateau compared with those who did, there were no differences in maximal HR, V˙O2peak, ventilation, and RER. Thus, although the absence of a plateau makes it difficult to determine whether a maximal oxygen consumption has been achieved, it is not necessarily indicative of a failure to attain a "true" maximal oxygen consumption.
Attainment of other criteria in overweight and obese individuals.
In the absence of a plateau, other criteria may be used to provide evidence that a maximal oxygen consumption has been achieved. In phase 1, the frequency with which individuals met each of the other criteria was greater than the frequency with which they attained a plateau in V˙O2 (Figs. 1 and 2). The number of other criteria attained was not different between those who did and did not attain a plateau in phase 1, and there were no differences in V˙O2peak or the number of the other criteria attained in phase 1 of testing in the 85% who attained a plateau in V˙O2 (phases 1 and 2) versus those who did not plateau (data not shown). Furthermore, neither gender nor fatness predicted the number of V˙O2 markers attained, and attainment of the other criteria did not differentiate whether a V˙O2 plateau was achieved (Figs. 1 and 2).
Given the questionable usefulness of the other criteria, it has been suggested that a verification or "booster" test may be used to determine whether a V˙O2peak is indicative of a true maximal V˙O2 (27). Although protocols vary between studies, the verification test typically involves participants undertaking a standard incremental exercise test, having a short rest, and then undertaking an additional verification test at a workload higher than that achieved at the end of the incremental exercise test (27). This two-phase test can be conducted within a single session and thus may provide a time-efficient means of verifying whether a V˙O2peak reflects a true V˙O2max without relying on other ill-defined and controversial criteria.
All participants in the present study undertook a verification test after the standard incremental exercise test. When analyzed as a group, there was a small, but significant, increase in work rate between phases 1 and 2 of the testing (0.48 METs, P < 0.0001; Table 3). Despite the higher workload, there were no differences in the peak HR attained during each phase of testing, and although peak RER, lactate concentration, and RPE differed between phases 1 and 2 (Table 3), these differences were likely related to the duration of the exercise and were unlikely to be clinically meaningful. In addition, the measured change in V˙O2 between phases 1 and 2 for the whole group (0.13 mL·min·kg−1, P = 0.55) was far less than the 1.68 mL·min·kg−1 that would be predicted for a 0.48-MET increase in mechanical work. Importantly, the change in V˙O2 between phases 1 and 2 was not different between those who did and did not attain a plateau in phase 1.
For each individual who completed both phases of testing, we also compared the measured change in V˙O2 with the expected change in V˙O2 on the basis of the change in workload. This individualized approach provides further evidence that a large number of participants had reached a "true" maximal oxygen consumption at the end of the incremental exercise test. Of the 132 participants who have complete V˙O2 data for both tests, 60 (46%) attained a plateau in V˙O2 at the end of the incremental treadmill test. The phase 2 test provided verification that an additional 52 participants (39%) had indeed attained maximum on the basis of an increase in V˙O2 of less than 50% of that expected on the basis of the change in workload between the final stage of the incremental exercise test (phase 1) and the final stage of the verification test (phase 2). This is consistent with previous research using a verification test in lean males where all participants failed to attain a plateau in V˙O2 at the end of a continuous incremental exercise test but had no further increase in V˙O2 at the end of a verification test performed 5 min later at either 95% or 105% of the previously attained maximal workload (27).
For both phases of testing, a plateau was defined as an increase in of less than 50% of that expected on the basis of the increase in mechanical work. In phase 1 (incremental treadmill test), the change in workload was a uniform increase in grade of 2.5% between the penultimate and ultimate stages of the test. For phase 2 (the verification test), the increase in workload was individualized for each participant and was achieved via increases in either speed or grade or a combination of speed and grade. For both phases, the expected change in V˙O2 was predicted from the appropriate ACSM walking or running equation (1), depending on speed. It is possible that this method may overestimate the incidence of a plateau, given that the equations are valid only for steady-state conditions and the workload in this study was increased each minute during exercise. A preferable way of determining the expected increase in V˙O2 would have been to establish the relationship between V˙O2 and workload from individual regression equations for submaximal workloads completed during the incremental test. However, because the test protocol was designed to avoid participants undertaking unduly long tests, there were too few data points to establish regression equations with any predictive power.
V˙O2max is commonly used to measure changes in cardiorespiratory capacity in overweight and obese individuals during exercise (or other) interventions. If successful, these interventions may result in considerable weight loss such that individuals who entered the intervention as obese may be classified as overweight or even "normal weight" by the end of the intervention period. Implicit in the use of a V˙O2max test to measure changes in cardiorespiratory capacity is that the test is limited by the same variable, cardiorespiratory power, at each time point. If the ability to reach V˙O2max (as defined by a plateau in V˙O2) is affected by excess body fat, then apparent changes in V˙O2max in response to an exercise intervention (with accompanying weight loss) may be attributable, at least in part, to a reduced fat mass. The findings of the present study, in combination with the limited existing evidence (15), suggest that this is not the case.
In the present study, 85% of participants attained a plateau in despite having %BF levels ranging from 22% to 47% in males and from 26% to 57% in females. Furthermore, although there was a moderate negative correlation between %BF and V˙O2peak (mL·min·kg−1) in both males and females in the present study, neither BMI nor %BF was different between those who did and did not achieve a plateau in V˙O2 (Table 4). This suggests that even a very high level of body fat does not prevent an individual from attaining a plateau in V˙O2. This is in agreement with the only longitudinal study addressing this question in which Goran et al. (15) reported no differences in V˙O2max when expressed relative to FFM in formerly obese women who had lost an average of ∼13 kg (16%) of their body weight and reduced their fat mass by 31%. Although this warrants further investigation, the available evidence suggests that a high level of body fat does not impede the maximum capacity of the cardiorespiratory system and does not preclude the use of V˙O2max testing in longitudinal studies in which individuals experience large changes in body composition.
There are two important findings from the present research. Firstly, 46% of overweight and obese individuals attained a plateau in V˙O2 at the end of a continuous incremental exercise test, and there were no differences in any of the measured variables between those who did and did not attain a plateau. Secondly, in an additional 39% of individuals who did not attain a plateau in V˙O2 in the continuous incremental test, the verification test failed to induce further increases in V˙O2 despite a significant increase in workload. Together, these findings suggest that the absence of a plateau in V˙O2 alone is not indicative of a failure to reach a true maximal oxygen consumption and that individuals with excessive body fat are no less likely than "normal-weight" individuals to exhibit a plateau in V˙O2 at the end of a continuous incremental exercise test, provided that the protocol is appropriate to the population and encouragement to exercise to maximal exertion is provided.
This research received financial support from Polar Electro Oy and HUR Oy. The results of this study do not constitute endorsement by ACSM.
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Keywords:©2010The American College of Sports Medicine
MAXIMAL AEROBIC POWER; OBESITY; CARDIORESPIRATORY FITNESS; EXERCISE TESTING