Maximal oxygen uptake (V˙O2max) represents the maximal ability of the body for O2 delivery and utilization. V˙O2max is an important health marker for establishing levels of cardiorespiratory fitness (5) and for assessing the effects of exercise training (27,28,35). Low levels of V˙O2max are associated with a poor cardiovascular disease risk profile (14), and higher levels of V˙O2max are associated with a reduced risk of myocardial infarction later in life (17). In children, the determination of V˙O2max provides information regarding cardiopulmonary and muscular growth and development (37). Therefore, obtaining accurate values of V˙O2max is important in a pediatric population.
V˙O2 has been traditionally measured using a ramp or incremental test protocol, and attainment of V˙O2max has been validated when V˙O2 displayed a plateau with an increase in work rate in adults (40). However, it has been difficult to validate V˙O2max using the plateau criterion in children because few children typically exhibit a plateau (6,12,36). In the absence of a plateau, secondary criteria such as RER ≥ 1.00, maximal HR ≥ 90% of age-predicted maximal HR, and signs of intense effort such as hyperpnea, facial flushing and grimacing, unsteady gait, and sweating have been suggested (5). However, the use of secondary criteria for validating V˙O2max significantly underestimate V˙O2 max in children, which has led researchers to suggest that even these criteria should be abandoned (7,34). The recommended approach for validating V˙O2max is to conduct a verification test at a workload that is higher than the highest incremental workload (29).
Supramaximal verification testing has typically been conducted at 105% of the maximal power achieved during the incremental test, 10–15 min after the maximal incremental test in nonobese children (7,34,36). There are no data comparing incremental and supramaximal verification tests in obese children. It is unclear whether the use of a verification test in obese children could yield a higher V˙O2max than the incremental test. This is important to ascertain because obesity affects 17.4% of 6- to 11-yr-old children in the United States (31). Hulens et al. (19) previously reported that obese adults achieved lower values of minute ventilation (V˙E), RER, HR, and perceived exertion at peak exercise when compared with lean adults, which could make it difficult to determine whether they attained V˙O2max, although similar findings have not been observed in obese children (26). Moreover, as noted previously, there are obvious problems associated with the validity of primary and secondary criteria in determining the attainment of V˙O2max during an incremental test, which could lead to incorrectly accepting submaximal V˙O2 values as V˙O2max (32,34). The inaccurate measurement of V˙O2max could lead to obese children being wrongly labeled as “deconditioned,” and they could be offered an exercise prescription of a lower intensity than suitable. Because the accurate determination of V˙O2max is important for the diagnosis and treatment of obese children (11), it is imperative that uniform testing procedures and standard criteria validating V˙O2max be established in lieu of accepting the highest values from an incremental test and reporting them as V˙O2peak, while lacking confidence in their validity.
The purpose of this study was to examine whether a supramaximal constant-load verification test at 105% of the highest work rate would yield a higher V˙O2max when compared with an incremental test in 10–12 yr nonobese and obese children. We hypothesized that obese children would achieve higher levels V˙O2 during verification testing when compared with incremental testing because the increased energetic costs of physical activity in obesity could potentially affect performance during an incremental test, with exhaustion occurring before the attainment of V˙O2max (19). We also hypothesized that nonobese children would achieve similar levels of V˙O2 during verification testing when compared with incremental testing based on previous reports (4,7,34,36).
All study procedures were explained in detail. The child's parent or guardian provided written, informed consent, and all children provided written assent. The UT Southwestern Institutional Review Board approved this study (approval number 052012-076).
Twelve obese children (9 boys, body mass index [BMI] > 95th percentile for age and sex) and 11 nonobese children (5 boys, BMI between the 16th and the 84th percentile for age and sex) qualified for this cross-sectional study. All participants were 10–12 yr old when they were recruited for the study and were considered otherwise healthy. Any participant with a history of smoking exposure or a diagnosis of heart disease, hypertension, sleep disorders, or diabetes was excluded. Girls who had started their menses were also excluded. Participants that showed evidence of bronchodilator response (improvement in forced expiratory volume in 1 s [FEV1] of ≥12% with a minimum volume improvement of 200 mL) were excluded from the study (1).
Height was recorded using a calibrated eye-level physical scale, and weight was recorded using a calibrated weighing scale. BMI percentile was calculated for each child using age and specific BMI tables from the Centers of Disease and Control and Prevention (23). Participants completed a Tanner pubertal stage assessment for the assessment of pubertal status. Participants also completed a physical activity readiness questionnaire (10). On a different day, participants completed a dual-energy x-ray absorptiometry (DXA) scan for assessment of body composition, a maximal incremental exercise test, and a supramaximal verification test.
All participants underwent a DXA scan for the assessment of body composition (Lunar Prodigy Advance; GE healthcare Lunar, Madison, WI). Whole-body fat mass and fat-free mass were estimated using Prodigy enCore software (GE healthcare Lunar).
Maximal incremental exercise test
Participants were asked not to consume a heavy meal at least 3 h before the test. The maximal incremental exercise test was performed on a cycle ergometer (Lode Corival, The Netherlands). Before the incremental exercise test, participants performed a 6-min constant-load exercise test at 40 W on a cycle ergometer, which served as a warm-up for the incremental test and was consistent with our protocol in adults (8). The initial work rate was 20 W, and the work rate was increased by 10 or 15 W every minute until volitional exhaustion. These data were part of a larger study designed to examine the respiratory effects of obesity in children.
When the study was designed, a 15-W increment protocol was established. After a few participants were tested, there was uncertainty regarding the attainment of V˙O2max during the incremental test based on secondary criteria (i.e., at least two of the following: RER ≥ 1.00, maximal HR ≥ 90% of age-predicted maximal HR, and signs of intense efforts) (5). To eliminate the need for secondary criteria, a supramaximal verification test was added to the study protocol. The next few tests showed that verification V˙O2max was substantially higher than the incremental V˙O2max and that the duration of the incremental test tended to be less than 8 min. Therefore, the increment of the incremental test was reduced from 15 to 10 W, and this was maintained for the remainder of the study. As part of a larger study, all children were invited to return for repeat testing 1 yr later, and all repeat incremental tests had 10-W increments (Fig. 1).
Participants were asked to maintain a cadence of 60–70 rpm and were allowed to increase cadence toward the end of the test. The test was stopped when participants were unable to maintain a cadence of 50 rpm for five consecutive seconds despite gentle but firm encouragement. HR, ratings of perceived breathlessness (Borg 0–10 scale), and RPE (Borg 6–20 scale) were monitored and recorded during the test. The Borg scales for exertion and breathlessness have been previously validated in a pediatric population (18,42). V˙E, V˙O2, and expired carbon dioxide (V˙CO2) were measured using the Douglas bag technique where expired gases were collected for 60 s in each stage in a polyurethane bags. Gas fractions were measured using a mass spectrometer (model 1100; Marquette Electronics, Milwaukee, WI), and expired volume was measured using a 200-L Tissot spirometer. The highest V˙O2 from a stage that lasted at least 20 s was recorded as maximal V˙O2 from the incremental test.
After at least 15 min of seated rest, participants performed a constant-load supramaximal verification test at 105% of the maximal work rate achieved during the incremental test. The participants warmed up by cycling for 2 min at 20 W. Work rate was increased to 105% maximal work rate within 30 s, and participants were encouraged to pedal at a cadence of their choice for as long as possible. The test was stopped when participants were unable to maintain a cadence of 50 rpm for 5 s despite encouragement. The procedure for the verification test was consistent with previous studies (7,34). After the first minute, expired air was collected for 30 s in polyurethane bags and analyzed as described previously. The highest V˙O2 from a bag that collected air for at least 20 s was recorded as maximal V˙O2 from the verification test.
Data were expressed as mean ± SD, unless otherwise specified. P < 0.05 was considered statistically significant using a two-tailed distribution. Using data from Poole et al. (32), we estimated that a sample size of 16 participants would detect differences between incremental and verification tests at 80% power (α = 0.05, effect size = 0.759, two-tailed hypothesis, based on a t-test analysis) (15). The sample size of this study is comparable with other studies that have examined differences in V˙O2 between incremental and verification tests (7,22,38) and to studies that have examined the validity of secondary criteria for establishing V˙O2max (7,32). All data were checked to ensure normal distribution.
V˙O2 plateau calculations were performed for each participant. A linear regression was fit to the linear portion of the V˙O2–work rate relationship after excluding the last minute of the incremental test. Regression equations for each participant were used to calculate predicted V˙O2 at maximal work rate. V˙O2 plateau attainment was accepted when the difference in measured V˙O2 from the penultimate stage and measured V˙O2 from the maximal stage was less than half of the difference between the measured V˙O2 from the penultimate stage and the predicted V˙O2 for the maximal stage (13). Similarly, regression equations for each participant were also used to calculate predicted verification V˙O2max at the verification work rate (i.e., 105% of maximal work rate). Measured verification V˙O2max was considered higher than measured incremental V˙O2max when the difference between measured verification V˙O2max and incremental V˙O2max was greater than the difference between predicted verification V˙O2max and incremental V˙O2max. This approach was considered because using an absolute cutoff value for the difference between incremental and verification V˙O2max (i.e., 150 mL·min−1) may not be appropriate for obese and nonobese children who may have different energy costs of cycling.
Sensitivity and specificity analyses as well as calculations regarding positive predictive value and negative predictive value were conducted to evaluate whether primary and secondary criteria can correctly identify the attainment of V˙O2max from the incremental test. The attainment of V˙O2max during the incremental test was confirmed when the difference between measured verification V˙O2max and incremental V˙O2max was less than the difference between predicted verification V˙O2max and incremental V˙O2max. True-positive, false-positive, true-negative, and false-negative rates were determined for each of the primary and secondary criteria. The following calculations were performed for each of the primary and secondary criteria (24):
Independent t-tests were used to compare differences between nonobese and obese children. A two-way repeated-measures ANOVA was used to examine group (obese vs nonobese)–test (incremental vs verification) differences on outcome variables. Bland–Altman plots were used to examine agreement between V˙O2max measured by incremental versus verification tests.
As explained in the methods, there were differences in the incremental test protocol at the start of the study, with some children undergoing tests with 15-W increments (n = 7) and others undergoing tests with 10-W increments (n = 8; Fig. 1). Verification V˙O2max was higher than incremental V˙O2max (P = 0.001) independent of the increment used (i.e., 10 or 15 W; test–increment interaction P = 0.528). However, for consistency, the following analyses only include data from children that underwent 10-W increment tests (N = 18) followed by supramaximal verification tests, either before or after 1 yr of starting the study (see shaded boxes in Fig. 1). Participant characteristics are presented in Table 1.
Performance in the verification tests
Time to exhaustion during the verification test ranged from 90 to 210 s for all children. Time to exhaustion was 135 ± 34 s for nonobese children and 125 ± 33 s for obese children, with no differences between groups (P = 0.549).
Comparing responses between incremental and verification tests
V˙O2 (L·min−1) was significantly higher during the verification test when compared with the incremental test in both nonobese and obese children (P = 0.001; Table 2). There was no significant group–test interaction (P = 0.747). Because there were no differences between nonobese and obese children, pooled data showed that verification test elicited higher V˙O2max to a similar extent for both boys and girls (△V˙O2 verification–incremental: 0.10 ± 0.12 and 0.14 ± 0.09 L·min−1, respectively, test–gender interaction P = 0.466). The verification test resulted in higher V˙O2 when expressed relative to body mass, fat-free mass, and as a percent of predicted V˙O2max based on ideal body weight (P < 0.001 for all comparisons), with no significant group–test interactions (P = 0.170, 0.534, and 0.686, respectively). Bland–Altman analysis showed wide limits of agreement (−92 to 329 mL·min−1), but there was no proportional bias and no obvious obesity status difference (Fig. 2).
There were no significant differences in maximal HR (beats per minute or expressed as a percent of age-predicted maximal HR) between incremental and verification tests for both nonobese and obese children (P = 0.071 and 0.070, respectively), with no significant test–obesity status interactions (P = 0.332 and 0.335, respectively; Table 2). RER and V˙E were also similar between incremental and verification tests for both nonobese and obese children (P = 0.363 and 0.430, respectively), with no significant group–test interactions (P = 0.750 and 0.743, respectively; Table 2).
Primary and secondary criteria for validity of an incremental exercise test
Table 3 shows individual subject data from the maximal exercise test, demonstrating the attainment of primary and secondary validity criteria as well as individual differences between the incremental and the verification tests. Seven of nine nonobese children and five of nine obese children achieved a verification V˙O2max that was higher than predicted verification V˙O2max (Table 3).
Using pooled data for nonobese and obese children, 7 of 18 children achieved a V˙O2 plateau as defined by <50% increase in V˙O2 with an increase in work rate based on the V˙O2–work rate regression from the incremental exercise test (13). Five of seven children that achieved a plateau during the incremental test had verification test V˙O2max values that were higher than predicted verification V˙O2max, and none of these seven children attained a maximal HR during the incremental test that was ≥ 90% of age-predicted maximal HR. Forty-four percent of children attained a maximal HR that was ≥ 90% of age-predicted maximal HR, 94% of children attained an RER ≥ 1.00, and 61% of children attained an RER ≥ 1.10 during the incremental test.
None of the primary or secondary criteria for validating an incremental test achieved high levels of both sensitivity and specificity, limiting their usefulness in validating an incremental test. Similarly, none of the primary or secondary criteria for validating an incremental test achieved high positive and negative predictive values. If participants achieved either a maximal HR that was ≥ 90% of age-predicted maximal HR or an RER ≥ 1.10, the sensitivity of detecting a valid V˙O2max was estimated at 56%, and specificity was 11%, but requiring both criteria to be present in the same test reduced the sensitivity to 11% and increase the specificity to 56%.
Comparing responses between incremental and verification tests
This is the first study to show that verification V˙O2max was significantly higher than incremental V˙O2max in obese children as well as in nonobese children. By contrast, four previous studies (4,7,34,36) have not shown a higher verification V˙O2max when compared with the incremental V˙O2max in nonobese children. Three (4,7,36) of these studies included a very active population of children, with average V˙O2max ranging from 50 to 62 mL·min−1·kg−1, which was substantially higher that the average V˙O2max in the current study for nonobese children (see Table 2). It is possible that the higher fitness levels and increased motivation for performance (36) during the incremental and supramaximal tests allowed participants to achieve V˙O2max during the incremental and supramaximal tests. The study by Robben et al. (34) was conducted in children with a broad range of fitness levels (i.e., 17–47 mL·min−1·kg−1), which was similar to the current study. Robben et al. showed that verification testing yielded V˙O2max values that were significantly (i.e., 150 mL·min−1) higher than values obtained using incremental testing in some nonobese prepubescent children (20). However, Robben et al. repeated the incremental test in this study until the final difference in V˙O2max between incremental and verification tests was less than 150 mL·min−1 (20), and consequently, there were no significant differences between incremental and verification V˙O2max. Therefore, the current study is the first to show that supramaximal verification testing yielded higher levels of V˙O2 when compared with incremental testing for obese and nonobese children that represented a broad range of fitness levels (i.e., 24–52 mL·min−1·kg−1).
In obese adults, a study by Sawyer et al. (38) did not find significantly higher verification V˙O2 when compared with incremental V˙O2, although they did show substantial individual variability in the results, with 13 of 19 subjects achieving a V˙O2 during verification testing that was 2% to 21% higher than the incremental test. The authors did not observe any relationship between BMI and performance on the verification test, which is consistent with the lack of differences between obese and nonobese groups in the present study. The authors also reported significantly higher maximal HR during the verification test compared with the incremental test, which is not consistent with other studies comparing the two tests in adults (22,43) or the present data that do not show significant differences in maximal HR between the two test protocols.
The average verification V˙O2 was significantly higher than the incremental V˙O2, which could be because the incremental test did not yield V˙O2max, whereas the slow component of V˙O2 during the supramaximal verification test yielded “true” V˙O2max (20). The difference in measured V˙O2 between the incremental and the verification tests exceeded the difference in predicted V˙O2 between the incremental and the verification tests, suggesting that the end-exercise gain in V˙O2 may have exceeded 9–16 mL·min−1·W−1 for obese and nonobese children (16,20). It is also possible that the incremental test produced a “priming” effect, which resulted in faster V˙O2 kinetics and increased time to exhaustion leading to the attainment of V˙O2max during the verification test (21).
Performance in the verification tests
Cardiorespiratory fitness is not affected by obesity as evidenced by similar levels of V˙O2max expressed relative to fat-free weight in obese and nonobese individuals (9). Nevertheless, obese children have lower physical fitness than their nonobese counterparts (41), which could raise questions regarding the feasibility of supramaximal verification testing for obese children. The average time to exhaustion during the verification tests in the present study (i.e., 130 ± 63 s) was longer than other studies that have reported average times of 66 and 91 s in children (7,34). There were no differences in the time to exhaustion during the verification test between obese and nonobese children, suggesting that obesity may not adversely affect performance at supramaximal work rates for cycle ergometer exercise. Moreover, studies have reported that there are no differences in V˙O2 kinetics or oxygen deficit during exercise between obese and nonobese children (25,33), suggesting that obese children may not need to exercise for a longer duration at supramaximal workloads to attain V˙O2max when compared with nonobese children.
Mortensen et al. (30) previously reported the achievement of peak cardiac output, leg blood flow, and oxygen delivery within 60–90 s of supramaximal cycling in adults, suggesting that verification tests may need to last at least 60 s to yield V˙O2max. Phase II V˙O2 kinetics, which represents the phase of rapid increase in V˙O2 during moderate to very heavy intensity exercise, may be faster in children when compared with adults (2,3). Therefore, it is possible that the minimum time needed for the attainment of V˙O2 max during a verification test could be lower than 60 s in children (34), including obese children that have similar kinetics as nonobese children (25,33). Finally, all the children in this study were able to complete incremental and verification tests without any adverse events. In fact, most children expressed that they found the verification testing both challenging and enjoyable, although it should be noted that this was a subjective assessment by the investigator rather than a formal validated assessment.
Primary and secondary criteria for validity of an incremental exercise test
The current study showed that the achievement of a V˙O2 plateau in an incremental test did not confirm the attainment of V˙O2max in five of seven participants. These data agree with Scharhag-Rosenberger et al. (39) who demonstrated higher verification V˙O2max values were possible in untrained adults despite achieving a plateau during the incremental test. Furthermore, 7 of 11 participants in the current study did not achieve a V˙O2 plateau but still had a verification test V˙O2 that was higher than predicted verification V˙O2max, suggesting that the absence of the plateau does not imply that V˙O2 max was not achieved during the incremental test. These data are consistent with Armstrong et al. (4) who showed that children (9.9 ± 0.4 yr) achieved V˙O2max during an incremental test as confirmed by two verification tests even in the absence of a plateau. Similarly in adults, Wood et al. (43) showed that 39% of overweight and obese participants did not attain a V˙O2 plateau during the incremental test and were not able to achieve higher V˙O2 during a verification protocol, confirming that the absence of a V˙O2 plateau during the incremental test did not indicate failure to reach V˙O2max.
Previously, Robben et al. (34) showed that using a two-step protocol that included an incremental and supramaximal constant-load verification test in prepubertal children with and without expiratory flow limitation allowed for the unambiguous determination of V˙O2max. They also reported that the RER criterion on 1.00 was erroneous for confirming the attainment of V˙O2max, which we confirm because 17 of 18 of the children in this study were able to achieve an RER > 1.00, although ≈70% of these children still achieved a verification V˙O2max that was higher than predicted verification V˙O2max. Robben et al. also showed that many of their prepubertal children were not able to achieve the HR criterion of ≥90% age-predicted maximal HR, which is consistent with the present data where less than half of our participants were able to reach this HR criterion. On the basis of the present data, none of the primary or secondary criteria meet an acceptable combination of sensitivity and specificity for predicting a valid incremental test in 10- to 12-yr-old children, and therefore, using a two-step approach (incremental and verification testing) may be the best approach for determining V˙O2max.
This study has a relatively small sample size (N = 18) and narrow age range (i.e., 10–12 yr). Future studies should include children of different fitness and obesity levels and a larger sample size to further support and encourage the use of verification testing in the assessment of V˙O2max. There could be an effect of pubertal status on the results, but the small sample size and varied pubertal stages in our participant population precluded any statistical comparisons based on pubertal status. Some studies have either repeated the supramaximal test (4,39) or repeated the incremental test (34) on a different day to confirm the attainment of V˙O2max. However, like Barker et al. (7), the present study only conducted one incremental test followed by a verification test on the same day. The verification test could be repeated on a different day to confirm V˙O2max, although a study that used a similar approach did not find significant differences in V˙O2max between verification tests performed on two separate days in adults (39).
Because there was a change in the work rate increment from 15 to 10 W after the study had already commenced, almost half of the participant population included children that returned for testing 1 yr later. Thus, the final sample reported in this article included only children that had undergone an incremental test with 10-W work rate increments, including two children that were 12.5 and 12.8 yr. We believe that comparing children that underwent the same incremental test protocol followed by the verification test was the best methodological approach. Our analyses demonstrated that the difference between incremental and verification test V˙O2 values was similar for tests that used 15- and 10-W increments, which supports our results. It is also difficult to ascertain whether the children who returned after 1 yr performed better during the incremental test because of habituation with the protocol. However, the difference between incremental and verification V˙O2 was similar for children that were tested either before or after 1 yr using a 10-W increment protocol (data not reported), suggesting that any effects of habituation may have been minimal.
We showed that the highest verification V˙O2 values were significantly higher than incremental values for 10–12 yr obese and nonobese children. We also showed that a two-step protocol that includes both incremental and verification testing should be used instead of primary or secondary criteria, which lack adequate sensitivity and specificity for validating an incremental test. Because cardiorespiratory fitness is an important marker for health and there are inherent difficulties in validating an incremental exercise test in children, researchers should use a two-step protocol that includes both incremental and verification testing for measuring V˙O2max.
This research was supported by the National Institutes of Health (grant no. HL096782), Dr. Pepper Snapple, King Charitable Foundation Trust, and Texas Health Presbyterian Hospital Dallas. The authors thank J. Todd Bassett, Rubria Marines-Price, Maria C. Roman, Andreas Kreutzer, Joseph Genovese, Jessica Pineda, and Raksa B. Moran who have helped with data collection on this project. They also thank the participants and their parents for their enthusiastic participation in the project.
All authors declare that they have no competing financial interests related to this publication. All authors declare that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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