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Maximal oxygen consumption and oxygen uptake efficiency in adolescent males

Background/objective Measures of oxygen uptake efficiency (OUE) have been used to evaluate cardiorespiratory fitness (CRF) in adolescents unable to perform maximal exercise. The oxygen uptake efficiency slope (OUES) and oxygen uptake efficiency plateau (OUEP) have been proposed as surrogates for maximal oxygen consumption (V̇O2max). We assessed the validity of the OUES and OUEP as predictors of V̇O2max in healthy male adolescents. Methods Sixty-three healthy male adolescents aged 15.40 ± 0.34 years underwent an incremental treadmill test to determine V̇O2max, OUES and OUEP. OUE throughout the test was assessed by dividing each V̇O2 value by the corresponding minute ventilation (V̇E) value. OUEP was determined as the 90 s average highest consecutive values for OUE. OUES was determined using data up to the ventilatory threshold (VT) by calculating the slope of the linear relation between V̇O2 and the logarithm of V̇E. Results Limits of agreement for V̇O2max predicted by OUES (±13.3 mL kg−1.min−1) and OUEP (±16.7 mL kg−1.min−1) relative to V̇O2max were wide and a magnitude bias was found for OUES and OUEP as predictors of V̇O2max (p < 0.001). Conclusion The OUES and OUEP do not accurately predict V̇O2max in male adolescents and should not replace V̇O2max when assessing CRF in this population.


Introduction
Cardiorespiratory fitness (CRF) is a powerful indicator of cardiovascular health in children and adolescents. 1 High CRF during childhood and adolescence is associated with a healthier cardiovascular profile during these years 1 and also later in adulthood. 2 Maximal oxygen consumption (V_O 2max ), measured during a standardized cardiopulmonary exercise test (CPET), is considered the gold standard measure of CRF and is defined as the highest rate of oxygen uptake and utilization by the body during intense, maximal exercise that no further increases in work rate result in additional rises in V_O 2 . 3 The conventional criterion for the attainment of V_O 2max during a CPET is a levelling-off or plateau in V_O 2, and relies entirely on data from the last segment of a CPET. In such cases where the criteria for V_O 2max attainment is not met, V_O 2peak is recorded, which is the highest V_O 2 value attained during an incremental or other high intensity exercise test, designed to bring the subject to the limit of tolerance. 4 The ability to attain V_O 2max during a CPET is strongly effort-dependent and sensitive to many factors including participant motivation. 5 Oxygen uptake efficiency (OUE) measurement, which is the reciprocal of the ventilatory equivalent for oxygen (V_O 2/ V_ E ), provides an estimation of the efficiency of minute ventilation (V_ E ) with respect to oxygen uptake (V_O 2 ). 6 As the OUE is not linear during the entire exercise period, the oxygen uptake efficiency slope (OUES), mathematically derived from the linear relation between V_O 2 and the logarithmic transformation of V_ E during incremental exercise (V_O 2 ¼ a log10 VE þ b, where a is the OUES and b is the intercept) has been proposed as an objective and effort-independent submaximal measure of exercise capacity 7 and a potential surrogate for V_O 2max . 8,9 The logarithmic transformation of V_ E reduces the curvature expected with increases in exercise intensity making the OUES resistant to disruption by the early termination of exercise.
Studies have shown that the OUES, calculated using only submaximal exercise data, is identical to the OUES calculated over the entire duration of a CPET. 10,11 Furthermore, both maximal OUES and submaximal OUES are significantly related to V_O 2peak in both normal weight and obese children and adolescents. 10,12e14 There is evidence however, that OUES at submaximal and maximal exercise intensities exhibits a significant magnitude bias when used to predict V_O 2peak in healthy adults 15 and adolescents 12 with varying levels of CRF, in that it over predicts V_O 2peak at low levels of CRF and under predicts V_O 2peak at high levels of CRF.
More recently, Sun et al. (2012) noted that the OUE when plotted against time, reaches its highest and briefly stable values/plateau (OUEP) at a moderate exercise intensity near the ventilator threshold (VT) before declining due to hyperventilation stimulated by the onset of metabolic acidosis. 16 A cross-sectional study involving 214 boys and girls aged between 8 and 19 years found that the OUEP was weak-to-moderately correlated with V_O 2peak and the OUES was strongly correlated with VO 2peak. 6 Similar findings have been observed in healthy adults. 16 The validity of submaximal measures of CRF including the OUES and OUEP to accurately predict V_O 2max in adolescents is of particular importance for disease risk identification and classification, given that V_O 2max is not often attained in this cohort. While the validity of the majority of submaximal measures of CRF including OUES and OUEP have been based on their correlations with V_O 2peak/max , a strong correlation between OUES/OUEP and V_O 2peak/max, does not take into account their interchangeability. 15 To date, only one previous study has assessed the interchangeability of OUES with measured V_O peak and found that the wide interindividual variation in OUES precluded its use in clinical practice as a predictor of V_O 2peak . 12 Furthermore, no previous studies have examined the interchangeability of the OUEP with V_O 2max .
The purpose of this study was to therefore assess the concurrent validity of the OUES and the OUEP as predictors of V_O 2max in apparently healthy male adolescents by using Bland and Altman method, 17 to compare the V_O 2max value predicted from OUES and OUEP to the V_O 2max measured during maximal exercise testing.

Participants and procedures
Sixty-three apparently healthy male adolescents (mean ± SD; age 15.40 ± 0.34 years) participated in the study. Initially, it was planned that females would participate in the study. However, during the recruitment phase of the study, many females were unwilling to participate and some females reported having an irregular menstrual cycle. Given that we wanted to control for the menstrual cycle in this study and that irregularity in the menstrual cycle would make this difficult, females were not included in the study. Exclusion criteria for male adolescents included current smokers, currently taking vasoactive medications, uncontrolled hypertension (SBP >180 mmHg, diastolic BP > 100 mmHg) or any condition that precluded participants from engaging in moderate to vigorous exercise. Informed parental consent and child assent were obtained prior to participation. The study was approved by Research Ethics Committee at Dublin City University. Participants made a single visit to the research laboratory during which body composition, pubertal status and CRF were assessed. Participants refrained from physical activity for 24 h prior to the visit.

Anthropometry and pubertal assessment
A stadiometer and electronic scale (Seca 797, USA) were used to measure height (cm) and body mass (kg), respectively. Body mass index (BMI) was calculated as body mass in kilograms divided by height in metres-squared (kg . m À2 ). Age-and gender-specific BMI percentiles were calculated and Centers for Disease Control and Prevention (CDC) standards were used to classify participants as normal weight (BMI < 85th percentile), overweight (BMI ! 85th and <95th percentile) and obese (BMI ! 95th percentile). 18 Participants self-reported their genital and pubic hair development using standardized Tanner drawings representing different stages of sexual maturity. 19 Participants were classified as pre-pubertal (Tanner 1), pubertal (Tanner II-IV) and post-pubertal (Tanner V) as previously described. 20 Cardiopulmonary exercise testing CPET was performed on a treadmill (Woodway ELG 55, Waukesha, WI) in a temperature-controlled laboratory environment (temperature 21.1 C, ambient humidity 60%, atmospheric pressure 750 mm Hg). The test consisted of a continuous incremental protocol designed to ensure that participants reached volitional exhaustion between 8 and 12 min. One month prior to the CPET, participants undertook a school-based 20 m shuttle run test to estimate their level of CRF. Based on the number of shuttle runs completed, participants were classified as low CRF (LCRF) (0e40th percentile), moderate CRF (MCRF) (40e70th percentile), and high CRF (HCRF) (70th-100th percentile), in accordance with European age and gender specific percentiles for aerobic fitness. 21 Three separate protocols were then specifically developed to accommodate LCRF, MCRF and HCRF. Briefly, participants undertook a 2 min warm up on a 0% grade, which consisted of 2 min at 6.0 for km.hr À1 for those with LCRF, 7.0 km h À1 for those with MCRF and 9.0 km h À1 for those HCRF After the warm-up, the treadmill velocity was increased to 7.0 for LCRF, 8.0 for MCRF and 11.0 km h À1 for HCRF for 1 min and was then increased to 8.0, 10.0 and 12.0 or 13 km h À1 in those with LCRF, MCRF and HCRF respectively, after which the speed remained constant and the grade was increased by 0.5% every 30 s until volitional exhaustion. Respiratory metabolic responses were determined using standard open-circuit spirometry techniques (Sensormedics Vmax 229, SensorMedics Corp., CA). Prior to testing, the gas analyzers were calibrated with standard gases of known concentration. A mass flow sensor was used to collect breath-by-breath measurements of ventilation. A 3.0 L volume syringe was used to calibrate the mass flow sensor prior to each test. Heart rate was continuously recorded throughout the test using telemetry (Polar team 2 Pro, Polar Electro Inc., NY, USA) and rating of perceived exertion (RPE) was recorded during the last 15 s of each min using the Borg 16 point category rating scale. Participants were verbally encouraged to give their maximal effort.

Criteria for maximal oxygen consumption
Maximal oxygen consumption (V_O 2max ) was determined using the highest consecutive 20 s value. The test was deemed maximal if at least two of the following four criteria were satisfied: (i) a plateau in oxygen consumption (defined as a 2.0 . mL kg À1. min À1 change in V_O 2 during the last min of exercise) (ii) heart rate >200 bpm, (iii) RER >1.0 or (iv) volitional fatigue. 14

Oxygen uptake efficiency
Oxygen uptake efficiency (OUE) throughout the CPET was assessed by dividing each V_O 2 value (ml/min) by the corresponding V_ E value (L/min). The OUEP was then determined as the 90 s average highest consecutive values for OUE. 16 OUES was determined on the respiratory data up to the VT by calculating the slope of the linear relation between V_O 2 (y-axis) and the logarithm of V_ E (x-axis) through single regression analysis. 7 The VO 2 at the VT (V_O 2 VT) was defined as the level of V_O 2 at which the linear relation between V_CO 2 and V_O 2 disappeared 22 and/or at which an increase in ventilatory equivalent for oxygen occurred (V_ E V_O 2 ) without a simultaneous increase in ventilatory equivalent for carbon dioxide (V_ E V_CO 2 ). The first minute of exercise was excluded because of the often irregular breathing pattern that is common at the onset of exercise. Relative values for V_O 2max , V_O 2 VT, OUES and OUEP were calculated by dividing the absolute values by body mass.

Statistical analysis
Statistical analyses was performed using SPSS for Windows statistical software (V22.0, SPSS Inc, IL). Continuous variables are expressed as mean ± SD. Estimates of V_O 2max were calculated from the regression equation of the relation between V_O 2max and OUES and V_O 2max and OUEP. Bland and Altman 17 analyses was then used to examine the level of agreement between measured and estimated V_O 2max . Specifically, 2-tailed paired t-tests were conducted at a ¼ 0.05, between the measured and estimated V_O 2 max. To determine if there was measurement bias between objectively measured V_O 2max and V_O 2max predicted using OUES and OUEP, the differences between measured V_O 2max and estimated V_O 2max measured from OUES and OUEP were regressed against the means of the measured and estimated V_O 2max from OUES and OUEP. Significance tests on the regression slope were applied to determine if there was measurement bias over the range of V_O 2max values for OUES and OUEP. Univariate correlation analysis was conducted between V_O 2max , OUES and OUEP. The significance level was set at a ¼ 0.05 using a Bonferroni correction factor of 0.00625 (0.05/8).

Participant characteristics and CPET results
Participant characteristics and CPET results are presented in Table 1. 42 participants (63%) were classified as normal weight, 4 participants were overweight (16%) and 17 participants were obese. 55 participants self-reported that they were in the pubertal stage and 8 participants self-reported that they were in the post-pubertal stage All exercise tests were completed without any contraindications. At least two of the four predetermined criteria used to verify attainment of V_O 2max were achieved by 94% of the participants. Almost all of the study participants (97%) achieved an RER value > 1.0, 65% had a plateau in oxygen uptake and 41% achieved a HR > 200 bpm. OUEP was obtained in all participants. OUES was obtained in 46 participants as the VT could not be determined in 17 participants. Absolute and relative values for V_O 2max, OUES and OUEP are summarized in Table 1.

Discussion
The present study assessed the concurrent validity of the OUES and the OUEP as predictors of V_O 2max in an adolescent population. The findings indicate that the large inter-individual variation and significant magnitude bias in both OUES and OUEP may impede their use as a predictor of V_O 2max in healthy normal weight and overweight adolescents with varying levels of CRF.
V_O 2max is considered to be the gold standard measurement for assessing cardiorespiratory fitness. The determination of V_O 2max is based on the attainment of predetermined criteria which children and individuals with poor motivation and chronic disease are often unable to meet due to its effort dependency. 23 Similar to V_O 2max, both the OUES and OUEP provide an objective measure of cardiorespiratory function, reflecting the efficiency of ventilation with regard to oxygen uptake during exercise, but do not require maximal effort. 16,24 Higher OUES and OUEP values indicate a more efficient VO 2 while lower values represent a lower VO 2 for any given V_ E . The OUEP represents the maximal and most efficient OUE which occurs at submaximal exercise intensities around the VT. 16 The OUES describes the slope of the linear relation between V_O 2 and the log transformation of V_ E . Theoretically, OUES should be robust to the early termination of exercise allowing it to be used as an objective, effort independent measure of cardiorespiratory fitness. 7,25 Studies in adults have found the OUES to be a reliable index of cardiorespiratory functional reserve. 5,19 Using correlation analysis, OUES was found to be an accurate replacement for V_O 2max in healthy individuals and heart disease patients. 4,18 OUES and OUEP were both significantly correlated with V_O 2max in the present study. The present findings are similar to previous studies involving healthy children and adolescents that reported a significant relation between both OUES and OUEP and V_O 2max or V_O 2peak . 7,10,11,13,14 However, a strong correlation between OUES and OUEP with V_O 2max or V_O 2peak does not take into account their interchangeability. 15 The current study is the first study to assess the interchangeability of both the OUES and OUEP with measured V_O 2max in an adolescent population. Although there were no significant differences between measured V_O 2max and predicted V_O 2max using OUES and OUEP, the limits of agreement varied greatly, indicating a significant magnitude bias. OUES and OUEP over predicted V_O 2max at lower levels of CRF and under predicted V_O 2max at higher levels of CRF. The heterogeneity in fitness levels in our study population, may have contributed to the variability in V_O 2max values predicted from OUES and OUEP. Drinkard et al. (2007) also found a significant magnitude bias when OUES at the lactate inflection point, OUES at 150% of lactate infection and OUES at V_O 2peak was used to predict V_O 2max in both normal weight and overweight adolescents varying in levels of CRF. 12 The fact that both the OUES and OUEP over predicted V_O 2max at lower levels of V_O 2 in the present study, suggests that they should not be used as a replacements for V_O 2max to assess CRF in clinical practice, particularly in adolescents with low CRF who are a target for primary prevention of CVD. The V_O 2 or workrate corresponding to the nonlinear increase in V_ E during exercise demarcates the ventilatory anaerobic threshold (VT). Like OUES and OUEP, the V_O 2 VT does not require a physiologically maximal exercise effort, and is considered to be more consistent with an individual's ability perform activities of daily living. 26 A major shortcoming of the VT measurement, is the fact Fig. 1. Difference between measured V_O 2max . (mL . kg À1. min À1 ) and (a) predicted V_O 2max using OUES.kg À1 and (b) predicted V_O 2max using OUEP.kg À1 . that often it is not identifiable, and its measurement is subject to substantial intra-and inter-observer variability. 27,28 Measurement of OUEP is an appealing alternative to V_O 2 VT as it does not rely on VT determination. Furthermore, while data up to the VT was used to calculate OUES in the present study, OUES does not necessarily need to rely on VT determination and can be calculated using a certain percentage of the max/peak exercise data i.e., 50%. A recent study calculated OUES using data up to 3, 4, 5 and 6 min of the exercise test and found that OUES determined from 5 min of exercise data was strongly related to V_O 2max in obese children and adolescents. 29 However, while the OUES and OUEP are both objective and effort-independent submaximal measures of exercise capacity and are related to V_O 2max , their potential application for assessing CRF in a group or population setting for surveillance purposes is limited as they rely on the administration of a CPET in a laboratory setting which is expensive and not very feasible.
Key strengths of the present study is that it is the first study to concurrently assess the interchangeability of both the OUEP and OUES in an adolescent population. Our findings revealed that the OUEP and the OUES do not accurately predict V_O 2max in a normal and overweight adolescents varying in CRF and should not be used as a replacement for V_O 2max when assessing adolescents CRF. Accurate and routine monitoring of CRF in adolescents is of paramount importance given that it has been recently shown that low CRF in adolescence is independently associated with risk of heart failure in adulthood. 30 The present study included both normal weight and overweight obese adolescents. Body mass has been shown to influence OUES, with a larger body mass resulting in increased absolute OUES values. 13 To account for the influence of body mass, V_O 2max , OUES and OUEP values were normalized relative to body mass. Moreover, our findings are similar to that of a previous study that found a significant bias in OUES expressed relative to lean body mass when used predict V_O 2max in normal and overweight adolescents with varying levels of CRF. 12 A limitation of this study is use of the VT to calculate the OUES as 17 participants could not be included in the analysis. Future studies should employ a certain percentage of exercise data or duration of the exercise test to calculate OUES rather than relying on VT determination and should assess the validity of these measures of OUE as predictors of V_O 2max Secondly, another limitation of the present study is that participants self-reported their pubertal status. The OUES has been shown to be influenced by maturation with an inverse relation observed between OUES and puberty stage. 31 While the majority of participants (87%) in the preset study selfreported that they were in the pubertal stage, 8% reported that they were post-pubertal. The disadvantage of using self-reported measures to assess pubertal status in adolescents is that there may be bias. While we adjusted for body mass in the current study, a previous study showed that adjusting for body mass did not eliminate differences across maturational groups suggesting that qualitative factors such as oxidative metabolism may be responsible for the inverse relation observed between pubertal stage and the OUES. Thirdly, while we expressed V_O 2max , OUES and OUEP values relative to body mass, expressing these values relative to lean body mass would have allowed for better control of the influence of adiposity. Finally, this study was conducted in male adolescents only and therefore the findings cannot be generalized and extrapolated to female adolescents.

Conclusion
The OUES and OUEP do not reliably predict V_O 2max and therefore should not be used in as a replacement for V_O 2max to assess CRF in male adolescents.

Funding/support statement
This work was supported by The Insight Centre for Data Analytics, Science Foundation Ireland (SFI 12/RC/2289) who assisted in analysis and interpretation of the study data.