Brazilian Cardiorespiratory Fitness Classification Based on Maximum Oxygen Consumption

Background Cardiopulmonary exercise test (CPET) is the most complete tool available to assess functional aerobic capacity (FAC). Maximum oxygen consumption (VO2 max), an important biomarker, reflects the real FAC. Objective To develop a cardiorespiratory fitness (CRF) classification based on VO2 max in a Brazilian sample of healthy and physically active individuals of both sexes. Methods We selected 2837 CEPT from 2837 individuals aged 15 to 74 years, distributed as follows: G1 (15 to 24); G2 (25 to 34); G3 (35 to 44); G4 (45 to 54); G5 (55 to 64) and G6 (65 to 74). Good CRF was the mean VO2 max obtained for each group, generating the following subclassification: Very Low (VL): VO2 < 50% of the mean; Low (L): 50% - 80%; Fair (F): 80% - 95%; Good (G): 95% -105%; Excellent (E) > 105%. Results Men VL < 50% L 50-80% F 80-95% G 95-105% E > 105% G1 < 25.30 25.30-40.48 40.49-48.07 48.08-53.13 > 53.13 G2 < 23.70 23.70-37.92 37.93-45.03 45.04-49.77 > 49.77 G3 < 22.70 22.70-36.32 36.33-43.13 43.14-47.67 > 47.67 G4 < 20.25 20.25-32.40 32.41-38.47 38.48-42.52 > 42.52 G5 < 17.54 17.65-28.24 28.25-33.53 33.54-37.06 > 37.06 G6 < 15 15.00-24.00 24.01-28.50 28.51-31.50 > 31.50 Women G1 < 19.45 19.45-31.12 31.13-36.95 36.96-40.84 > 40.85 G2 < 19.05 19.05-30.48 30.49-36.19 36.20-40.00 > 40.01 G3 < 17.45 17.45-27.92 27.93-33.15 33.16-34.08 > 34.09 G4 < 15.55 15.55-24.88 24.89-29.54 29.55-32.65 > 32.66 G5 < 14.30 14.30-22.88 22.89-27.17 27.18-30.03 > 30.04 G6 < 12.55 12.55-20.08 20.09-23.84 23.85-26.35 > 26.36 Conclusions This chart stratifies VO2 max measured on a treadmill in a robust Brazilian sample and can be used as an alternative for the real functional evaluation of physically and healthy individuals stratified by age and sex.

of physical exercise. [2][3][4][5] In Brazil, CPET is preferably performed on a treadmill, but, in many countries, a cycle ergometer is preferred. Maximum oxygen consumption (VO 2 max) reflects the individual's maximum capacity to absorb, transport and consume oxygen. 2 The major determinants of normal VO 2 max are: genetic factors, muscle mass amount, age, sex and body weight. 1,2 In practice, VO 2 max is considered to be equivalent to the highest VO 2 value obtained in peak exertion, which is usually used to classify cardiorespiratory fitness (CRF) in a population. In this study, for practical purposes, we named VO 2 peak, which was actually measured, VO 2 max.
Few studies have provided reference CRF charts for populations, and it is yet to be clarified whether the existing classifications can be extrapolated to other populations. Most published studies have been based on small samples, and the profiles of the populations studied have significantly differed. 6,7 The CRF classification charts most used in Brazil are as follows: that of the American Heart Association (AHA), published in 1972 (Table 1), and that by Cooper, of 1987. Brazil does not have a solid and widely used CRF classification for CPET; therefore, this study proposes a classification based on Brazilian population data. Such data, resulting from a recently published study, were used as reference for CPET on a treadmill (ramp protocol) for sedentary and physically active men and women. 8

Methods
This study's sample comprised 9,250 CPET performed at a large cardiology referral center in southern Brazil. 8 Based on a questionnaire completed during the test, individuals with the following characteristics were excluded from the study: any symptom suggesting disease or pathology; amateur or professional athletes; smokers; users of any medication; obese individuals (body mass index -BMI > 30); and tests with the ratio between the amount of carbon dioxide produced and of oxygen used (respiratory exchange ratio -RER) < 1.1.
After applying the exclusion criteria, 3,922 CPET were identified, of which, 2,837 CPET, corresponding to healthy and active individuals, were selected. Those individuals, aged between 15 and 74 years, were of both sexes and different ethnicities, and practiced leisure-time aerobic physical activity for at least 30 minutes a day, three times a week. 8 All exercise tests were conducted by cardiologists trained in ergometry and CPET by the Brazilian Society of Cardiology Department of Ergometry and Cardiovascular Rehabilitation. The tests were performed on a treadmill (Inbrasport -ATL™, Brazil, 1999, Software ErgoPC Elite Version 3.3.6.2, Micromed Brazil, 1999), using the ramp protocol. A mixing chamber gas analyzer (MetaLyzer II, Cortex TM -Leipzig, Germany, 2004) was used to collect the expired gases. For descriptive statistics, central trend measures, such as means, were used, in addition to dispersion measures (standard deviation). Excel software, Microsoft 2008, was used for statistical analyses and charts.
To internally validate our proposed CRF classification, sedentary individuals of both sexes from the study population sample were assessed, according to previous publication. 8 This study was approved by the Ethics Committee in Research of the Instituto de Cardiologia de Santa Catarina.   Results Tables 2 and 3 show the mean VO 2 max values of the original population and the number of CPET performed, stratified by sex and age groups, of physically active and sedentary individuals. The VO 2 max levels were higher in the active groups as compared to the sedentary ones, and men had greater VO 2 max levels than women did. Tables 4 and 5 show our proposed CRF classification, with five different categories, stratified by sex and age group, of apparently healthy individuals. Table 6 shows the classification of the sedentary population (men and women) from the original sample, considering the new CRF chart proposed in this study. It is worth noting that the CRF of sedentary individuals is always classified as either fair or low.
As expected, VO 2 max levels dropped throughout the age groups for both sexes (Figures 1 and 2).

Discussion
We elaborated a CRF classification chart based on VO 2 max levels measured during CPET (ramp protocol) performed on an ergometric treadmill, to more accurately classify a solid Brazilian sample of healthy and physically active individuals of both sexes. We chose to base our analysis on data of physically active individuals, who would provide CRF in the "good" category, corresponding to mean CRF values. Not using data of sedentary individuals allowed us to validate our proposed CRF classification chart, observing in which category sedentary individuals would fit.
According to our CRF classification chart, we confirmed that the CRF of active men is higher than that of active women of the same age group, and, for both sexes, active individuals had a better CRF as compared to sedentary ones. According to Nunes et al., 7 mean VO 2 max values of women are lower than those of men, the mean VO 2 max values of the former corresponding to only 70% of those of the latter. The present study showed a mean VO 2 max of women corresponding to 76% to 83% of the mean VO 2 max of men of the same age group.
Sedentary individuals not only had a lower VO 2 max as compared to physically active ones, but also a twice higher decrease in VO 2 max as age advanced. 9,10 Regular exercise practice reduces the VO 2 max rate of decrease as compared to a sedentary lifestyle, 11 and, the greater the VO 2 , the greater the protection against cardiovascular events. An increase in aerobic capacity is associated with an increase in survival, as reported by Myers et al., 12 who have demonstrated a significant increase in the relative risk of death from any cause as functional capacity decreased, regardless of the risk factors involved. In addition, those authors have reported a 12%-increase in survival for each 1-MET increase in the CRF level. 12 Most CRF classification charts used in clinical practice have been elaborated in other countries and have not been validated for the Brazilian population. Extrapolating those classifications to the Brazilian population can lead to relevant discrepancies. Belli et al. 13 have shown significant discrepancies when comparing international charts with Brazilian data.  VO 2 max depends on a frequent and constant physical activity and can be enhanced with treinos. 14 However, despite the volume or intensity of the workout raise VO2 max by 10 to 30%, there is also an important genetic influence. Research has shown that genetic inheritance is the main responsible for max VO2 each individual and may be responsible for up to 25% to 50% of the variation in the values of VO 2 max, ie, alone accounts for almost half of ACR. 15 VO 2 max can be measured directly by analyzing the gases expired during CPET, or indirectly, by using calculations. Although some prediction equations provide an acceptable  association with values obtained via direct measurements, the difference varies, depending on the population studied. The error for one certain individual can be extremely high, ranging from 15% to 20% in some studies, and can even reach or exceed 30%, a high margin of error, considering other measurements in the biological area 16 .
According to data obtained in this study, VO 2 max drops with age. That drop in women varies less from one age group to the other as compared to that in men. We observed a higher drop in VO 2 max among active women from group 3 to group 4, with a mean of 0.38 mL.kg -1 .min -1 per year. Among sedentary women, that drop was sharper

Herdy & Caixeta Cardiorespiratory fitness classification
Arq Bras Cardiol. 2016; 106(5):389-395 from group 2 to group 3, with a mean of 0.4 mL.kg -1 .min -1 per year. Among both active and sedentary men, however, the VO 2 max drop was more marked from group 5 to group 6, with a mean of 0.53 mL.kg -1 .min -1 per year among active men, and of 0.69 mL.kg -1 .min -1 per year among sedentary men. Nunes et al. 7 have shown a VO 2 max drop of 0.4 mL.kg -1 .min -1 per year among men aged 20 to 60 years. Belli et al., 13 using indirect VO 2 max measurement, have evidenced a drop of 20% to 25% per decade in mean VO 2 max from the age of 50 years onward, that drop being sharper after the age of 60 years. An approximate drop in VO 2 max of 0.4 mL.kg -1 .min -1 per year is estimated to occur from the age of 25 years onward, and that VO 2 max decline is twice greater in sedentary individuals as compared to physically active ones. 8,9 We used the new CRF classification chart to classify sedentary individuals undergoing CPET under the same conditions of the physically active ones from the original population. This would allow us to validate our proposed classification, considering how the VO 2 max values of those individuals would fit. Differently from the studies estimating VO 2 max indirectly, the direct measurement of VO 2 max shows that CRF in sedentary individuals is classified, at the most, as fair, regardless of age and sex (Table 6). From the practical viewpoint, sedentary individuals have decreased tolerance to exertion, and, thus, physical exercise prescription to active and sedentary individuals should differ. 17 The CRF chart by Cooper 18 and that of the AHA 19 (Table 1) are the most commonly used tools to classify CRF in CPET programs in Brazil. However, the literature lacks data concerning sampling methods and sample types used to elaborate the AHA chart. Therefore, the comparison of data obtained in this study with the AHA chart is limited. Our classification comprises a wider age range, from 15 to 74 years, as compared to that of the AHA (20 to 69 years). The VO 2 max analysis in both charts evidences, in younger age groups, very similar VO 2 max values. However, in the other age groups, a greater difference is observed between our data and the VO 2 max values of the AHA chart.
Most CRF charts published so far have been elaborated with CPET performed on a cycle ergometer. The VO 2 max obtained in tests performed on a treadmill, as opposed to those performed on a bicycle, is approximately 5% to 17% higher (mean of 8%). 20,21 The difference is attributed to the amount of active muscle mass involved in the test, which is greater for the inclined treadmill. Another important factor relates to the pedaling effect, which causes localized muscle fatigue by using the large muscle groups of the thigh, and that fatigue can occur before maximum exertion is imposed to the circulatory and respiratory systems, generating a lower VO 2 max. 2 In our study sample, the age range was wide, including adolescents older than 15 years. We believe that from that age on, individuals already have muscle maturation and performance close to those of young adults under the age of 25 years. 22,23 The classification chart proposed should be assessed as an instrument to predict risk for morbidity and mortality, according to each individual's functional profile. Further studies are required.
This study has limitations, such as the lack of standardization of ramp protocols. Individuals classified as physically active practiced different types of activities and sports, making the comparison of the results in different populations difficult. Further studies are required, using the same intensities and inclinations in the protocol ramp and with individuals practicing the same type of aerobic exercise, because that would improve the analysis and comparison of the results. Individuals with hypertension, diabetes or dyslipidemia, those on any type of medication, and those with a BMI greater than 30 (obese) were excluded, making the applicability of that classification in those subgroups uncertain. The Brazilian population is known to be diversified, and, in southern Brazil, the European colonization predominates (smaller percentage of Afrodescendant and Native individuals, as compared to other Brazilian regions). New studies should be developed, including different ethnicities and individuals from other Brazilian regions, aiming at comparing with the classification proposed to verify whether the values differ.

Conclusion
This is one of the few Brazilian studies to propose a CRF chart with data extracted from a robust population sample, and based on VO 2 max measured via CPET on a treadmill. These data can be used for functional capacity classification according to sex and age group and considering different risk profiles.

Author contributions
Conception and design of the research: Herdy AH e Caixeta A. Acquisition of data: Herdy AH. Analysis and interpretation of the data: Herdy AH e Caixeta A. Statistical analysis: Herdy AH e Caixeta A. Obtaining financing: Herdy AH. Writing of the manuscript: Herdy AH e Caixeta A. Critical revision of the manuscript for intellectual content: Herdy AH e Caixeta A.

Potential Conflict of Interest
No potential conflict of interest relevant to this article was reported.

Sources of Funding
There were no external funding sources for this study.

Study Association
This study is not associated with any thesis or dissertation work.