Evaluación de la aptitud cardiorrespiratoria en jóvenes obesos

Autor(es):

	David Thivel
	Universidad de Clermont Auvergne, EA 3533, Laboratorio de Adaptaciones Metabólicas al ejercicio bajo condiciones fisiológicas y patológicas (AME2P), BP 80026, F-63171 Aubière cedex, France
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	Julien Aucouturier
	Universidad de Derecho y Salud Lille 2, EA 4488 “Actividad Física, Musculo, Salud”, Facultad de Ciencias del deporte y de la Educación Física, 59790 Ronchin, France
	Ver la biografía del autor

Cardiorespiratory fitness in obese children and adolescents

Obese children and adolescents usually have lower overall physical abilities and especially lower cardiorespiratory fitness (CRF) when compared to their normal-weight peers. This is mainly because of the increased effort required to move their larger body mass and carry an excessive amount of body fat ¹. It is only among extremely obese children that the lower CRF can partly result from impaired lung function, with decreased expiratory reserve volume and functional residual capacities due to their lower chest wall and lung compliance ^2-5. He and al. did not observe any difference of pulmonary functions between lean and obese children, despite a higher prevalence of respiratory symptoms that may occasionally impair cardiorespiratory fitness in obese youth ⁶. Although lower cardiorespiratory performances are observed in obese children and adolescents compared to those of lean children and adolescents when adjusted to body mass, absolute performances are similar or higher, and these differences disappear when performances are adjusted to fat free mass, suggesting that muscle maximal oxidative ability is not impaired with obesity in youth ^{7, 8}. As an example, Lazzer et al. reported a maximal oxygen uptake (VO₂max) approximately 27% higher in 12-16 years old obese youth when expressed in absolute terms (L.min^-1) , but when VO₂max was adjusted to Fat-Free Mass there was no difference between the obese adolescents and their normal weight controls as illustrated by the Figure 1 ⁹. Using an incremental treadmill test conducted to exhaustion and the measurement of VO_2max, Watanabe et al. observed an inverse and significant relation between 12-15 years old obese adolescents’ CRF and their body fat mass ⁷. CRF results are similar when VO_2max cannot be directly assessed and indirect methods are used to assess CRF (Queen’s college step test) ¹⁰. Excessive body fat is also thought to contribute to the exercise intolerance and low CRF in obese youth ⁷. Some studies suggest different effects of obesity in girls and boys, as Mota et al. did not observe any CRF difference between lean, overweight or obese 8 years old boys, whereas overweight and obese girls were more likely to be unfit compared to lean girls ¹¹. This is in accordance with previous findings from a longitudinal study showing that CRF among girls but not boys was significantly associated with the incidence of overweight and obesity ¹².

Figure 1. Illustration of the VO2 differences between lean and obese youth depending on its expression during an incremental exercise test (relative to Body Mass (A) or Fat Free Mass (B).

Although exercise training represent the best method to improve CRF in obese youth, their initial low fitness level is a barrier to their engagement in regular physical activity, contributing to the poor compliance usually observed in physical activity interventions ¹³. An important clinical challenge to track the changes in physical fitness with these interventions is to properly assess CRF in obese youth by using validated and accurate tests.

How to measure CRF in pediatric obesity?

Maximal oxygen uptake (VO_2max): the “Gold Standard”

Laboratory tests to assess cardiorespiratory fitness either measure or predict oxygen uptake (VO_2max) and are accepted as reference methods ^14-17. Basically, VO_2max is assessed during a cycling or running graded exhaustive test with increasing workload (in Watt) when performed on a cycle ergometer or speed and/or slope when performed on a treadmill. Whether it is on a cycle ergometer or a treadmill, the stage duration at each workload or speed ranges between 1 to 3 minutes of duration. In children and adolescents, the criteria for achievement of VO_2max are subjective exhaustion, heart rate above 195 beats.min^-1 and/or Respiratory Exchange Ratio (RER, VCO₂/VO₂) above 1.02 and/or a plateau of VO₂ ¹⁸. Although this method is widely used the completion of maximal test requires strong encouragement from the investigation or medical team and remains difficult to perform in obese subjects. Obese children and adolescents specifically have been shown to express a significantly higher rate of perceived exertion during incremental test compared to their normal-weight peers ¹⁹, with pain and fatigue considered as the main causes. Children who rarely engage in physical activity of high intensity, often fail to reach the required VO_2max criteria during a maximal CRF test, and if the aforementioned criteria are not met, the maximal oxygen uptake measured is termed VO_2peak rather than VO_2max ²⁰. VO_2peak represents the highest value of oxygen consumed by participants in a maximal protocol but with less stringent criteria than VO_2max. As an example, Breithaupt et al. reported that only 18 out of 62 obese children who performed a maximal CRF test were able to achieve VO_2max based on the criteria presented above ²¹.

In addition to the measurement of VO_2max, two ventilatory thresholds (VT1 and VT2) can be determined during incremental test and are each characterized by a disproportionate increase in minute ventilation (VE) relative to the increase in VO₂. Also VT1 and VT2 are considered as good physiological indicators of cardiorespiratory endurance, they are difficult to determine accurately in obese children and adolescents due to frequently erratic breathing. Some authors indicate that the thresholds are almost undetectable in up to 20% of children and adolescents ^{22, 23}. Despite these limitations, VT determination can be used for exercise prescription. For example, training below VT1 will represent a moderate intensity of exercise that will favor fat oxidation ²⁴, training alternating moderate and high intensities (between VT1 and VT2) has been shown to reduce cardiovascular risk factors ²⁵ and exercising at the VT2 can reduce post-exercise energy consumption ²⁶.

Maximal laboratory tests with gas exchange measurement may represent the most accurate method to asses CRF, but these tests are often expensive and not accessible to all obese youth. Submaximal tests have been therefore been developed ²⁷ for and validated in the general pediatric population, and are now applied to obese youth.

Submaximal tests: from the laboratory to the field setting

When using validated tests, submaximal exercise testing offers a valuable and reliable alternative to estimate VO_2max. Submaximal measures do not require the participants to exercise until exhaustion and may thus overcome some of the limitations of maximal testing and are better tolerated by patients experiencing physical limitation, important fatigue and pain while exercising ¹⁷. Basically, extrapolations of VO_2max or maximal power output are performed from the theoretical maximal heart rate (HR), and the linear relationship between power output (or VO₂) and heart rate measured during at least two bouts of exercise performed at two different submaximal intensities of exercise ²⁸. Estimation of CRF can also be done using other predictive variables such as HR recovery during step tests ^{29, 30} or validated predictive equations with parameters such as age, gender, body weight or rest heart rate among others ^31-33.

Recently, Breithaupt et al. proposed a new submaximal protocol adapted to obese youth (The HALO protocol: Healthy Active Living and Obesity research group Protocol) that they compared with a direct progressive maximal test to exhaustion in 21 obese adolescents ³⁴. The test consists in a walking test at constant speed (brisk but confortable) during 4-min stages to ensure that steady state of VO₂ and HR is reached. After a 4-min warm-up, the incline of the treadmill is increased by 3% over each subsequent stage. The test ends when: i) the participant reaches 85% of his maximal estimated HR; ii) he completes 20 minutes of exercise; iii) he indicates that he can no longer continue. Then VO_2peak is predicted by extrapolating the HR-VO₂ linear relationship to age-predicted HRmax. While only 29% of their sample reaches a VO₂ plateau during the maximal test, all the participants ended the HALO protocol and expressed lower RPE. According to their results, the HALO submaximal protocol provides an accurate and valid method to estimate VO_2peak compared with a classical maximal test. Furthermore, this test results in better estimates of VO2_peak compared with previously validated submaximal estimations in obese youth ³⁴.

A shorter submaximal protocol has also been validated in obese adolescents against a laboratory-bases maximal VO_2max measure. Nemeth et al. asked 113 obese 12 years old boys and girls to complete a 4-minute treadmill exercise ³³. After a 4-minute warm-up at a self-selected confortable walking speed (treadmill grade = 0%); the participants were asked to maintain this speed for 4 minutes while the treadmill grade increased to 5%. Heart rate was recorded at rest and at the end of the 4 minutes as well as the self-selected speed. Based on these two variables, the authors proposed an equation that also included sex, weight (kg) and height (cm) to estimate VO_2max. These simple methods requiring only HR measurement accurately predict VO_2max in overweight and obese children and adolescents ³³ and offers then practitioners feasible and simple methods to assess cardiorespiratory fitness.

Several inexpensive, easy to implement and reproducible field-methods initially validated in non-obese youth are commonly used obese youth ^{17, 35-37}. The two main field tests used among pediatric obese populations are the Six-Minute Walk Test (6MWT) and the 20-Meter Shuttle Run Test (20MST).

The 6-minute walk test is an accurate and convenient method to assess CRF at submaximal intensity in children ³⁸ and has been shown to better reflect daily living activities than any other functional walk test ³⁹. The recently established reference values have facilitated the use of the 6MWT and allow determining whether a child has a good or a poor CRF ^40-44. Not surprisingly, several studies have shown lower distances completed during the 6MWT in obese compared to lean children and adolescents ^{45, 46}. Elloumi et al. confirmed the validity of the 6MWT in obese adolescents by comparison with a validated incremental submaximal protocol with gas exchange measurements ⁴⁷. The 6MWT has also been shown to be sensitive to changes in physical fitness of obese adolescents following a 2-month physical activity ^{47, 48}. The 6MWT has also been used to estimate the maximal fat oxidation point (Fatmax or lipoxmax), when gas exchange measurement – with the VO₂ and VCO₂ data used to calculate the rate of fat oxidation – is not available (using the distance performed during the test as a central values in a predictive equation) ⁴⁹.

The 20-meter shuttle run test (20MST) developed by Leger et al. is among the most used field tests to assess cardiorespiratory fitness in youth ⁵⁰. During this test, children are instructed to run for as long as possible between two lines drawn 20 meters apart at an increasing speed imposed by a recording emitting tones at appropriate intervals. The test starts at 8 km/h and increases by 0.5 km/h every minute. The test ends when the participant is not able anymore to complete a whole stage. Castro-Pineiro et al. showed that overweight and obese children performed less well than lean ones at this test ⁵¹, and this is partly explained by the excessive start speed during the original test (8 km/h). This led to the development of an adapted version of the test, with the use of an incremental shuttle walk test with 15 levels from 1.8 to 10.3 km/h over a 10-meter distance ³⁶. More recently, an adapted version of the 20MST has been developed in obese children and adolescents ⁵². 10 stages have been added at the beginning of the original 20MSRT in order to reduce the starting speed and speed over the duration of the test ⁵². Then, the participants are asked to start at 4 km/h (walk speed) with an increment of 0.5 km/h every minute. The original start speed of 8 km/h is reach after the first 10 minutes. The authors reported a strong correlation between the maximum speed obtained and laboratory-assessed peak VO₂ (r=0.81), which indicates the validity of this adapted version of the test in obese youth ⁵².

Conclusion and recommendations

Cardiorespiratory fitness is impaired in obese children and adolescents, and is among the main reasons for their low engagement into physical activities. There is a double interest in properly assessing CRF in this population. First, CRF is an important clinical parameter for the diagnosis and follow-up of the current and future functional and metabolic health of obese youth. A laboratory-based direct and maximal assessment should thus be encouraged.

Second, from a more practical point of view, CRF is necessary information to obtain when implementing interventions for the treatment of obesity, especially when it is based on physical activity-based programs. Disposing of CRF indicators will help the practitioners and/or educators to properly prescribe physical activities by determining the appropriate exercise intensities and controlling their progression through the intervention. When direct VO_2max measurement is not available, submaximal and field testing are reliable alternatives. Thanks to their easy feasibility, these tests can be repeated many times during the program to and bring adaptation to the exercise prescription if necessary.

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