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September 2010

Impaired Cardiac Function Among Obese Adolescents: Effect of Aerobic Interval Training

Author Affiliations

Author Affiliations: Department of Circulation and Medical Imaging (Drs Ingul, Tjonna, Stolen, Stoylen, and Wisloff) and Centre for Sports and Physical Activity Research (Dr Wisloff), Norwegian University of Science and Technology, and Department of Cardiology, St Olav University Hospital (Dr Stoylen), Trondheim, Norway.

Arch Pediatr Adolesc Med. 2010;164(9):852-859. doi:10.1001/archpediatrics.2010.158

Objectives  To measure cardiac function before and after 3 months of aerobic interval training in obese adolescents and to compare the findings with those in lean counterparts.

Design  Exercise intervention study. Cardiac function was assessed by echocardiography and maximal oxygen uptake by ergospirometry.

Setting  The obese adolescents were referred from general practice to the St Olav University Hospital, Trondheim, Norway, and the control group was recruited from 2 schools.

Participants  Ten obese adolescents (mean [SD] age, 14.8 [1.2] years; mean [SD] body mass index {BMI; calculated as weight in kilograms divided by height in meters squared}, 33.5 [4.3]) and 10 lean counterparts (mean [SD] age, 14.9 [1.3] years; mean [SD] BMI, 20.4 [3.0]) participated.

Intervention  Aerobic interval training (4 × 4 minutes at 90% of maximal heart rate, 40 minutes of training in total) was performed twice per week for 13 weeks among the obese adolescents, whereas the lean counterparts only performed the tests.

Main Outcome Measures  Left ventricular end-diastolic volume, stroke volume, and maximal oxygen uptake.

Results  Maximal oxygen uptake was 41.4% lower among the obese adolescents compared with the lean counterparts, but the maximal oxygen uptake increased by 8.6% (P = .008) after intervention. Obese adolescents initially had 7.8% and 14.5% lower left ventricular end-diastolic and stroke volumes, 21.3% reduced global strain rate and 16.3% global strain, reduced mitral annulus excursion and systolic/diastolic tissue velocity, longer isovolumic relaxation time, and longer deceleration time compared with the lean counterparts. No group difference was observed after the intervention. Aerobic interval training increased the ejection fraction but was lower compared with the lean counterparts. Aerobic interval training reduced fat content by 2.0% (P = .005) among the obese adolescents.

Conclusions  Aerobic interval training almost restored an impaired systolic and diastolic cardiac function among obese adolescents when compared with lean counterparts. These results may have implications for future treatment programs for obese adolescents.

Trial Registration  clinicaltrials.gov Identifier: NCT00184236

Obesity in adolescence is a major threat to public health and is associated with reduced longevity.1,2 The prevalence of overweight and obesity in adolescents has nearly tripled in the past 2 decades in the United States,3 and overweight adolescents often become obese as adults.4 Parental obesity increases the risk of adult obesity by 2- to 3-fold among both obese and nonobese children younger than 10 years.5 Obesity in adolescence increases the risk for premature death and disease, independent of obesity during adulthood.6 The associations are stronger in boys than in girls and increase with the age of the child in both sexes.7

Obesity in adolescence is associated with an increased risk of type 2 diabetes mellitus, hypertension, stroke, certain cancers, disability, sleep apnea, metabolic syndrome, liver disease, and cardiovascular disease.8 Cardiovascular risk factors are elevated in overweight youth with body mass index (BMI; calculated as weight in kilograms divided by height in meters squared) in the 85th to 95th percentile, with further abnormalities in those with obesity (BMI >95th percentile), suggesting that even modest degrees of excess adiposity contribute to cardiovascular risk.9

New echocardiographic techniques such as tissue Doppler imaging can quantify myocardial function and have been shown to detect subclinical heart disease.10 Strain and strain rate quantify regional myocardial deformation.11 An Italian study showed a significant impairment of longitudinal myocardial deformation (strain and strain rate) in obese children involving both the left ventricle (LV) and the right ventricle compared with healthy, lean children.12 Other myocardial deformation imaging studies in obese adults have shown the same.13

Obese adolescents have a reduced maximal oxygen uptake (VO2max) compared with normal-weight controls.14 Physical activity protects against disease independent of body shape and size, and an increase of physical fitness in children and adolescents could reduce the risk of obesity-related comorbidities.15 Although promotion of physical activity is recommended in obesity treatment and preventive interventions in adolescence, limited data are available for the quantity and quality of physical activity to prevent or treat obesity.16,17 Aerobic interval training is an effective type of exercise for improved cardiac function in healthy individuals18 and in individuals with postinfarction heart failure,19 but to our knowledge this has never been studied in obese adolescents.

The purposes of this study were to evaluate the effect of aerobic interval training on LV performance among obese adolescents and to compare the findings with those in lean counterparts.

Study population

We investigated 10 obese adolescents (mean [SD] age, 14.8 [1.2] years; age range, 13-16 years; 4 girls) and compared them with 10 lean, healthy subjects of comparable age and sex (mean [SD] age, 14.9 [1.3] years; age range, 13-16 years; 4 girls). The obese adolescents were referred to St Olav University Hospital, Trondheim, Norway, from general practice for management of overweight and obesity, and the inclusion criterion was BMI for age greater than +2 SDs.20 The control group was recruited from 2 randomly chosen schools. All adolescents who underwent the test for study purposes gave written informed consent together with their parents, and the protocol was approved by the Regional Committees for Research Ethics in Norway. The clinical details of the study population are given in Table 1.

Table 1. 
Baseline Characteristics Among Obese and Lean Adolescents
Baseline Characteristics Among Obese and Lean Adolescents
Anthropometric measurements and blood pressure

The BMI was calculated as weight in kilograms divided by height in meters squared. Dual-energy x-ray absorptiometry (Discovery A; Hologic, Inc, Bedford, Massachusetts) was used to measure total body composition and fat content. Waist circumference was measured in expiration, midway between the lower lateral costal margin and the iliac crest, with the subject standing. Blood pressure was measured 3 times after 5 minutes of rest in a sitting position and the last 2 measurements were averaged.

Testing of maximal oxygen uptake and maximal heart rate

Before measurements of maximal oxygen uptake (VO2max) were taken, the subjects were informed about the test and instructed to exercise to their maximal limit. The test started on a flat treadmill and the speed and inclination were individually adjusted (3-6 km/h and 0%-5%, respectively) for a 10-minute warm-up. After the warm-up period, a mask was placed on the subject's face for metabolic measurements using MetaMax II (Cortex, Leipzig, Germany). The VO2max test was performed using a ramp protocol where the speed was constant and the incline increased 2% every second minute until the VO2max was reached. The test has been previously described in detail.21 Heart rate was continuously recorded using a Polar Sport Tester (Polar Electro OY, Kempele, Finland) to obtain the maximal attainable heart rate. The criteria for VO2max were a leveling off in oxygen uptake despite increased workload, plus a respiratory exchange ratio of 1.05 or higher, which was achieved in all individuals.

Aerobic interval training

Aerobic interval training was performed only among the obese adolescents as walking or running with elevation on a treadmill twice a week for 13 weeks. The exercise started with a warm-up period of 10 minutes at 70% of the maximal heart rate followed by an interval of 4 minutes at 90% to 95% of the maximal heart rate and then 3 minutes of active recovery at 70% of the maximal heart rate, for a total of 4 intervals. The training session ended with a 5-minute cool-down period, giving a total of 40 minutes of training. Every training session was supervised by an instructor. The criterion for a completed exercise program was 80% compliance with the training program.

Image Acquisition

Full resting echocardiography was performed with a Vivid 7 scanner (GE Vingmed Ultrasound, Horten, Norway) using a phased-array matrix transducer. Three cine loops from the 3 standard apical planes (4-chamber, 2-chamber, and long-axis planes) were recorded in tissue Doppler mode with simultaneous grayscale harmonic images. The pulse repetition frequency was between 1.0 and 1.5 kHz. At rest, the grayscale mean frame rate was 36.1/s (range, 31-51/s) and the mean tissue Doppler frame rate was 131.2/s (range, 100-160/s). At peak exercise echo grayscale, the mean frame rate was 27.8/second (range, 25-39/s) and the mean tissue Doppler frame rate was 110.4/s (range, 100-154/s). The LV standard Doppler echocardiographic indices were measured, and body surface area (in meters squared) by the Dubois formula was used to normalize cardiac dimensions for differences in body size. Ejection fraction and LV volumes were determined using a biplane (apical 4- and 2-chamber) modified Simpsons method. Stroke volume was determined by pulsed-wave Doppler imaging of the aortic outlet in the 4-chamber view. Peak early (E) and late (A) mitral inflow velocities and deceleration time of early filling velocity were measured using pulsed-wave Doppler imaging at the leaflet tips. Isovolumic relaxation time (IVRT) was also measured by pulsed-wave velocity. Peak mitral annular systolic (S′) and diastolic early (e′) and late (a′) tissue velocities were obtained in the pulsed-wave Doppler mode at rest and by color tissue Doppler imaging at exercise. The ratio E/e′ was calculated for a measurement of LV filling pressure. Mitral annulus excursion by color tissue Doppler and pulsed-wave tissue Doppler velocities were measured at the atrioventricular plane in 4- and 2-chamber views and a mean of the 4 points was used.

Analysis of Myocardial Deformation

For automated identification of myocardial segments, we used a customized postprocessing system (GcMat; GE Vingmed Ultrasound) that runs under Matlab (MathWorks, Inc, Natick, Massachusetts).22 In each apical view, the apex and mitral ring points were identified and the endocardial border was drawn automatically. The myocardium was divided into 6 equal segments, subject to manual adjustment. Tracking was done axially (along the ultrasound beam) by tissue Doppler data and laterally by speckle tracking.23 The timing of aortic valve opening and closure was automatically defined using tissue Doppler imaging.24 Strain rate was calculated from the velocity gradient along the ultrasound beam, and strain was calculated as the temporal integral of strain rate, corrected from Eulerian strain rate to Lagrangian strain. The strain length was 10 to 15 mm.

Tissue Doppler imaging measurements included peak systolic strain rate, determined as the maximal negative strain rate value during ejection time, and end-systolic strain, determined as the magnitude of strain at aortic valve closure. Measurements were made in 16 segments from 3 apical views for all 20 subjects. The apical septal segments and the apical lateral segments of the 4-chamber and long-axis views were averaged to achieve 16 segments. Global strain rate and global strain were calculated as the mean value of all 16 segments or the number of segments available per individual.

Intraobserver and interobserver variabilities have been tested in earlier studies.22,25,26

In all, 1620 myocardial segments were analyzed. Strain rate could be analyzed in 88% of the segments, and strain could be analyzed in 86% of the segments.

Exercise stress echocardiography

Exercise echocardiography was performed on a cycle ergometer to submaximal level. After the resting images had been obtained, the individuals exercised on a cycle ergometer in an upright position starting at 50 W and increasing every 3 minutes to 75 W and 100 W. Recordings were done at baseline and at 100 W assessing apical 4-chamber, 2-chamber, and long-axis views in B-mode and tissue Doppler imaging. Electrocardiography was monitored during the test. All study subjects achieved the workload of 100 W.

Statistical analysis

Analyses were carried out using a standard statistical software program (SPSS version 15.0 statistical software; SPSS Inc, Chicago, Illinois). Measurements are presented as mean (standard deviation). Analysis of variance was used to compare continuous variables between groups, with the Scheffe method to correct for multiple comparisons. Pearson χ2 test was used for categorical data. Multiple regression analysis was used to establish which parameters were independent correlates of peak systolic strain rate. The method of Bland and Altman was used for variability analysis. A value of P < .05 was considered statistically significant.

Baseline characteristics

The clinical characteristics of the obese and lean adolescents are summarized in Table 1. Among the obese subjects, 6 had a BMI for age greater than +3 SDs (4 boys) and 4 had a BMI for age greater than +2 SDs (2 boys). The mean (SD) BMIs were 33.5 (4.3) for the obese adolescents and 20.4 (3.0) for their lean counterparts (P < .001). Aerobic interval training reduced waist circumference by 3.2% (P = .001) and fat content from 41.5% to 39.5% (P = .005), whereas BMI remained unchanged among the obese subjects (P = .45) (Table 1).

All of the obese adolescents completed the training, and the compliance was 85.0%.

At both pretest and posttest, the oxygen uptake leveled off despite an increased work load and the respiratory exchange ratio was greater than 1.10 for all subjects, indicating that the true VO2max was reached. In the obese subjects, aerobic interval training increased VO2max by 8.6% (P = .008), but the VO2max was still lower than that observed in lean counterparts (32.8 vs 51.5 mL/kg/min, respectively; P < .001). Systolic and diastolic blood pressures as well as resting heart rate were lower for the lean subjects (Table 1). There was no significant difference in maximal heart rate between the obese adolescents and the lean counterparts (mean [SD], 200.6 [10.0] vs 202.0 [6.2] beats/min, respectively; P = .71).

Lv volumes and systolic function

Compared with lean control subjects, obese adolescents had larger absolute LV end-diastolic volumes (EDVs) (114.4 mL vs 100.0 mL, respectively; P = .04) but smaller relative LVEDVs when normalized to body surface area (54.2 vs 58.8 mL/m2, respectively; P = .02). The LV end-systolic volume expressed absolutely (in milliliters) was 34.6% larger in obese adolescents but similar to that observed in lean counterparts when normalized to body surface area (Table 2). The LVEDV index, but not the LV end-systolic volume, was normalized after the exercise training period (Table 2 and Figure 1).

Figure 1.
Effect of exercise on diastole. Changes after exercise intervention for diastolic echocardiographic variables stroke volume index (SVI) (A), left ventricular end-diastolic volume index (LVEDVI) (B), and peak early diastolic tissue velocity (e′) (C), with SVI an end-diastolic volume between obese adolescents (n = 10) and lean counterparts, with supine values. Mean values for obese adolescents are shown before and after intervention and mean values are shown for the lean counterparts. Error bars indicate standard deviation.

Effect of exercise on diastole. Changes after exercise intervention for diastolic echocardiographic variables stroke volume index (SVI) (A), left ventricular end-diastolic volume index (LVEDVI) (B), and peak early diastolic tissue velocity (e′) (C), with SVI an end-diastolic volume between obese adolescents (n = 10) and lean counterparts, with supine values. Mean values for obese adolescents are shown before and after intervention and mean values are shown for the lean counterparts. Error bars indicate standard deviation.

Table 2. 
Systolic Echocardiographic Variables and Left Ventricular Volumes
Systolic Echocardiographic Variables and Left Ventricular Volumes

All indices of systolic function were impaired in obese adolescents compared with lean counterparts: 14.5% reduction in stroke volume (in milliliters per meters squared), 11.1% reduction in ejection fraction, 10.2% reduction in fractional shortening, 12.3% lower mitral annulus excursion, 12.0% lower peak systolic tissue velocity, 21.3% reduction in global strain rate, and 16.3% reduction in global strain.

After 13 weeks of aerobic interval training in obese adolescents, all markers of systolic function were improved to levels comparable to those of their lean counterparts (Table 2 and Figure 2).

Figure 2.
Effect of exercise on systole. Changes after exercise intervention for the systolic echocardiographic variables global strain rate (A), global strain (B), mitral annulus excursion (MAE) (C), and peak systolic tissue velocity (S′) (D) are shown between obese adolescents (n = 10) and lean counterparts, with supine values. Mean values for obese adolescents are shown before and after intervention and mean values are shown for the lean counterparts. The P values are between the lean counterparts and the obese adolescents after intervention. Error bars indicate standard deviation.

Effect of exercise on systole. Changes after exercise intervention for the systolic echocardiographic variables global strain rate (A), global strain (B), mitral annulus excursion (MAE) (C), and peak systolic tissue velocity (S′) (D) are shown between obese adolescents (n = 10) and lean counterparts, with supine values. Mean values for obese adolescents are shown before and after intervention and mean values are shown for the lean counterparts. The P values are between the lean counterparts and the obese adolescents after intervention. Error bars indicate standard deviation.

Diastolic function

The IVRT and deceleration time were significantly prolonged by 12.8% and 30.5%, respectively, and e′ was 12.9% lower in the obese adolescents compared with the lean counterparts at baseline (Table 3). The E/A value was equal among the groups (Table 3). After training, there were no differences between the groups in e′, IVRT, and deceleration time (Figure 1). The E/e′ value was higher at baseline among the obese adolescents but within normal limits, with no effect of the exercise intervention (P = .76) (Table 3).

Table 3. 
Diastolic Doppler and Tissue Doppler Imaging Variables
Diastolic Doppler and Tissue Doppler Imaging Variables
Exercise stress echocardiography

Table 4 summarizes the hemodynamic response to exercise stress echocardiography. The target workload of 100 W was achieved in all study subjects. All subjects had a fusion of the E and A waves at peak exercise and were therefore comparable at peak, but not with resting values with separate E and A waves. The impaired cardiac function among the obese subjects was even more pronounced during exercise, with a lower mitral annulus excursion of 24.1%, a maximal velocity of the LV outflow tract (VmaxLVOT) of 32.1%, a flow velocity time integral of the LV outflow tract (VTILVOT) of 16.4%, an S′ of 19.3%, a global strain rate of 31.8%, and a global strain of 21.8% compared with lean counterparts.

Table 4. 
Exercise Stress Echo With Tissue Doppler and Doppler Flow Velocities for Lean and Obese Adolescents
Exercise Stress Echo With Tissue Doppler and Doppler Flow Velocities for Lean and Obese Adolescents

Furthermore, e′ was reduced by 18.1% and E/e′ increased by 19.8%. At peak exercise, mitral annulus excursion, VTILVOT, S′, global strain, global strain rate, and e′ increased after intervention and there were no differences after 3 months between the obese and lean adolescents except for VmaxLVOT. After the intervention, the E/e′ value in obese adolescents decreased to the same level as that of the lean counterparts at peak exercise.


The main findings in this study were that obese adolescents had substantially reduced LV size and function both at rest and during exercise when compared with lean counterparts. Additionally, 26 exercise sessions of aerobic interval training improved maximal oxygen uptake and LV function with increased peak velocities and a more efficient ejection and relaxation.

Cardiac function

To our knowledge, this is the first study to assess the effect of aerobic interval training on LV function in obese adolescents using tissue Doppler imaging, strain, and strain rate. Our data of impaired myocardial function in obese adolescents are in line with data from Di Salvo et al12 that also show reduced myocardial function by strain rate imaging in 300 obese children. However, aerobic interval training seems to be able to reverse this.

The obese adolescents had a small LVEDV for their body size, probably because of a sedentary lifestyle, compensated for by a higher resting heart rate. However, the opposite cause and effect relationship could be that their resting heart rate was higher owing to their lifestyle, decreasing diastolic duration and thus reducing their LVEDV. The obese adolescents had a high fat content, and this excess adipose tissue generates an increase in oxygen consumption (both directly and indirectly) and subsequently requires an increase in cardiac output.27 The elevated cardiac output at baseline in the obese adolescents can be a consequence of either increased stroke volume or higher heart rate.28 In our study, both the resting heart rate and stroke volume were higher among the obese adolescents, but the stroke volume index was lower. Aerobic interval training normalized the LVEDV and stroke volume indexed to body surface area to similar volumes as in the lean counterparts. The exercise-induced increase in stroke volume was a consequence of increased LVEDV and increased ejection fraction. On the other hand, training decreases the basal heart rate and thus increases the EDV and stroke volume, with the larger diastolic filling inducing an increase in mitral annulus excursion.

Despite increased stroke volume, resting cardiac output did not increase because the resting heart rate decreased after interval training. Although the ejection fraction increased after intervention, there was still a significant difference compared with the lean counterparts. This indicates that ejection fraction is less sensitive than mitral annulus excursion, S′, global strain rate, and strain, which were all normalized after the exercise intervention. Submaximal exercise showed that the obese adolescents had not fully normalized the systolic reserve by an improved but significantly lower VmaxLVOT compared with the lean counterparts.

The obese adolescents also showed a reduced diastolic function, which is consistent with the study by Di Salvo et al12 showing prolonged IVRT and deceleration time, a higher E/e′, and a reduced e′ at baseline. After the intervention period, E/e′ was normalized not at rest but during submaximal exercise. Both deceleration time and IVRT decreased after intervention as a sign of a quicker relaxation. Systolic blood pressure decreased after intervention, resulting in a shorter IVRT. There was no significant difference in mitral early or late diastolic flow between the obese and lean adolescents. Diastolic function by tissue Doppler imaging has been shown to be a more sensitive marker of diastolic function in subclinical heart disease,10,29 and e′ was initially reduced but increased and was comparable to that of the lean adolescents after the intervention.

Aerobic capacity, resting heart rate, and body composition

The initial cardiorespiratory fitness level measured as VO2max of 30.2 mL/kg/min, being equivalent to nearly a 70-year-old person,30 was not fully normalized after intervention. However, VO2max is heavily influenced by body weight, and by expressing VO2max relative to body mass (in milliliters per kilogram per minute), one overestimates the effect of body mass differences between individuals or changes due to an intervention. In a study including 546 healthy teenagers (aged 13-19 years), investigators found no increase in VO2max with age.31 However, even after using appropriate scaling procedures to correctly adjust for differences in body mass (VO2max expressed as milliliters per kilograms to the 0.75th per minute) in the obese adolescents, the maximal oxygen uptake increased after a training period (Table 1). When treating adolescents for obesity, the intervention goal should not be weight reduction but weight stagnation. Muscles are heavier than fat and the initial effect of training is a decrease in fat content, not in weight, explaining why we observed no reduction in BMI. The increased waist circumference, as observed among the obese adolescents in our study, is associated with a greater risk of metabolic and cardiovascular disorders,32,33 but the waist circumference decreased after training. We have previously shown that overweight adolescents who increase physical activity can decrease fat content, reduce cardiovascular risk factors, and increase cardiorespiratory fitness and thereby decrease the risk of developing obesity-related comorbid conditions despite minimal weight loss.34 Other studies in adults have shown the same using exercise as a single treatment with a reduction of total body fat, abdominal fat, visceral fat, and insulin resistance.35,36 Aerobic interval training among the obese adolescents was well accepted with a high compliance. Initially, approximately a third of the group had an aversion toward the training, but that disappeared with increased fitness. We consider the aerobic interval training to be highly feasible among obese adolescents, and it can be performed on a treadmill or in other activities using dynamical work with large muscle groups.

Limitations and strengths

This is a small single-center study and needs to be tested in a multicenter study. This study was not an intent-to-treat study but rather an effect-of-treatment study. Owing to the small size of the study, linear regression could not be performed and correlations with blood pressure, heart rate, age, sex, BMI, etc could not be determined. The usual limitations to strain rate and strain apply, such as angle dependency and influence of noise, especially at exercise.

A strength of the study is the well-controlled and highly defined exercise intensity intervention regimen. The power of the physiological determinants is strong and results are significant with few subjects.


Impaired cardiac function in obese adolescents can be improved by 3 months of aerobic interval training, almost to the same level as lean counterparts. These results should be considered in future studies and treatment programs.

Correspondence: Charlotte Bjork Ingul, MD, PhD, Department of Circulation and Medical Imaging, Norwegian University of Science and Technology, Olav Kyrresgt 9, 7489 Trondheim, Norway (charlotte.b.ingul@ntnu.no).

Accepted for Publication: March 11, 2010.

Author Contributions:Study concept and design: Ingul, Tjonna, Stoylen, and Wisloff. Acquisition of data: Ingul, Tjonna, and Stolen. Analysis and interpretation of data: Ingul, Tjonna, Stoylen, and Wisloff. Drafting of the manuscript: Ingul. Critical revision of the manuscript for important intellectual content: Ingul, Tjonna, Stolen, Stoylen, and Wisloff. Statistical analysis: Tjonna. Administrative, technical, and material support: Ingul, Tjonna, Stolen, Stoylen, and Wisloff.

Financial Disclosure: None reported.

Funding/Support: This study was supported by grants from the Norwegian University of Science and Technology (Dr Ingul), the Norwegian Council on Cardiovascular Disease and Norwegian Research Council Funding for Outstanding Young Investigators (Dr Wisloff), and the Foundation for Cardiovascular Research at St Olav University Hospital (Drs Tjonna and Wisloff).

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