Effects of High-Intensity Intermittent Training on Vascular Function in Obese Preadolescent Boys

Abstract

Background:

High-intensity intermittent training (HIIT) may serve as an effective alternative to traditional endurance training, since HIIT has been shown to induce greater improvements in aerobic fitness and health-related markers in adult populations. Our objective was to determine whether HIIT and supramaximal high-intensity intermittent training (supra-HIIT) would improve vascular structure and function in obese preadolescent boys.

Methods:

Before the baseline testing, 48 obese preadolescent boys, aged 8–12 years, were randomly assigned into control (CON; n = 16), HIIT (8 × 2 minutes at 90% peak power output, n = 16), and supra-HIIT (8 × 20 seconds at 170% peak power output, n = 16) groups. Both exercise groups performed exercises on a cycle ergometer three times/week for 12 weeks.

Results:

After 12 weeks, both HIIT and supra-HIIT did not affect body mass, body fat percentage, and waist circumference. Peak oxygen consumption (VO₂peak) increased in both HIIT and supra-HIIT groups (p < 0.05). Both HIIT and supra-HIIT groups had higher resting metabolic rate than the control group (p < 0.05). A measure of arterial stiffness, brachial-ankle pulse wave velocity, and carotid intima-media thickness decreased after 12 weeks of HIIT and supra-HIIT program (all p < 0.05). Flow-mediated dilation, a measure of endothelium-dependent vasodilation, increased in both HIIT and supra-HIIT groups (all p < 0.05).

Conclusions:

It is concluded that both HIIT and supra-HIIT have favorable effects on aerobic capacity, metabolic rate, vascular function and structure, and blood lipid profile in obese preadolescent boys. HIIT may be a time efficient and effective exercise for preventing future cardiovascular disease in obese children.

Introduction

The prevalence of overweight and obesity has increased in recent decades and remains high among children and adolescents.¹ Childhood levels of cardiovascular risk factors predict early atherosclerosis and cardiac pathology,² and children with a greater number of cardiovascular disease risk factors develop atherosclerosis in adulthood at an accelerated rate.³

Two of the earliest and detectable manifestation of vascular changes that leads to cardiovascular disease are arterial stiffness and endothelial function that can be assessed noninvasively in children.⁴ Many of the childhood cardiovascular risk factors and lifestyles are associated with increased carotid intima-media thickness (IMT), decreased carotid elasticity, and impaired brachial endothelial function in adulthood.^5,6 Thus, early treatment of obesity and vascular dysfunction in obese children is needed to prevent future development of cardiovascular disease.

Regular aerobic exercise (endurance exercise training) is recommended for the prevention and treatment of obesity and vascular dysfunction associated with childhood obesity.⁷ However, low-to-moderate intensity endurance training may not be effective in improving vascular function.^8,9 To achieve substantive health benefits in school-aged children and youth, physical activity should be of at least a moderate intensity, but vigorous intensity activities may provide even greater benefit.¹⁰ In addition, dropout from traditional exercise programs is substantial in children, particularly in obese children.¹¹

In this context, high-intensity intermittent training (HIIT) may serve as an effective alternative to traditional endurance training, since HIIT, involving shuttle runs and treadmill sprinting, has been shown to induce greater improvements in aerobic fitness and a range of physiological and health-related markers in adult populations.¹² A recent meta-analysis provides preliminary evidence of improvements in cardiorespiratory fitness, BMI, and body fat percentage even for adolescents following HIIT interventions.¹³ In one of the limited number of studies conducted in obese children, 12 weeks of HIIT performed at 100%–110% of maximal aerobic speed resulted in significant improvements in blood lipid profile and adiponectin.¹⁴

In recent years, exercise training performed at even higher intensity, supramaximal high-intensity intermittent training (supra-HIIT) and high-intensity “all-out” exercises alternated with brief resting period, has been reported as a feasible form of exercise prescription in obese adults,¹⁵ as well as in lean adolescent boys.¹⁶ We recently assessed the feasibility of supra-HIIT by examining the acute effect of supra-HIIT and demonstrated that supra-HIIT was well tolerated and feasible even in obese preadolescent boys.¹⁷ More importantly, vascular function improved acutely (transiently) and significantly following supra-HIIT performed at the highest exercise intensity of 170% of peak power output.¹⁷

To the best of our knowledge, no studies have determined the effects of supra-HIIT intervention on adiposity and vascular structure and function in obese children despite the potential benefits and added advantages that this exercise modality brings. Accordingly, the primary aim of the present study was to determine the chronic effects of HIIT and supra-HIIT on arterial stiffness and endothelial function in obese preadolescent children. In addition to vascular function, the impacts on adiposity, basal metabolic rate, physical fitness, and traditional risk factors for cardiovascular disease were addressed as the secondary aim. We hypothesized that 12 weeks of HIIT would induce favorable effects on vascular function and other health-related parameters in obese preadolescent children.

Methods

Subjects

A total of 48 obese preadolescent boys, aged 8–12 years, were recruited from Yaowaluck Wittaya Thonburi School in Thailand. The inclusion criteria included BMI ≥2 standard deviation above the growth reference data for boys,¹⁸ no cardiovascular disease or asthma, no orthopedic problems that prohibit them from exercising, and no regular exercise program or sport activities within the last 3 months. Medical history and physical activity readiness questionnaire for children were obtained. All participants and their parents gave their written informed consents before participation in the study. The study was approved by the local ethics review committee.

Study Design and Protocol

The eligible subjects were randomly allocated into one of the three groups as follows: Sedentary control (n = 16), HIIT (n = 16), and supra-HIIT (n = 16). Randomization was conducted using a randomization table generated by a computer software. The subjects and the parents were asked to maintain their regular dietary habit throughout the study period. To confirm this, dietary intake was assessed with a self-reported 7-day food record filled in by the parents. To demonstrate that post-exercise physical activity was similar between groups, daily physical activity was measured for 7-days using unsealed pedometers (HJ-113; Omron Healthcare, Kyoto, Japan),¹⁹ which have been shown to be valid and reliable.²⁰ The sedentary control group was advised to maintain their usual activities of daily living during the study.

The training protocol consisted of cycling on mechanically-braked cycle ergometer (Ergomedic 894E Peak Bike; Monark, Stockholm, Sweden), thrice a week on alternate days for 12 weeks. The HIIT group performed eight sets of 2-minute exercises at 90% of peak power output, interspersed by a 1-minute rest. The supra-HIIT group performed eight sets of 20-second high-intensity exercises at 170% of peak power output, followed by 10 seconds of resting periods. The intensity of supra-HIIT exercise was calculated by peak power output (watt) multiplied by percentage of target intensity. Peak power output was determined by maximal work load at peak oxygen consumption (VO₂peak) during the incremental exercise test. The same absolute work rate obtained at the baseline testing was used throughout the exercise intervention periods. Heart rate (HR) was continuously monitored using HR monitors (FT40; Polar) during exercise. These exercise sessions conducted in afterschool settings were supervised by both investigators and school teachers.

Before and after the interventions, on the first visit, the subjects reported to the laboratory in the morning after an overnight fast for 8 hours. A blood sample was collected from a forearm vein. After having breakfast for 2 hours and resting for 15 minutes, medical history and physical activity readiness questionnaire for children, general physiological characteristics (body composition, resting HR, and blood pressure), and VO₂peak were assessed. On the second visit, brachial-ankle pulse wave velocity (baPWV), carotid artery IMT, brachial artery flow-mediated dilation (FMD), and resting metabolic rate (RMR) were measured. Enjoyment of physical activity was assessed in the HIIT and supra-HIIT groups at the posttest.

Measurements

General physiological characteristics

Body composition was determined by bioelectrical impedance method using a standardized body composition analyzer (bioelectrical impedance analyzer; Jawon Medical). Waist–hip ratio was calculated from waist and hip circumference measurements using a nonelastic tape measure. HR and blood pressure were recorded after >5 minutes of rest with the subject in seated position using a semiautomated blood pressure device (CARESCAPE V100 monitor; GE Healthcare, Milwaukee, WI).

VO₂peak was determined during graded exercise test using a mechanically-braked cycle ergometer (Ergomedic 894E Peak Bike; Monark).¹⁷ Briefly, after a 3-minute warm up, the subjects started with a workload of 20 watts. The work rate was increased by 20 watts every 3 minute until volitional exhaustion.²¹ In order for the VO₂peak test to be valid, at least two of the following criteria were fulfilled: a plateau in VO₂ increase, maximal Respiratory Exchange Ratio (RER) of >1.0, maximal Rate of Perceived Exertion (RPE) of >17, or attainment of age-predicted maximal HR. Pulmonary ventilation and gas exchange were measured using breath-by-breath cardiopulmonary gas exchange system (Stationary Gas Analyzer: Vmax Encore 29 system; Yorba Linda). HR and electrocardiography (ECG) were continuously recorded. VO₂peak was expressed relative to body weight, and different expressions of VO₂peak (absolute unit, relative to lean body mass, or allometric scaling) had no impact on the main results. Leg muscle strength was measured using a muscle dynamometer (TKK 5002; Takei Scientific).

Resting metabolic rate

RMR was measured using the stationary gas analyzer (Vmax Encore 29 system; CareFusion, San Diego, CA) equipped with an infrared CO₂ sensor, an electrochemical O₂ sensor, and a variable mass flow. After a resting period (>10 minutes), the canopy was positioned over the participant's head, and indirect calorimetry measurement was started with an initial 10-minute period to accustom the participant to the device and for equilibration. Subsequently, a 20-minute recording period followed while the subject remained under strict resting conditions.

Physical activity

Spontaneous physical activity was determined by sealed pedometers (Omron HJ-113; Omron Healthcare, Lake Forest, IL) for 7 consecutive days. The teachers and parents were asked to remind participants to wear the pedometer during all activity hours. Nonwearing periods (e.g., during all water activities and sleeping) were recorded.

Enjoyment score

Enjoyment of physical activity was assessed using the modified physical activity enjoyment scale (PACES).²² The rating scale was a 5-point Likert type scale that ranged from 1 to 5.

Arterial stiffness

The baPWV, an index of arterial stiffness, was measured using a noninvasive vascular testing device (VP-1000plus; Omron Healthcare, Kyoto, Japan).¹⁷ Electrodes for electrocardiogram were placed on the wrists, the phonocardiographic microphone was attached on the sternum, and the limb cuffs that combined with a plethysmographic sensor and an oscillometric pressure sensor were placed on all extremities. Electrocardiogram, phonocardiograms, bilateral brachial, and ankle blood pressures were measured simultaneously. PWV was calculated as the distance between two arterial recording sites divided by transit time using a foot-to-foot method.²³

Arterial wall thickness

Carotid artery IMT was measured from images derived from an ultrasound machine (CompactXtreme CX50 with QLAB's IMT measurement plug in; Philips Healthcare, Andover, MA) equipped with a high-resolution linear-array transducer. The longitudinal ultrasound images were obtained at the proximal 1- to 2-cm straight portion of the common carotid artery. Ultrasound images were digitally recorded and analyzed by use of automated computerized software (Carotid Analyzer; Medical Imaging Applications, Coralville, IA). The IMT was the distance between the leading edge of the first bright line (lumen–intima interface) and the second line (media–adventitia interface) of far wall.

Endothelium-dependent vasodilation

Brachial artery FMD, an index of endothelium-dependent vasodilation, was measured using the ultrasound machine (CompactXtreme CX50; Philips Healthcare) equipped with a high-frequency linear-array ultrasound probe (4–12 MHz) as previously described.²⁴ Briefly, the brachial artery was imaged above the antecubital fossa. After the baseline data were obtained, the cuff placed around the forearm was inflated rapidly to 50 mmHg above systolic blood pressure for 5 minutes and deflated for 5 minutes of recovery.¹⁷ Ultrasound images were acquired and transferred to digital viewing software (Brachial Analyzer; Medical Imaging Applications), where pulsatile changes in brachial artery diameter were analyzed offline. FMD was calculated using the equation: (maximum diameter − baseline diameter)/baseline diameter × 100.

Blood chemistry analyses

A venous blood sample was collected from the antecubital vein after an 8-hour fasting period. Serum concentrations of lipid profiles (total cholesterol, triglyceride, high-density lipoprotein cholesterol and low-density lipoprotein (LDL) cholesterol), and creatine kinase were measured with the standard procedure of the certified clinical laboratory (Faculty of Allied Health Sciences Lab, Thammasat University, Prathumthani, Thailand). Leptin and adiponectin were measured in plasma samples with the commercial assay kit (Leptin/Adiponectin Human ELISA Kit; PromoKine, Heidelberg, Germany). Malondialdehyde (MDA) concentration was measured by thiobarbituric acid reactive (TBAR) substances method. MDA was utilized as a marker of oxidative stress.²⁵

Statistical Analyses

The data are expressed as mean ± standard error of the mean. The normality and homogeneity of each variable were confirmed before the analyses. The dependent variables were compared using a 3 × 2 (groups × times) repeated-measures analysis of variance or analysis of covariance (ANCOVA). A post-hoc test with the Fisher least significant difference (LSD) was used for multiple comparison purposes. Differences in dropout rates from the exercise program were evaluated using nonparametric chi-square test. Associations of interest were examined using Pearson correlation coefficients. ANCOVA was used to assess changes in baPWV and FMD after adjusting for mean blood pressure and baseline brachial artery diameter as covariate, respectively. Statistical significance was inferred at a two-tailed probability value of 0.05.

Results

A total of 11 subjects (5 in control, 5 in HIIT, and 1 in supra-HIIT) dropped out in the course of the study because of their personal and/or family reasons (e.g., a lack of time). Supra-HIIT had a lower dropout rate than HIIT (6.3% vs. 31.3%, p < 0.001). The general characteristics of the subjects are shown in Table 1. Before the intervention, there were no significant group differences in most variables. Body mass, body fat percentage, and waist circumference did not change in any of the groups. After 12 weeks, VO₂peak, leg muscle strength, and RMR increased in both HIIT and supra-HIIT groups (all p < 0.05). Systolic blood pressure decreased only in the supra-HIIT group (p < 0.05). There were no significant changes in spontaneous physical activity in any of the groups. The supra-HIIT group had a higher enjoyment (PACES) score than the HIIT group (p < 0.05).

Table 1.

General Physiological Characteristics of Obese Prepubescent Boys Who Participated in the Control, High-Intensity Intermittent Training, and Supramaximal High-Intensity Intermittent Training Groups

	Control (n = 11)		HIIT (n = 11)		Supra-HIIT (n = 15)		ANOVA statistics
Parameter	Pre	Post	Pre	Post	Pre	Post	Time	Group	Time × Group	Effect size
Age (year)	10.6 ± 0.3 (10.1, 11.1)	10.6 ± 0.3 (10.1, 11.1)	11.0 ± 0.3 (10.5, 11.5)	11.0 ± 0.3 (10.5, 11.5)	11.1 ± 0.2 (10.6, 11.5)	11.1 ± 0.2 (10.6, 11.5)	1.000	0.406	1.000	0.05
Height (cm)	144 ± 3 (138, 150)	146 ± 3 (140, 152)	155 ± 3 (149, 163)	157 ± 3 (151, 163)	155 ± 3 (150, 160)	157 ± 2 (152, 162)	0.000	0.010	0.927	0.01
Body weight (kg)	53.6 ± 4.0 (45.5, 61.6)	54.3 ± 3.9 (46.3, 62.3)	58.6 ± 4.0 (50.5, 66.6)	59.7 ± 3.9 (51.7, 67.7)	64.4 ± 3.4 (57.5, 71.3)	64.8 ± 3.4 (58.0, 71.7)	0.031	0.132	0.677	0.02
BMI (kg/m²)	26.1 ± 1.0 (24.0, 28.2)	25.1 ± 1.1 (23.0, 27.3)	24.2 ± 1.0 (22.1, 26.3)	24.1 ± 1.1 (21.9, 26.2)	26.5 ± 0.9 (24.7, 28.3)	26.1 ± 0.9 (24.3, 27.9)	0.051	0.277	0.469	0.04
Body fat (%)	25.3 ± 1.6 (22.2, 28.5)	24.4 ± 1.6 (21.2, 27.7)	22.9 ± 1.6 (19.7, 26.0)	21.5 ± 1.6 (18.3, 24.8)	26.2 ± 1.3 (23.5, 29.0)	24.8 ± 1.4 (22.1, 27.6)	0.001	0.253	0.803	0.01
Muscle mass (kg)	36.6 ± 2.1 (32.4, 40.9)	37.8 ± 2.1 (33.5, 42.1)	41.5 ± 2.1 (37.2, 45.7)	43.1 ± 2.1 (38.8, 47.4)	43.2 ± 1.8 (39.5, 46.8)	44.5 ± 1.8 (40.8, 48.2)	0.000	0.064	0.631	0.03
Waist (cm)	83.6 ± 2.8 (78.0, 89.2)	84.7 ± 2.7 (79.1, 90.2)	86.1 ± 2.8 (80.5, 91.7)	84.2 ± 2.7 (78.7, 89.8)	90.5 ± 2.4 (85.7, 95.3)	88.6 ± 2.3 (83.9, 93.4)	0.109	0.271	0.059	0.15
Waist/Hip	0.9 ± 0.0 (0.9, 1.0)	0.9 ± 0.0 (0.9, 1.0)	0.9 ± 0.0 (0.9, 1.0)	0.9 ± 0.0 (0.9, 1.0)	1.0 ± 0.0 (0.9, 1.0)	1.0 ± 0.0 (0.9, 1.0)	0.039	0.359	0.398	0.04
Heart rate (bpm)	88.9 ± 4.0 (80.7, 97.1)	88.9 ± 3.4 (82.0, 95.6)	85.5 ± 4.0 (77.3, 93.8)	76.4 ± 3.4 (69.4, 83.3)	88.9 ± 3.5 (81.9, 96.0)	81.6 ± 2.9 (77.3, 93.8)	0.003	0.267	0.103	0.13
Systolic BP (mmHg)	121 ± 4 (112, 129)	121 ± 4 (114, 129)	128 ± 4 (120, 137)	125 ± 4 (117, 132)	127 ± 4 (119, 134)	116 ± 3^* (110, 122)	0.004	0.489	0.007	0.25
Diastolic BP (mmHg)	73 ± 2 (68, 78)	74 ± 2 (70, 78)	77 ± 2 (72, 82)	77 ± 2 (73, 81)	76 ± 2 (72, 81)	71 ± 2 (68, 75)	0.357	0.390	0.223	0.08
Mean BP (mmHg)	87 ± 3 (82, 93)	86 ± 3 (80, 92)	91 ± 3 (86, 97)	92 ± 3 (86, 98)	89 ± 2 (84, 94)	81 ± 3 (76, 87)	0.036	0.163	0.057	0.15
Brachial artery diameter (mm)	3.1 ± 0.1 (2.9, 3.3)	3.1 ± 0.1 (2.9, 3.3)	3.2 ± 0.1 (3.0, 3.4)	3.2 ± 0.1 (3.0, 3.3)	3.2 ± 0.1 (3.0, 3.3)	3.2 ± 0.1 (3.0, 3.5)	0.699	0.810	0.825	0.01
VO₂peak (mL/kg/min)	25.6 ± 1.3 (23.1, 28.2)	26.4 ± 1.2 (23.9, 28.9)	28.8 ± 1.4 (25.9, 31.6)	35.5 ± 1.4^,^* (32.7, 38.3)	26.1 ± 1.1 (23.9, 28.3)	30.8 ± 1.0^,^* (28.7, 33.0)	0.000	0.003	0.006	0.28
RMR (kcal)	1544 ± 88 (1365, 1723)	1535 ± 94 (1345, 1725)	1689 ± 88 (1510, 1868)	1860 ± 94^,^* (1670, 2050)	1744 ± 75 (1591, 1897)	1882 ± 80^,^* (1510, 2045)	0.001	0.057	0.041	0.17
Muscle strength (kg)	53.5 ± 4.1 (45.1, 61.9)	55.3 ± 3.6 (47.9, 62.7)	51.7 ± 4.1 (43.3, 60.1)	60.4 ± 3.6^* (53.0, 67.8)	53.3 ± 3.7 (45.7, 61.0)	66.8 ± 4.1^,^* (60.1, 73.6)	0.000	0.492	0.015	0.25
PA (steps/day)	8805 ± 425 (7857, 9753)	8864 ± 691 (7325, 10,404)	8952 ± 531 (7769, 10,136)	8583 ± 626 (7187, 9978)	8297 ± 465 (7300, 9294)	8046 ± 599 (6762, 9331)	0.561	0.559	0.862	0.08
Enjoyment score (U)				3.62 ± 0.12		4.0 ± 0.1		0.021		0.38

p < 0.05 vs. Pre; ^**p < 0.05 vs. Control.

ANOVA, analysis of variance; BP, blood pressure; HIIT, high-intensity intermittent training; PA, physical activity; RMR, resting metabolic rate; supra-IIHT, supramaximal high-intensity intermittent training; VO₂peak, peak oxygen consumption.

As shown in Table 2, blood concentrations of total cholesterol, LDL-cholesterol, and triglyceride decreased in both HIIT and supra-HIIT groups (p < 0.05). There were no significant changes in any of the lipid profile variables in the sedentary control group. Blood nitric oxide (NO) level increased in both HIIT and supra-HIIT groups (p < 0.05). There were no significant changes in leptin, adiponectin, malondialdehyde, and creatine kinase in any of the groups.

Table 2.

Blood Chemistry of the Control, High-Intensity Intermittent Training, and Supramaximal High-Intensity Intermittent Training Groups

	Control (n = 11)		HIIT (n = 11)		Supra-HIIT (n = 15)		ANOVA statistics
Parameter	Pre	Post	Pre	Post	Pre	Post	Time	Group	Time × Group	Effect size
Total cholesterol (mg/dL)	186 ± 10 (166, 205)	172 ± 9^* (154, 191)	181 ± 9 (164, 199)	146 ± 8^,^* (129, 162)	166 ± 8 (150, 181)	137 ± 7^,^* (122, 151)	0.000	0.057	0.046	0.18
Triglyceride (mg/dL)	119 ± 10 (99, 140)	109 ± 10 (89, 128)	126 ± 11 (104, 148)	78 ± 10^,^* (57, 99)	122 ± 9 (103, 140)	86 ± 9^* (69, 104)	0.000	0.638	0.001	0.34
HDL cholesterol (mg/dL)	56 ± 3 (50, 61)	52 ± 3 (46, 58)	53 ± 3 (47, 58)	46 ± 3 (40, 52)	49 ± 2 (44, 54)	43 ± 3 (38, 48)	0.000	0.103	0.424	0.05
LDL cholesterol (mg/dL)	111 ± 7 (97, 125)	104 ± 6 (92, 116)	112 ± 6 (99, 125)	88 ± 6^* (77, 99)	105 ± 6 (94, 117)	81 ± 5^,^* (70, 91)	0.000	0.181	0.025	0.23
Nitric oxide (μmol/L)	4.8 ± 0.5 (3.8, 5.8)	5.7 ± 0.8 (4.0, 7.3)	4.8 ± 0.5 (3.8, 5.8)	7.5 ± 0.8^* (5.9, 9.2)	5.0 ± 0.4 (4.1, 5.8)	7.4 ± 0.7^* (6.0, 8.8)	0.000	0.450	0.046	0.17
Leptin (ng/mL)	7.9 ± 1.1 (5.6, 10.2)	6.6 ± 1.1 (4.2, 8.9)	5.3 ± 1.1 (3.0, 7.5)	4.2 ± 1.1 (2.0, 6.5)	6.8 ± 1.0 (4.8, 8.8)	6.2 ± 1.0 (4.3, 8.2)	0.009	0.240	0.692	0.02
Adiponectin (μg/mL)	89 ± 19 (51, 128)	100 ± 16 (67, 133)	103 ± 18 (67, 140)	101 ± 15 (70, 132)	101 ± 15 (69, 132)	118.4 ± 13.2 (91.5, 145.2)	0.226	0.777	0.478	0.04
MDA (μmol/L)	0.33 ± 0.03 (0.27, 0.38)	0.37 ± 0.02 (0.32, 0.41)	0.29 ± 0.03 (0.24, 0.34)	0.29 ± 0.02 (0.25, 0.34)	0.32 ± 0.02 (0.28, 0.36)	0.31 ± 0.02 (0.28, 0.35)	0.466	0.130	0.435	0.05
CK (U/L)	163 ± 16 (129, 196)	157 ± 21 (115, 199)	159 ± 16 (126, 192)	153 ± 21 (112, 195)	179 ± 15 (149, 208)	152 ± 18 (115, 189)	0.222	0.914	0.600	0.03

p < 0.05 vs. Pre; ^**p < 0.05 vs. Control.

C, cholesterol; CK, creatine kinase; HDL, high-density lipoprotein; LDL, low-density lipoprotein; MDA, malondialdehyde.

Figure 1 shows no group differences in measures of vascular structure and function before the start of the interventions. After 12 weeks of the interventions, baPWV and carotid artery IMT decreased in both HIIT and supra-HIIT groups (all p < 0.05). Reductions in baPWV remained significant in both groups even when adjusted for changes in mean blood pressure using ANCOVA. Baseline brachial artery diameter at rest did not change in any of the groups. Brachial FMD increased in both HIIT and supra-HIIT groups (all p < 0.05). There was no significant group difference in the magnitude of changes between the groups.

Figure 1.

The bar graph (mean ± SEM) and scatterplots demonstrating paired data of individual changes in carotid artery IMT (A, B), baPWV (C, D), and brachial FMD (E, F) in response to control (CON; n = 11), high-intensity interval exercise at 90% VO₂peak (HIIT; p < 0.05 vs. Control; n = 11), and supra high-intensity interval exercise at 170% VO₂peak (supra-HIIT; n = 15) in obese preadolescent boys. *p < 0.05 vs. Pre.; ^†p < 0.05 vs. Control; baPWV, brachial-ankle pulse wave velocity; FMD, flow-mediated dilation; HIIT, high-intensity intermittent training; IMT, intima-media thickness; SEM, standard error of the mean; supra-IIHT, supramaximal high-intensity intermittent training; VO₂peak, peak oxygen consumption.

Discussion

The major findings of the present study are as follows. First, both HIIT and supra-HIIT were well tolerated for 12 weeks in obese preadolescent boys. Second, arterial stiffness and carotid artery IMT decreased in both HIIT and supra-HIIT groups. Third, FMD improved significantly in both HIIT and supra-HIIT groups. Fourth, RMR increased and blood lipid levels decreased in both HIIT and supra-HIIT groups although body weight and adiposity did not change. Our present findings indicate that HIIT is an effective and time-efficient lifestyle modification strategy for obese preadolescent boys.

Arterial stiffness is considered as one of the early and easily-detectable measures of vascular dysfunction.²⁶ A number of studies have observed premature arterial stiffening in obese children.^27,28 In the present study, arterial stiffness as assessed by baPWV decreased 7%–8% in both exercise training groups. Previous study demonstrated that moderate-intensity exercise has failed to reduce PWV, whereas HIIT reduced it in adult patients with essential hypertension.²⁹ Similarly, interval exercise training decreased arterial stiffness, but continuous exercise training did not in treated hypertensive subjects.³⁰ Thus, training programs of higher exercise intensity may be more effective in reducing arterial stiffness in populations that already exhibit some alterations in vascular elasticity.²⁹ The mechanisms mediating the reduction in arterial stiffness with exercise training are not clear but may be related to the removal of chronic restraint on the arterial smooth muscle cells provided by the sympathetic adrenergic vasoconstrictor tone,³¹ as well as endothelin-1.³²

High-resolution B-mode ultrasound measurement of carotid IMT is an established marker of early preclinical atherosclerosis.³³ Several studies have reported elevated IMT in obese children compared with lean children.^34–36 The present study found that carotid artery IMT decreased after 12 weeks of both HIIT and supra-HIIT. We also found that changes in carotid IMT were associated with the corresponding changes in peak aerobic fitness. These results may be surprising given a relatively short duration of the exercise interventions, but are in agreement with a previous study reporting that a regular physical activity program, including mixed aerobic and strengthening exercises >60 minutes, three times/week for 6 months, decreased carotid artery IMT in obese prepubescent boys.⁸ Taken together, these results suggest that subclinical atherosclerosis can be reversed with regularly-performed exercise programs in obese children.

A previous study found that 8 weeks of low-volume high-intensity exercise improved endothelium-dependent vasodilation as assessed by FMD in patient with coronary artery disease.³⁷ In a previous study in young sedentary adults, 6 weeks of sprint interval training led to a greater increase in microvascular endothelial nitric oxide synthase content than endurance training.¹⁵ Consistent with these previous studies in adults, FMD increased significantly following interval training in the present study. The improvement in functional measure of endothelial function (i.e., FMD) was associated with the corresponding changes in biochemical measure of endothelial function as assessed by plasma NO levels determined by plasma nitrite and nitrate levels. Thus, both functional and biochemical measures of endothelial function consistently indicate that endothelium-dependent vasodilation can be improved by HIIT in obese children.

Regular exercise is well recognized to be essential for obese children, but dropout from the traditional exercise programs is substantial and is very well documented in the literature.¹¹ A previous study indicates that child's adherence to the exercise program depends largely on the attendance and availability of the parents.¹¹ Typical exercise training programs require 45–60 minutes for one exercise session. In contrast, the HIIT protocol required 23 minutes of total time and in the case of the supra-HIIT protocol, it took only 5 minutes to complete. Thus, the time commitment of the children, as well as the parents, is small. In addition, supra-HIIT was perceived by obese preadolescent boys to be more enjoyable. Thus, shorter exercise protocols may be more favorable over longer duration sessions. These factors could explain as to why the dropout rate was much lower in the supra-HIIT group in the present study. A previous HIIT exercise intervention study³⁸ has also reported a high attendance rate in male adolescents. It is important to determine in future studies if these HIIT exercises can be sustainable for longer periods of time and whether preadolescent girls would experience similar enjoyment from this kind of exercises.

Both HIIT and supra-HIIT programs produced significant increases in RMR. RMR accounts for the largest portion of daily energy expenditure. Additional energy expenditure should have been provided by physical activity thermogenesis during the performance of HIIT and supra-HIIT. However, body weight and measures of adiposity did not change in the present short-term exercise training intervention study. It is possible that a longer duration of interval training could produce significant reductions in body weight and adiposity through chronic elevations in basal metabolic rate. We also cannot exclude the possibility that the subjects increased their dietary intake although the dietary intake data are not consistent with such possibility.

Muscle strength and aerobic capacity are important components of physical fitness in children. VO₂peak increased with both HIIT and supra-HIIT, and the relative (%) increases were not different between the two groups. This finding was not expected because the primary energy system utilized in supra-HIIT is largely anaerobic energy system involving phosphagens and anaerobic glycolysis. In addition, the improvements in cardiovascular fitness are related to total amount of work completed, as determined by the intensity, duration, and frequency of exercise training. The HIIT program had substantially greater exercise training volume than the supra-HIIT program in the present study. It may be that very strenuous exercise could tax oxidative phosphorylation sufficiently during recovery periods if exercise bouts were repeated multiple times.³⁹

There are a number of limitations in the present study that should be emphasized. First, the number of subjects studied is relatively small, resulting small effect sizes. Second, the study had lack of puberty ascertainment. We did not measure the Tanner stage to determine the exact pubertal status since the exposure of sexual organs is highly inconvenient. Third, it is unknown why body mass and body fat percentage remained unchanged even though RMR increased significantly. It is possible that eating habits of participants may have changed although both children and their parents were instructed not to change their eating routine. Fourth, because no girls were recruited in this study, it remains unknown whether supra-HIIT exercise would be effective in obese preadolescent girls.

Conclusions

The results of the present study indicate that both HIIT and supra-HIIT were effective in improving cardiovascular fitness and vascular function in obese preadolescent children. In particular, supra-HIIT was more time effective in modifying some key functions and more enjoyable than HIIT and demonstrated a lower dropout. The supra-HIIT exercise fits with the children's activity style that is inherently intermittent and sporadic. It can counter the concern that obese children do not appear to engage in prolonged continuous exercise programs. It can be completed in <5 minutes and minimizes the time that parents would accompany children. Supra-HIIT may be an effective exercise modality that can be prescribed to obese children who could suffer from atherosclerotic vascular diseases in the future if they are not prevented.

Footnotes

Acknowledgments

This study was supported by Ratchadaphiseksomphot Endowment Fund at Chulalongkorn University. The authors gratefully acknowledge all subjects for participation. The authors thank Saowalak Siripanya, Saowaluck Suntraluck, Tussana Charujata, and Woradej Wibunjaroenkitja for assistance with data collection.

Author Disclosure Statement

No competing financial interests exist.

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