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Previous reports have demonstrated that children’s physical activity is typically intermittent in nature. Accelerometers are reliable and valid tools for quantifying the pattern of activity in children. However, in order to interpret accelerometer output it is necessary to apply appropriate accelerometer thresholds for classifying physical activity intensity. Currently multiple accelerometer thresholds are available in the literature and it is unclear which are the most appropriate or whether thresholds should be sample specific. Additionally, there is little information regarding how the pattern of activity in children varies across groups and how activity patterns relate to health. The overall aim of the thesis was to describe the pattern of habitual physical activity, using objectively-measured physical activity data, in relation to health outcomes in pre-adolescent boys. The first objective was to characterize the pattern of boys’ habitual physical activity, using objectively-measured physical activity data. The second objective was to investigate the relationship between habitual physical activity and specific aspects of the activity pattern and health outcomes in boys. The final objective was to investigate the effects of continuously- versus intermittently- accumulated physical activity on acute health outcomes in boys, using an intermittent activity protocol based on the measured pattern of habitual activity in boys. The first four studies used a sample of 54 boys, aged 8 to 10 years. The final study used a different sample of 10 boys, aged 9 to 11 years. Both samples were from the county of Devon, UK. The first study of this thesis established sample-specific accelerometer-intensity thresholds through calibration research with ActiGraph accelerometers (counts•2s-1) in boys. The second study in the thesis demonstrated that relationships between time accumulated at different activity intensities and health (fatness, peak oxygen consumption and resting blood pressure) in boys were similar irrespective of whether sample-specific or published thresholds were employed as long as the threshold was at least equivalent to a brisk walk (i.e. >4 METs). However, the prevalence of children reported as meeting activity guidelines did differ according to thresholds employed. Study three showed that, despite little difference between overweight and normal weight boys in overall activity, time spent sedentary and minutes of light, moderate and vigorous intensity activity accumulated, aspects of the activity pattern (frequency, intensity and duration of ≥4 s (short) and ≥5-min (long) bouts of ≥light, ≥moderate, ≥vigorous and ≥hard intensity activity) differed by weight status. Overweight boys accumulated fewer and shorter bouts of activity, particularly sustained bouts of activity which were of moderate intensity or greater. Study four examined the relationship between activity pattern and health in boys further, focusing on body fatness, aerobic fitness, blood pressure and microvascular function. Results demonstrated that summary measures of activity were negatively related to body fatness, and positively related with both aerobic fitness (i.e., total, moderate, vigorous and hard activity) and endothelial function (i.e., total and light activity). Time spent sedentary was negatively related to endothelial function. The frequency and duration of activity bouts of ≥moderate intensity and the intensity of all activity bouts (i.e., ≥light intensity) were most important for body fatness and aerobic fitness. The frequency of all bouts (short and long) of at least light intensity was most important for endothelial health. Finally, study five moved away from chronic measures of health and examined the acute physiological effects of the recommended daily amount of physical activity (60 minutes of physical activity of ≥moderate intensity) on postprandial lipaemia and microvascular function in boys the following day. Furthermore, the study aimed to assess whether the effects differed depending on whether the activity was accumulated continuously or in a manner more similar to the children’s typical activity patterns identified in studies 3 and 4. In contrast to findings from adolescents and adults, 60 minutes of >moderate intensity activity did not impact on postprandial lipaemia or microvascular function the following day in healthy, active boys, regardless of how it was accumulated. The results of the present thesis indicate that aspects of the activity pattern are significantly related to health in boys and differ according to type of day and weight status. Furthermore, the benefits of sporadically-accumulated activity are equally as strong as continuously-accumulated activity to body fatness, aerobic fitness and endothelial health in boys. Since children typically accumulate short, intermittent bouts of activity, the promotion of sporadic activity (i.e., in intervention research and current physical activity guidelines for children and youth) might increase enjoyment and adherence. The benefits of acute intermittent exercise (which simulates free-living activity) on postprandial lipaemia and microvascular function in inactive children with risk factors for cardiovascular disease should be investigated. Longitudinal investigations into the activity pattern of a much larger and more age-diverse sample of boys and girls are needed to determine whether any changes in aspects of the activity pattern might alter these and other health outcomes (i.e., cardiovascular risk factors).

Introdução: O exercício físico tem sido indicado como uma importante intervenção contra fatores de risco cardiovasculares, uma vez que pode atenuar o aumento da lipemia e estresse oxidativo após o consumo de uma refeição hiperlipídica (RH). Em contrapartida, pouco se sabe sobre a intensidade do exercício mais eficaz para causar redução da curva lipêmica e estresse oxidativo. Objetivo: Comparar o efeito de duas sessões isocalóricas de exercício físico em diferentes intensidades sobre a curva lipêmica e marcadores de estresse oxidativo em sujeitos saudáveis submetidos à refeição hiperlipídica. Metodologia: Onze sujeitos eutróficos do sexo masculino, fisicamente ativos, com idade média de 23 ± 3 anos participaram de um experimento composto por 3 diferentes protocolos com dois dias consecutivos cada. No dia 1, os sujeitos realizavam um dos três protocolos que foram realizados de forma randomizada: exercício em baixa intensidade (BI); exercício em moderada intensidade(MI); e repouso (Rep). No dia 2, 12h após a realização dos protocolos, os sujeitos consumiam uma RH (50% de Lipídeos, 35% de Carboidratos e 15% de Proteínas). As coletas de sangue para análise de Triglicerídeos (TG), Colesterol Total (CT), HDL, LDL, Substâncias Reativas ao Ácido Tiobarbitúrico (TBARS), Nitritos e Nitratos (NOx) e Tióis Totais foram realizadas no momento basal e a cada hora de 1 à 5h após o consumo da refeição. Resultados: O BI apresentou menores níveis de TG comparado ao Rep no momento 4h, já o MI atenuou os níveis de TG, em comparação ao Rep, nas horas 3, 4 e 5 (p<0,05). Não houve diferença entre BI e MI para TG. Em relação à área abaixo da curva (AUC) de TG, ambos os protocolos de exercício reduziram significativamente esta variável em comparação ao Rep (p<0,05). Houve diferença significativa entre as concentrações de TBARS nos protocolos BI e MI comparado ao Rep no momento 1h (p<0,01), dessa forma o exercício foi capaz de reduzir os níveis de TBARS no momento pós-prandial. O BI apresentou menores níveis de AUC de TBARS e maiores níveis de AUC de NOx em comparação ao Rep (p<0,02). No momento basal, o MI apresentou maiores valores de TBARS comparado ao BI (p<0,04). No mesmo momento, o BI apresentou maiores concentrações de NOx do que o Rep (p<0,01). Não foram encontradas diferenças entre os protocolos para os níveis de CT, LDL, HDL e Tióis Totais. Conclusão: Tanto o exercício em BI quanto em MI são eficazes para atenuar a lipemia e os marcadores de estresse oxidativo pós-prandiais. Além disso, existe um efeito subagudo do exercício de BI, que reduz os níveis basais de TBARS e aumenta os níveis de NOx em relação ao exercício de MI e Rep. / Introduction: Exercise has been considered as an important intervention against cardiovascular risk factors, since that exercise may attenuates postprandial lipaemia and oxidative stress after the consumption of a high-fat meal. Still, it remains the debate about the most effective exercise intensity that can reduce lipaemic curve and oxidative stress. Objectives: The aim of this study was to compare the effect of two isocaloric sessions of exercise performed in different intensities on the lipaemic curve and oxidative stress markers after a high-fat meal. Methods: Eleven eutrophic men, physically active, mean average 23 years old, participated in three protocols with two-days trials each. On day 1, participants performed one of the three randomized protocols: Low Intensity (LI); moderate intensity (MI); or Rest. On day 2, 12h after the exercises or rest protocols, it was provided a high-fat meal. Blood collections for analysis of Triglycerides (TG), Total Cholesterol (CT), HDL, LDL, Thiobarbituric Acid Reactive Substances (TBARS), Nitrites and Nitrates (NOx) and Total Thiols were taken in fasted state and from 1 to 5 hours after the consumption of the meal. Results: Lower levels of TG were observed in LI compared with Rest at 4h (p<0,05) and in MI compared with Rest at 3, 4 and 5h (p<0,05). No differences were found between LI and MI. There were differences between LI and MI compared with Rest for the area under the curve (AUC) of TG (p<0,05), thus both exercise intensities reduced this variable. Levels of TBARS in LI and MI were lower than Rest at 1h (p<0,01). A low AUC of TBARS and a high AUC of NOx was observed between LI and Rest (p<0,02). At baseline, the concentrations of TBARS for LI were significative lower than MI (p<0,04) and concentrations of NOx for LI were higher than Rest (p<0,01). There were no differences between the protocols for CT, LDL, HDL and Total Thiols. Conclusion: Both exercise intensities were effective to reduce postprandial lipaemia and oxidative stress. Yet, there is an acute effect of LI exercise that can reduce baseline concentration of TBARS and increase NOx compared to MI and Rest.

Introdução: O exercício físico tem sido indicado como uma importante intervenção contra fatores de risco cardiovasculares, uma vez que pode atenuar o aumento da lipemia e estresse oxidativo após o consumo de uma refeição hiperlipídica (RH). Em contrapartida, pouco se sabe sobre a intensidade do exercício mais eficaz para causar redução da curva lipêmica e estresse oxidativo. Objetivo: Comparar o efeito de duas sessões isocalóricas de exercício físico em diferentes intensidades sobre a curva lipêmica e marcadores de estresse oxidativo em sujeitos saudáveis submetidos à refeição hiperlipídica. Metodologia: Onze sujeitos eutróficos do sexo masculino, fisicamente ativos, com idade média de 23 ± 3 anos participaram de um experimento composto por 3 diferentes protocolos com dois dias consecutivos cada. No dia 1, os sujeitos realizavam um dos três protocolos que foram realizados de forma randomizada: exercício em baixa intensidade (BI); exercício em moderada intensidade(MI); e repouso (Rep). No dia 2, 12h após a realização dos protocolos, os sujeitos consumiam uma RH (50% de Lipídeos, 35% de Carboidratos e 15% de Proteínas). As coletas de sangue para análise de Triglicerídeos (TG), Colesterol Total (CT), HDL, LDL, Substâncias Reativas ao Ácido Tiobarbitúrico (TBARS), Nitritos e Nitratos (NOx) e Tióis Totais foram realizadas no momento basal e a cada hora de 1 à 5h após o consumo da refeição. Resultados: O BI apresentou menores níveis de TG comparado ao Rep no momento 4h, já o MI atenuou os níveis de TG, em comparação ao Rep, nas horas 3, 4 e 5 (p<0,05). Não houve diferença entre BI e MI para TG. Em relação à área abaixo da curva (AUC) de TG, ambos os protocolos de exercício reduziram significativamente esta variável em comparação ao Rep (p<0,05). Houve diferença significativa entre as concentrações de TBARS nos protocolos BI e MI comparado ao Rep no momento 1h (p<0,01), dessa forma o exercício foi capaz de reduzir os níveis de TBARS no momento pós-prandial. O BI apresentou menores níveis de AUC de TBARS e maiores níveis de AUC de NOx em comparação ao Rep (p<0,02). No momento basal, o MI apresentou maiores valores de TBARS comparado ao BI (p<0,04). No mesmo momento, o BI apresentou maiores concentrações de NOx do que o Rep (p<0,01). Não foram encontradas diferenças entre os protocolos para os níveis de CT, LDL, HDL e Tióis Totais. Conclusão: Tanto o exercício em BI quanto em MI são eficazes para atenuar a lipemia e os marcadores de estresse oxidativo pós-prandiais. Além disso, existe um efeito subagudo do exercício de BI, que reduz os níveis basais de TBARS e aumenta os níveis de NOx em relação ao exercício de MI e Rep. / Introduction: Exercise has been considered as an important intervention against cardiovascular risk factors, since that exercise may attenuates postprandial lipaemia and oxidative stress after the consumption of a high-fat meal. Still, it remains the debate about the most effective exercise intensity that can reduce lipaemic curve and oxidative stress. Objectives: The aim of this study was to compare the effect of two isocaloric sessions of exercise performed in different intensities on the lipaemic curve and oxidative stress markers after a high-fat meal. Methods: Eleven eutrophic men, physically active, mean average 23 years old, participated in three protocols with two-days trials each. On day 1, participants performed one of the three randomized protocols: Low Intensity (LI); moderate intensity (MI); or Rest. On day 2, 12h after the exercises or rest protocols, it was provided a high-fat meal. Blood collections for analysis of Triglycerides (TG), Total Cholesterol (CT), HDL, LDL, Thiobarbituric Acid Reactive Substances (TBARS), Nitrites and Nitrates (NOx) and Total Thiols were taken in fasted state and from 1 to 5 hours after the consumption of the meal. Results: Lower levels of TG were observed in LI compared with Rest at 4h (p<0,05) and in MI compared with Rest at 3, 4 and 5h (p<0,05). No differences were found between LI and MI. There were differences between LI and MI compared with Rest for the area under the curve (AUC) of TG (p<0,05), thus both exercise intensities reduced this variable. Levels of TBARS in LI and MI were lower than Rest at 1h (p<0,01). A low AUC of TBARS and a high AUC of NOx was observed between LI and Rest (p<0,02). At baseline, the concentrations of TBARS for LI were significative lower than MI (p<0,04) and concentrations of NOx for LI were higher than Rest (p<0,01). There were no differences between the protocols for CT, LDL, HDL and Total Thiols. Conclusion: Both exercise intensities were effective to reduce postprandial lipaemia and oxidative stress. Yet, there is an acute effect of LI exercise that can reduce baseline concentration of TBARS and increase NOx compared to MI and Rest.

Introdução: O exercício físico tem sido indicado como uma importante intervenção contra fatores de risco cardiovasculares, uma vez que pode atenuar o aumento da lipemia e estresse oxidativo após o consumo de uma refeição hiperlipídica (RH). Em contrapartida, pouco se sabe sobre a intensidade do exercício mais eficaz para causar redução da curva lipêmica e estresse oxidativo. Objetivo: Comparar o efeito de duas sessões isocalóricas de exercício físico em diferentes intensidades sobre a curva lipêmica e marcadores de estresse oxidativo em sujeitos saudáveis submetidos à refeição hiperlipídica. Metodologia: Onze sujeitos eutróficos do sexo masculino, fisicamente ativos, com idade média de 23 ± 3 anos participaram de um experimento composto por 3 diferentes protocolos com dois dias consecutivos cada. No dia 1, os sujeitos realizavam um dos três protocolos que foram realizados de forma randomizada: exercício em baixa intensidade (BI); exercício em moderada intensidade(MI); e repouso (Rep). No dia 2, 12h após a realização dos protocolos, os sujeitos consumiam uma RH (50% de Lipídeos, 35% de Carboidratos e 15% de Proteínas). As coletas de sangue para análise de Triglicerídeos (TG), Colesterol Total (CT), HDL, LDL, Substâncias Reativas ao Ácido Tiobarbitúrico (TBARS), Nitritos e Nitratos (NOx) e Tióis Totais foram realizadas no momento basal e a cada hora de 1 à 5h após o consumo da refeição. Resultados: O BI apresentou menores níveis de TG comparado ao Rep no momento 4h, já o MI atenuou os níveis de TG, em comparação ao Rep, nas horas 3, 4 e 5 (p<0,05). Não houve diferença entre BI e MI para TG. Em relação à área abaixo da curva (AUC) de TG, ambos os protocolos de exercício reduziram significativamente esta variável em comparação ao Rep (p<0,05). Houve diferença significativa entre as concentrações de TBARS nos protocolos BI e MI comparado ao Rep no momento 1h (p<0,01), dessa forma o exercício foi capaz de reduzir os níveis de TBARS no momento pós-prandial. O BI apresentou menores níveis de AUC de TBARS e maiores níveis de AUC de NOx em comparação ao Rep (p<0,02). No momento basal, o MI apresentou maiores valores de TBARS comparado ao BI (p<0,04). No mesmo momento, o BI apresentou maiores concentrações de NOx do que o Rep (p<0,01). Não foram encontradas diferenças entre os protocolos para os níveis de CT, LDL, HDL e Tióis Totais. Conclusão: Tanto o exercício em BI quanto em MI são eficazes para atenuar a lipemia e os marcadores de estresse oxidativo pós-prandiais. Além disso, existe um efeito subagudo do exercício de BI, que reduz os níveis basais de TBARS e aumenta os níveis de NOx em relação ao exercício de MI e Rep. / Introduction: Exercise has been considered as an important intervention against cardiovascular risk factors, since that exercise may attenuates postprandial lipaemia and oxidative stress after the consumption of a high-fat meal. Still, it remains the debate about the most effective exercise intensity that can reduce lipaemic curve and oxidative stress. Objectives: The aim of this study was to compare the effect of two isocaloric sessions of exercise performed in different intensities on the lipaemic curve and oxidative stress markers after a high-fat meal. Methods: Eleven eutrophic men, physically active, mean average 23 years old, participated in three protocols with two-days trials each. On day 1, participants performed one of the three randomized protocols: Low Intensity (LI); moderate intensity (MI); or Rest. On day 2, 12h after the exercises or rest protocols, it was provided a high-fat meal. Blood collections for analysis of Triglycerides (TG), Total Cholesterol (CT), HDL, LDL, Thiobarbituric Acid Reactive Substances (TBARS), Nitrites and Nitrates (NOx) and Total Thiols were taken in fasted state and from 1 to 5 hours after the consumption of the meal. Results: Lower levels of TG were observed in LI compared with Rest at 4h (p<0,05) and in MI compared with Rest at 3, 4 and 5h (p<0,05). No differences were found between LI and MI. There were differences between LI and MI compared with Rest for the area under the curve (AUC) of TG (p<0,05), thus both exercise intensities reduced this variable. Levels of TBARS in LI and MI were lower than Rest at 1h (p<0,01). A low AUC of TBARS and a high AUC of NOx was observed between LI and Rest (p<0,02). At baseline, the concentrations of TBARS for LI were significative lower than MI (p<0,04) and concentrations of NOx for LI were higher than Rest (p<0,01). There were no differences between the protocols for CT, LDL, HDL and Total Thiols. Conclusion: Both exercise intensities were effective to reduce postprandial lipaemia and oxidative stress. Yet, there is an acute effect of LI exercise that can reduce baseline concentration of TBARS and increase NOx compared to MI and Rest.

The thesis investigated the effect of exercise on endothelial function (measured as flow-mediated dilation (FMD)) and triacylglycerol concentrations following the ingestion of a high-fat breakfast and lunch in adolescent boys. The validity of measuring lipid and lipoprotein concentrations from a capillary blood sample, and the reproducibility of the postprandial FMD and triacylglycerol concentration responses to the high-fat meals, was established. The effects of prior continuous moderate-intensity exercise (60 min walking at 60% V̇O₂peak), repeated very short duration sprints (40 x 6 s maximal effort cycle sprints) and accumulated moderate-intensity exercise (6 x 10 min running at 70% V̇O₂peak) on endothelial function and triacylglycerol concentrations in adolescent boys were then established across three studies, each consisting of two, 2-day main trials (control and exercise). On day 1, participants were either inactive or completed the prescribed exercise. On day 2, FMD and triacylglycerol concentrations were measured prior to, and following, ingestion of a high-fat breakfast and lunch. In each control trial FMD was reduced (signifying endothelial dysfunction), compared to fasting, by 20-32% and 24-33% following the high-fat breakfast and lunch. Following continuous moderate-intensity exercise, repeated very-short duration sprints and accumulated moderate-intensity exercise these reductions were only 8% and 10% (main effect trial, P = 0.002; main effect time, P = 0.023; interaction effect trial x time, P = 0.088), 2% and 5% (main effect trial, P = 0.012; main effect time, P = 0.004; interaction effect trial x time, P = 0.003) and 1% and 3% (main effect trial, P = 0.020; main effect time, P < 0.001; interaction effect trial x time, P = 0.014) respectively. The continuous moderate-intensity exercise and repeated very short duration sprints also significantly reduced the total area under the triacylglycerol concentration versus time curve by 22% (Control vs. Exercise; 12.68 (sem 1.37) vs. 9.84 (sem 0.75) mmol L-1 6.5h, P = 0.018) and 13% (Exercise vs. Control: 8.65 (sem 0.97) vs. 9.92 (sem 1.16) mmol L-1 6.5h, P = 0.023). The accumulated moderate-intensity exercise also reduced the total area under the triacylglycerol concentration versus time curve by 11%, but this reduction was not significant (Control vs. Exercise: 10.71 (sem 0.94) vs. 9.56 (sem 0.67) mmol L-1 6.5h, respectively, P = 0.183). The experimental evidence from these studies emphasise that exercise might offer an acceptable, non-pharmacological means of influencing CHD risk when individuals are young. The results of these studies can help shape future physical activity guidelines.

Cardiovascular diseases (CVD) are the leading cause of death, and the underlying atherosclerotic process has its origin in youth. Physical activity lowers future CVD risk, however few adolescents achieve the recommended minimum amount of daily activity and interventions fail to meaningfully increase activity levels in this group. It is therefore essential to identify how small volumes of exercise can be optimised for the primary prevention of CVD. The purpose of this thesis is to identify the influence of exercise intensity on vascular health outcomes in adolescents, and to assess the efficacy of 2 weeks of low volume, high-intensity interval training on CVD risk factors in this population. Chapter 4 demonstrates that a single bout of high-intensity interval exercise (HIIE) performed one hour before a high fat meal elicits comparable reductions in postprandial lipaemia as a work-matched bout of moderate-intensity exercise (MIE) in girls. However, neither exercise attenuated postprandial lipaemia in the boys. Additionally, HIIE elicited a superior increase in postprandial fat oxidation and decrease in blood pressure, and this was sex independent. These findings are furthered in Chapter 5, which identified that accumulating HIIE, but not MIE, favourably modulates glycaemic control, postprandial blood pressure and fat oxidation in adolescents irrespective of sex. A high fat meal was included in Chapter 6 in order to impair vascular function via oxidative stress. Postprandial vascular function was preserved following MIE, but improved after HIIE, and these changes were not related to changes in postprandial lipaemia or total antioxidant status. Chapter 7 addressed the time course of the changes in vascular function post exercise, and identified that HIIE promotes superior changes in vascular function than MIE. Finally, Chapter 8 identified that 2 weeks of high-intensity interval training improved novel (endothelial function and heart rate variability), but not traditional CVD factors in adolescent boys and girls. However, most of these favourable changes were lost 3 days after training cessation. Thus, this thesis demonstrates that vascular health outcomes are positively associated with exercise intensity. Given that HIIE was perceived to be more enjoyable than MIE in Chapters 4, 6 and 7, performing HIIE appears to be an effectual and feasible alternative to MIE for the primary prevention of CVD.