Physical activity, sedentary behaviour and cardiometabolic health in children, adolescents and young adults

The overarching aims of this thesis were to: (i) to determine the validity of the activPAL 3 micro for measuring physical activity (PA) intensity in adolescents; (ii) to quantify differences in free-living activity behaviours across cohorts of children, adolescents and young adults using the activPAL 3 micro; (iii) to examine the associations between free-living activity behaviours and indices of cardiometabolic health in a representative sample of Irish adolescents; and, (iv) to model the effects of reallocating time between activity behaviours on cardiometabolic health markers in Irish adolescents. The activPAL monitor is the world’s first single-site instrument validated to accurately measure sedentary time, standing time and postural changes. Despite this, the device output has been limited to classifying an individual’s free-living activity into periods of sitting/lying, stepping and standing. A count-to-activity threshold for the activPAL 3 micro was developed and validated to provide accurate measures of MVPA and subsequently LIPA. Habitual activity behaviours were assessed within cohorts of children, adolescents and young adults (N = 194). Waking sedentary time, MVPA and prolonged sedentary bouts increased across the different aged cohorts. Standing time differed significantly, while LIPA and the time spent in short sedentary bouts decreased. The level, duration and pattern of habitual activity behaviours differed significantly between weekdays and weekend days and between school/university hours and out-of-school/university hours. Free-living activity behaviours and cardiometabolic health indices were examined in a representative sample of 15 to 18-year-old Irish adolescents (N = 222). Sedentary waking time was detrimentally associated with diastolic blood pressure, LDL-C and glucose. Increased standing time was only associated with lower systolic and diastolic blood pressure. Increased LIPA time was beneficially associated with sum-of-skinfold thickness, handgrip strength, cardiorespiratory fitness, diastolic blood pressure, total cholesterol and triglycerides and negatively associated with waist-to-hip ratio and HDL-C. Sedentary Bouts < 20 minutes were beneficially associated handgrip strength and cardiovascular fitness and negatively associated with waist-hip-ratio. Sedentary bouts > 60 minutes were detrimentally associated with diastolic blood pressure. Reallocating the time spent in one activity behaviour with another significantly influenced the cardiometabolic health indices. The evidence presented in this thesis highlights the importance of increasing LIPA in addition to MVPA to improve and maintain adolescent cardiometabolic health. Future interventional and longitudinal research should be conducted to determine the implication of replacing sedentary time with LIPA, with the aim of informing public health initiatives and future PA guidelines.