Objectively measured sedentary behaviour, physical activity and health in adolescent females

The primary purpose of this thesis was to determine the associations between low intensity physical activities and indices of health in a particularly inactive population, adolescent females. To achieve this, methodologies which enabled the accurate, reliable and objective measurement of low intensity physical activities were examined and developed. The inclinometer-based activPAL activity monitor was selected as the objective measure of physical activity due to its ability to distinguish between sitting/lying and standing. The validity of the activPAL as a measure of free-living physical activity was investigated, and a count to activity threshold was developed to quantify moderate to vigorous physical activity. Protocols which allowed the examination of both sedentary and physical activity variables were developed and implemented based on activity monitoring best practice. Three sets of cross-sectional data were pooled to examine whether relations existed between low intensity physical activity variables and body composition in Irish adolescent females (n=195). No associations were evident between body composition and sedentary variables. However, negative associations were observed between light intensity physical activity and body composition. Furthermore, in this population, light intensity physical activity appears to protect adolescent females against sedentary time to a greater extent than moderate to vigorous physical activity. The associations between low intensity physical activity variables and more powerful indices of cardiovascular health, including blood lipids, carotid intima-media thickness and arterial stiffness, were also examined in a smaller sample of adolescent females (n=64). Similarly, no significant relationships were evident between sedentary variables and cardio-metabolic risk factors, while light intensity physical activity was negatively associated with body mass index. In summary, the activPAL has been identified as a valid measure of physical activity in an adolescent female population. The novel examination of light intensity physical activity, which distinguishes between sitting and standing activities, has been described and presented. Negative associations have been observed between activPAL determined light intensity physical activity and body composition in this population. To conclude, the evidence presented in this thesis suggests that a worthwhile public health initiative may be to encourage Irish adolescent females to increase the amount of time spent in light intensity physical activity by reducing the amount of time spent in sedentary behaviours