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The objectives of The Identification and Prevention of Dietary and Lifestyle Induced Health
Effects in Children and Infants study (child sample) and the Kenya Adolescent Physical Activity
study (adolescent sample) conducted in this thesis was firstly, to assess the impact of
methodological and practical decisions such as the appropriate epoch length and cutpoints to use
in accelerometry studies involving children and adolescents across the physical activity
continuum and the reliability of these accelerometer outcomes in predicting habitual physical
activity. Secondly, the performance of uniaxial ActiTrainer accelerometry with heart rate (HR)
monitoring was compared to triaxial GT3X accelerometry against indirect calorimetry during
structured activities in the predominantly active Kenyan adolescent cohort. Similarly, the
performance of uniaxial ActiTrainer accelerometry with HR monitoring vs. triaxial 3DNX
accelerometry was compared against DLW under free living conditions in both children and
adolescent cohorts. Finally, the validated uniaxial ActiTrainer was used to assess the impact of
physical activity and the environment on energy expenditure and indices of adiposity in the two
cohorts. The main findings of the thesis were: (a) that 15 s epoch reports significantly higher
engagement in physical activity compared to a 60 s epoch in both the children and adolescents
cohorts (b) choice of cutpoints significantly affected classification of physical activity and
sedentary behaviour in both cohorts (c) a minimum of 6 h for 7 - 9 days in the cohort involving
children and a minimum of 6 h for 4 - 5 days in the adolescents were required to reliably
measure physical activity (d) triaxial accelerometry reported better predictive validity compared
to uniaxial accelerometry during structured activities. In addition, HR monitoring did not
improve the predictive validity of either accelerometer during structured activities (e) during free
living activities, however, uniaxial and triaxial accelerometry reported comparable predictive
validity. The addition of HR monitoring improved the predictive validity of uniaxial
accelerometry by approximately 4% in both cohorts (f) in both cohorts, total volume of physical
activity and patterns (time engaged in light and moderate to vigorous physical activity) were
significantly associated with energy expenditure (g) physical activity and sedentary behaviour
were significantly related to indices of adiposity in both cohorts (h) the environment was a
significant predictor of physical activity and indices of adiposity in the adolescents but not
children. The findings of this thesis have important implications on strategies to standardise
accelerometry field protocols and future studies on the validation of accelerometers and the
association between physical activity, the environment and health. |
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