Despite of growing research on sedentary behavior and health in preschool children, few studies have measured sedentary behavior objectively. This dissertation project consists of three studies that were conducted to provide scientific evidence on the reliability, level, and effect of objectively measured sedentary behavior in preschool children. The specific aims of this dissertation were: 1) to determine how many days of monitoring are necessary to reliably estimate sedentary behavior in preschool children, 2) to determine the levels of sedentary behavior in children attending Montessori and traditional preschools, and 3) to determine if sedentary behavior is associated with BMI z-score in preschool children. All three studies employed accelerometry as an objective measure of sedentary behavior.

In study one, the accelerometry-derived sedentary behavior was summarized using three different cutpoints (< 37.5 cts/15s, <200 cts /15s, and <373 cts /15s). Across different cutpoints, 2 – 4 days and 6 – 9 days of accelerometry monitoring was needed to reliably measure in-school and total day sedentary behavior respectively. In study two, the amount of time spent in sedentary behavior was described across different time segments of the day (in-school, after-school, and total day). Children attending Montessori preschools were significantly less sedentary compared to those attending traditional preschools during in-school (44.4 min/hr vs. 47.1 min/hr), after-school (43.0 min/hr vs. 44.9 min/hr), and total day (43.7 min/hr vs. 45.5 min/hr). School type (Montessori or Traditional), preschool setting (private or public), socio-demographic factors (age, gender, and socioeconomic status) were found to be significant predictors of preschoolers' sedentary behavior. In study three, the time spent in sedentary behavior was not associated with BMI z-score in two independent samples of preschool children, after adjusting for potential confounders. This null association persisted when the time spent in sedentary behavior was estimated using different accelerometry cutpoints.

In conclusion, accelerometry is a reliable method to objectively measure sedentary behavior in preschool children. Using accelerometry, the levels of time spent in sedentary behavior were significantly lower among children attending Montessori preschools than those attending traditional preschools. There was no independent association between accelerometry-derived sedentary behavior and BMI z-score in preschool children. Findings from this dissertation provide important information for designing epidemiologic research and developing public health policy for sedentary behavior in preschool children.