The accurate measurement of total energy expenditure and physical activity in daily life in the children is important because they are related to obesity and deceased physical fitness. The daily physical activity might also prevent autonomic imbalance. The aims of the present study were to validate the newly developed activity monitor with tri-axial accelerometer ( Actimarker ) using doubly labeled water ( DLW ) method and to examine the effect of physical activity on autonomic nervous system ( ANS ) function in children. A total of 12 active children were paticipated. Body composition and physical fitness were measured. Two-week total energy expenditure ( TEE ) were measured by DLW method and estimated by Actimarker. The ANS activities were assessed during the resting condition by heart rate variability power spectral analysis. The low frequency of the heart rate variability ( LF; 0.03 - 0.15 Hz ) is reflecting mixed sympathetic ( SNS ) and parasympathetic nervous system ( PNS ) activity, and the high frequency ( HF; 0.15-0.5 Hz ) is mainly associated with PNS activity. The TEE estimated by Actimarker were significantly and highly correlated ( r = 0.899, P＜0.001 ) with the TEE measured by DLW method with no significant difference ( P = 0.592 ). The duration of low-to-moderate physical activity was significantly and positively correlated with PNS activity. The duration of vigorous activity was significantly and negatively corelated with heart rate. The results suggest that the Actimarker can be used TEE measurement of daily life in active children, and the objectively measured physical activities by Actimarker are associated with ANS function in the children.
