Date Published: April 15, 2019
Publisher: Public Library of Science
Author(s): Katherine L. Downing, Xanne Janssen, Dylan P. Cliff, Anthony D. Okely, John J. Reilly, Matthew M. Schubert.
Despite growing scientific interest in the benefits of breaking up sedentary time with intermittent standing or walking, few studies have investigated the energy cost of posture transitions. This study aimed to determine whether posture transitions are associated with increased energy expenditure in preschool children.
Forty children (mean age 5.3 ± 1.0y) completed a ~150-min room calorimeter protocol involving sedentary, light, and moderate- to vigorous-intensity activities. This study utilised data from ~65-min of the protocol, during which children were undertaking sedentary behaviours (TV viewing, drawing/colouring in, and playing with toys on the floor). Posture was coded as sit/lie, stand, walk, or other using direct observation; posture transitions were classified as sit/lie to stand/walk, sit/lie to other, stand/walk to other, or vice versa. Energy expenditure was calculated using the Weir equation and used to calculate individualised MET and activity energy expenditure (AEE) values. Spearman’s rank correlations were used to compare the number of posture transitions, in the individual activities separately and combined, with corresponding MET and AEE values. Participants were divided into tertiles based on the number of posture transitions; MET and AEE values of children in the lowest and highest tertiles of posture transitions were compared using unpaired t-tests. Effect sizes (Cohen’s d) were calculated.
There was a positive correlation between the total number of posture transitions and average METs (rs = 0.42, p = 0.02) and AEE (rs = 0.43, p = 0.02). MET differences between the lowest and highest tertiles of posture transitions resulted in a small effect size for playing with toys (d = 0.27), and moderate effect sizes for TV viewing, drawing and all three activities combined (d = 0.61, 0.50 and 0.64 respectively). Similar results were found for AEE.
Results from this study showed that variation in posture transitions may be associated with variation in energy expenditure in preschool children. The findings suggest that the concept that variation in posture transitions may have meaningful biological or health effects in early childhood is worth investigating further.
Sedentary behaviour, defined as any activity undertaken in a sitting or lying posture and requiring fewer than 1.5 metabolic equivalents (METs) , has an impact on several major health outcomes in adults. High levels of sedentary behaviour are associated with an increased risk of all-cause mortality, cardiovascular disease, cancer, and type 2 diabetes, largely independent of time spent in moderate- to vigorous-intensity physical activity (MVPA) . Breaking up prolonged bouts of sedentary time has been shown to be associated with more favourable cardiometabolic profiles in adults [3, 4], with similar evidence emerging in children [5–8]. Although the direct mechanisms are still largely unknown, these associations are likely due to distinct and important physiological differences in skeletal muscle metabolism and energy expenditure that exist between sitting and standing still .
Of the 40 children who completed the WRC protocol, two had missing data due to calorimeter malfunction. Of the remaining 38 children, 32 (84.2%), 36 (94.7%), and 35 (92.1%) had valid energy expenditure data for TV viewing, drawing and playing with toys, respectively. Thirty-one children had valid data for the combined analyses. Descriptive characteristics of the current sample (n = 36) are presented in Table 2. The mean (SD) measurement time for the three activities was 56.6 (7.3) minutes.
Despite growing interest in the metabolic health effects of breaking up sitting, few studies have examined the energy cost of changes in posture and none have done so in children aged 4–6 years. Emerging evidence in adults suggests that sit/stand transitions have a significantly higher energy cost than sitting still . Findings from the present study suggest that children who have more frequent changes in posture during typical sedentary behaviours may expend more energy than those who have less frequent posture changes. With the exception of time spent playing with toys, the effect sizes for the differences in energy expenditure between the lowest and highest tertiles of posture transitions in individual activities (i.e., watching TV and drawing), and for the combined activities, were moderate. Additionally, there was a statistically significant positive correlation between the total number of posture transitions during sedentary time and energy expenditure, suggesting that the notion that variations in posture transitions produces meaningful variation in energy expenditure in young children is worth investigating further.
Findings from this study suggest that posture transitions may be associated with increased energy expenditure in preschool children. Despite the inherent limitations, the findings are encouraging and provide preliminary evidence to suggest that the concept that variation in posture transitions may have meaningful biological or health effects in early childhood is worth investigating further.