Breaks in sedentary time in young children : measures and methodological issues

There is evidence that sedentary behaviour and breaks in sedentary time, independent of physical activity levels, influence human health. In order to explore this relationship, accurate and validated measurement instruments are required. Such tools are also required for exploring the effects of factors such as the differences in sedentary behaviour between groups, e.g. overweight/obese vs. healthy weight children. One promising instrument for making such measurements is the activPAL™ monitor. This is an event-based accelerometer with inbuilt inclinometer, and it may be more accurate for the measurement of sitting time and breaks in sitting than earlier instruments. An important setting in the monitor depends on the time required to define a new posture – the minimum sitting/upright period (MSUP). At present, the optimum activPAL™ MSUP setting is not known, particularly for children, who are likely to change posture faster than adults. This thesis includes four studies: Study 1 (Chapter 3): Using the activPAL™, we investigated the effect of variations in MSUP on total sitting time and breaks in sitting. Methods: Study 1a: In this in vitro experiment, the activPAL™ monitor was turned from a horizontal position to a vertical position manually (by hand) to simulate 5 sitting bouts. The length of the sitting time was varied from 1s to 10s. The number of the true events (i.e. 50) and the data from the activPAL™ (the number of sitting bouts for each bout length) was compared and represented in a graph. Study 1b: in data collected from children in a free-living environment (23 children (mean (SD) age 4.5yrs (0.7)) who wore the activPAL™ (24 hr/d) for 5-7d), we varied the setting of the MSUP. For each child, we calculated the following measures of sitting behaviour: volume (total time in sit/lie postures); number of breaks (number of sit/lie to stand transitions); number of sitting bouts (number of discrete periods spent sitting/lying); and pattern of accumulation of sitting (represented by accumulation curves and fragmentation index). We first studied the activPAL™ using the default setting of 10s MSUP, and then reduced this to 5s, 2s, and 1s. Results: Study 1a: the analysis software did not count sitting bouts of a shorter duration than the user defined MSUP in the new posture. For example, the sitting bouts with a period less than 10s were not counted when we used the activPAL™ setting of 10s MSUP. Study 1b: Comparing settings of 10, 5, 2, and 1s, there were no significant differences in total sitting time (6.2 hr (1.0), 6.3 hr (1.0), 6.4 hr (1.0), and 6.3 hr (1.6), respectively) between settings, but there were significant increases in: the apparent number of breaks - 8(3), 14(2), 21(4), and 28 (6)/hr) total number of bouts (118(18) vs 382 (80)); and Fragmentation Index (19.3 (3.7) vs 61.6 (16.4)), with a reduction in 50% bout length from 80s (14.7) to 42s (7.7) at 10s and 1s setting, respectively. Conclusion: With the activPAL™, breaks in sitting, but not total sitting time, are highly sensitive to the setting of the MSUP. Additional studies will be required to confirm these findings and to define the most appropriate MSUP for different age groups. Simple measures can characterise sitting behaviour in young children using the activPAL™. Study 2 (Chapter 4): Posture transitions are likely to be much more rapid in young children than in adults. We investigated the optimum activPAL™ setting of MSUP to define a change in posture for measurement of sitting time and breaks in sitting (not previously known). We evaluated the validity of different minimum event duration settings against direct observation as the criterion method. Methods: In a convenience sample of 30 pre-school children (mean age 4.1yrs (SD 0.5)), we validated the activPAL™ measures of sitting time and breaks in sitting at different MSUP settings against direct observation. Results: In comparison with direct observation, a 2s setting had the smallest error relative to direct observation (95% limits of agreement: -14 to +17 sitting bouts/hr, mean difference 1.83, p = 0.2). Conclusion: For pre-school children, 2s appears to be an appropriate MSUP to define breaks in sitting using the activPAL™. Study 3 (Chapter 5): The identification of risk factors for obesity is considered key to obesity prevention. Differences in time sitting compared to standing have been observed in obese and non-obese adults. Whether such differences are present between obese and non-obese children has not yet been examined. In a pilot study, we investigated differences in sitting behaviour between overweight/obese and healthy weight children. Methods: Overweight/obese children were recruited from weight management and dietetic clinics, the Active Children Eating Smart programme, and from three primary schools, while healthy weight children were recruited from schools. The participants wore the activPAL™ (24 hr/d) for 5-7d. During waking time, the time spent sitting, number of sitting bouts, the 50% and 90% sitting bouts length, and the Fragmentation Index were measured in both groups using the activPAL™ with a 2s MSUP setting. Results were available for 26 healthy weight children (mean age 6.4yrs (SD 0.9), median BMI Z-score 0.04 (range -3.24 - 0.66)) and 13 overweight/obese children (mean age 6.4 yrs (SD 0.9), median BMI Z-score of 1.38 (range 1.14 - 3.10). Results: In healthy weight children during the waking hours, the mean (SD) percentage of waking time spent sitting was 53.0 % (6.4) representing 6.8 hrs (0.9) per day; mean (SD) total number of sitting bouts per day was 280 (65). The median (range) of 50% and 90% of sitting bouts were ≤50.0s (40.0-50.0) and ≤3.5 min (2.0-6.0), respectively. The mean (SD) Fragmentation Index was 42.1(12.7). In the 13 overweight/obese children, the mean (SD) percentage of waking time spent sitting was 52.4% (5.2), representing 6.9 hrs per day (SD 0.8). The total number of sitting bouts was 284 per day (66). The median (range) of 50% and 90% of sitting bouts length were ≤50.0s (40.0-50.0) and ≤3.5 min (3.0-6.0), respectively. The mean (SD) Fragmentation Index was 41.5 (9.6). Conclusion: Both healthy weight children and overweight/obese children in this study spent the majority of their waking time sitting. Furthermore, there were no significant difference in the sitting time, number, or duration of sitting bouts and the Fragmentation Index between the two groups in this study. Study 4 (Chapter 6): Previous studies have shown differences in the total sedentary time between boys and girls using accelerometers where sedentary behaviour was defined as low movement or low energy expenditure. In the present study, we examined whether there were also differences in breaks in sitting time. Methods: A convenience sample of 62 (32 girls: 30 boys) free-living healthy children (mean age 5.8yrs (SD1.3)) was recruited from nurseries and schools in Glasgow and Edinburgh, Scotland, who each wore the activPAL™ monitor continuously for 5–7 days. For each child, the components of sedentary behaviour were measured. Results: The percentage of waking time spent sitting was significantly higher in girls, the mean (SD) (54.4% (5.6)) compared to boys (50.9 (5.6)), (2-sample t-test, p-value <0.02). The total sitting time in girls vs. boys respectively per day was 6.9 (0.8) vs. (6.5) (0.9), (p-value <0.08). There were no significant differences in the number of sitting bouts, Fragmentation Index, or in 50% and 90% sitting bout length between girls and boys. Conclusion: This study suggested that girls spend more time in sitting than boys. However, there were no significant gender differences in the number or duration of sitting bouts. Summary Conclusion: These studies have validated the activPAL™ as an instrument for measuring breaks in sedentary time in young children, and have established appropriate settings for making accurate measurements in this age group. Using these monitors to explore differences between obese and non-obese children, and between boys and girls, we found that, while there were differences between boys and girls in the total sitting time, there were no differences in the number of breaks between girls and boys, or between obese and non-obese children. These studies suggest that the activPAL™ may be useful to identify between group differences in sitting time and sitting fragmentation in future studies.