Physical activity is beneficial in relation to several life style diseases and the association between physical activity and early predictors of life style diseases seem to be present already in preschool age. Since physical activity and other health behaviours are established during childhood and track from childhood into adult life, it is relevant to address physical activity already in the preschool age. The research in preschool children’s physical activity is relatively new, and because of methodological
inconsistencies, the associations between physical activity and health are less clear in
this age group.
The objective of this thesis was to contribute to the knowledge base regarding physical activity in preschoolers; How active are preschoolers? Are activity levels related to specific settings during a typical week? And are the activity levels related to a range of health outcomes? Furthermore, three different methods to clean and analyse 24-hour accelerometer data was studied. Lastly, the need and requirements for a physical activity recommendation for preschoolers is discussed.
The thesis is based on 3-year data from the prospective observational cohort study
SKOT. Children (n=330) were examined at ages 9 and 18 months and at age 3 years. At the 3-year examination physical activity was monitored by 24-hour accelerometry
(ActiGraph GT3X accelerometers) for 7 days. Anthropometry and body composition
were measured using standard methods and bio-electrical impedance-analysis, while
bone mass and bone density was measured by dual-energy X-ray absorptiometry
(DEXA). Background data and information regarding TV viewing and computer use
was obtained from parent completed questionnaires.
Physical activity during waking hours was filtered from body movements during
nocturnal sleep using three different methods to define time of falling asleep and time of waking up: 1) Individual and day specific times were defined based on accelerometer data, 2) individual times of waking up and falling asleep were generated from questionnaire data and 3) one fixed time-slot was calculated based on population means.
There are several methodological inconsistencies in accelerometry including a lack of
consensus regarding accelerometer cutpoints to define intensities of physical activity.
Therefore no final conclusions could be reached regarding the amount of time these
preschool children spent in physical activity and sedentary behaviour. However, the
results from this study indicate that a low cutpoint of 25 counts / 15 seconds (100 counts per minute) is more useful than a high cutpoint of 302 counts / 15 seconds to separate sedentary behaviour from physical activity in this age group. By application of the low cutpoint more than 50% of waking hours are defined as sedentary behaviour, and just below half of the time as time spend active. Only a very small amount of this time, 6-11% depending on the cutpoint applied, was spend in activity of moderate-to-vigorous physical intensity. Similarly, the proportion of children meeting the recommendation of at least 3 hours activity of at least light intensity was highly dependent of cutpoint.
When the high cutpoint defining sedentary behaviour was used, only few percentages of children complied with this recommendation, while all children were sufficiently active when the low cutpoint was applied.
The children were more active on days when they attended daycare, compared to days
when they did not attend daycare. This was mainly due to the time in daycare, which
was the time spent most active. The children were least active during morning hours
before daycare. There were large differences in activity levels between the least active
and the most active quartile of children. For the least active children there were no
significant differences in activity levels across settings. This also applied to the most
active children. Accordingly, some children were highly active irrespective of setting,
whereas the least active children maintained a constant lower level of physical activity
irrespective of setting.
Time spent in light, moderate and vigorous physical activity, and time spent sedentary
was independently associated with a range of health outcomes. Sedentary behaviour was positively related to higher levels of low-density lipo-proteins (LDL) and total
cholesterol, as well as increased heart rate. These associations were independent of time spent in moderate-to-vigorous physical activity. Light physical activity was positively associated with higher fat-free mass index, independently of total sedentary time. This probably reflected increased muscle mass. Moderate-to-vigorous physical activity was positively associated with fat-free mass and growth markers and inversely associated with heart rate independent of sedentary behaviour. Vigorous activity was positively associated with increased bone mass independent of sedentary behaviour. Likewise, complying with the recommendations of 1 hour of daily moderate-to-vigorous physical activity for children older than 5 years, was associated with more beneficial body composition outcomes, lower heart rate and, surprisingly, increased blood pressure.
The method used to separate physical activity during waking hours from body
movement during nocturnal sleep, had high impact on the final results of physical
activity levels. Individual and day specific times or individual times of falling asleep
and waking up are recommended over one fixed time-slot defining day time.
It seem reasonable to highlight the importance of healthy active living from the early
childhood by given recommendations for physical activity. However, the
recommendation of at least 3 hours daily activity of at least light intensity is less
suitable for this age group. A recommendation for preschool children should aim at
increasing physical activity in general and decreasing sedentary behaviour. Furthermore the importance of increased activity of moderate and vigorous intensity should be stresse