Abstract
Physical activity (PA) in infancy is an under researched area despite the known relevance of PA to health. This thesis combines three distinct studies to achieve a positive contribution to knowledge in the area of PA and its measurement in the first year of life. The aims were to: (1) determine if accelerometers could measure PA in six month old children and (2) to determine if behavioural intervention could increase (a) tummy time at four and six months of age and (b) physical activity as measured by accelerometer at twelve months of age.
In the validation study three accelerometers were placed on six month old infants (n = 34) at the wrist, waist and ankle. Four activities were then performed by the infants and their caregivers. Accelerometer output was compared with that obtained from a doll wearing similarly placed accelerometers performing the same activities to clarify how much activity was attributed to the infant and how much was derived from the caregiver. Comparison with the accelerometers on the doll indicated that movement of the caregiver accounted for 23% to 55% of the infants’ activity counts during some activities. The accelerometer counts from site of placement differed for all comparisons (P<0.05). Accelerometer use at this young age may be restricted to sleep/wake cycles due to the lack of independent mobility of the infant.
Participants in the Prevention of overweight in infancy study (POI.nz) activity intervention groups Feeding Activity and Breastfeeding (FAB) and Combo (FAB and Sleep), (n=401) were offered group activity education sessions where the focus was on encouraging parents to be active with their children. Basic activities were suggested that could be carried out with the infant, such as tummy time, to enhance both infant activity and participation with the family. The other two groups received usual well child care (Control) or another intervention focussing on the establishment of health sleep patterns (Sleep). Parental questionnaires were administered when infants were 19 weeks, 27 weeks, and one year old to assess the effect of the activity intervention. The two outcome variables of interest in this thesis were the amount of tummy time per day offered at 19 and 27 weeks, and PA measured using accelerometry at 12 months. Tummy time was used by 95% of families in the study however the intervention groups used it more frequently at both time periods (19 weeks IRR 1.15 95% CI 1.03-1.28, P = 0.012, and 27 weeks IRR 1.15, 95% CI 1.02-1.30, P = 0.027). Total tummy time was significantly related to milestone achievement for crawling (19 weeks OR 1.14 95% CI 1.08-1.20, P<0.001; 27 weeks OR 1.06 95% CI 1.02-1.09, P = 0.001), and walking assisted (19 weeks OR 1.36 95% CI 1.13-1.63, P = 0.001; 27 weeks OR 1.15 95%CI 1.07-1.24, P<0.001). The POI.nz study was successful in achieving a large database of valid accelerometer data at 12 months of age (n=408). Intervention group effects were evident for the outcomes PA counts per minute (CPM), and time restrained. The COMBO group had significantly higher CPM on three of the five days measured and less time restrained in car seats or strollers (P = 0.041). Of most significance to the literature in this area was that there were significant differences in CPM between walking and non-walking infants (P<0.001), highlighting the difficulties observed in the validation study of using objective measures of PA at this age when infants’ movement is dependent on caregivers if they are not walking independently.
Conclusion: Measurement of PA in infancy is difficult due to developmental immaturity and objective measures used to date do not accurately capture infants’ PA. Intervention to increase PA appears to have been effective on some levels and may have wider application in the public health arena.