Daily physical activity is essential for children’s health and wellbeing. Physical inactivity is becoming increasingly evident in children from developed countries. Alongside these declines in physical activity are increased rates of childhood obesity. Children with intellectual disabilities (ID) are at a greater risk for obesity and this risk is associated with inactivity. However, due to measurement difficulties, limited studies have assessed physical activity in children with ID. Their refusal to be instrumented and discarding of devices are just some of the problems commonly encountered by researchers when attempting to objectively measure physical activity in children with ID. The purpose of this thesis was to explore an objective measure of physical activity that is accurate, tolerable and reliable for use in children with ID. The new technology may be employed to monitor trends in physical activity levels and to assess the effects of health interventions.

A Sensor Pressure System (SPS) was the result of a collaborative effort between the AUT Health Science and Engineering Schools. The device consisted of three force sensors, positioned under the inner metatarsal, outer metatarsal and heel, which measured participants’ applied weight during walking. Physical activity data were presented in steps. Twenty six children without ID were recruited from intermediate schools and holiday programmes. Nine children with mild ID were recruited from special schools. Out of the 26 typically developing children, ten participated in the test-retest reliability study of the SPS in laboratory conditions, ten participated in the validity study in laboratory conditions and ten participated in the feasibility study in free living conditions. Validity and feasibility of the SPS were assessed against direct observation in nine children with ID during a PE class.

In children without ID the inner metatarsal position displayed the highest reproducibility (ICC = 0.86-0.95). Compared to collection at other positions, data collected by the force sensor on the inner metatarsal were the least likely to contain measurement error. Overall, the SPS was more reliable during slow speeds (ICC = 0.65-0.95). Significant Pearson’s correlations were observed between inner and outer metatarsal positions and directly observed steps at all speeds (r =0.84-0.96). Overall, the SPS captured ± 12 % of directly observed steps. Most children without ID wore the SPS for seven days. Weather, household rules and activity type were the main limitations to the device. In children with mild ID, low Pearson’s correlations were observed between the SPS and directly observed steps (r =0.13 to -0.37). The SPS underestimated directly observed steps by 8 to 28 % in children with ID. While there were no issues with children refusing to be instrumented, the SPS was not completely inconspicuous, and some children with ID touched the device while it was on their shoe.

The SPS is a valid and reliable measure of physical activity in children without ID. It is a promising tool for use in children with ID. Further work is required for the SPS to become completely inconspicuous, and accurately measure step counts in children with ID.