Abstract
We have developed an instrument (GPAC) for measuring physical activity and providing a simultaneous measurement of location. The GPAC is comprised of an accelerometer, CSA Model 73–1024 Actigraph and a global positioning statellite receiver (Trimble Lassen SK-8). PURPOSE: To establish the validity and reliability of the GPAC for measuring physical activity and geographic location. METHODS: 20 subjects wore both the GPAC and an Actigraph while walking on a treadmill at 3,4, or 5 mph in a randomized order. In addition 24 subjects were asked to wear the GPAC and to record times, places, and activities. Typical locations visited were their homes, a classroom building, the outdoor running track, the Mall, a restaurant, etc. Activities included were walking, jogging, driving (car and/or bicycle). RESULTS: The Actigraph had significantly higher activity counts at all speeds than the GPAC (p < 0.001) but there was a significant correlation between the Actigraph and GPAC activity counts (r = 0.976, p < 0.001). Estimates of intrasession reliability for the GPAC ranged from 0.95 to 0.98 across the three speeds. Of 120 different waypoints visited by the subjects subsequent analysis of the latitude/longitude file enabled the naïve investigator to correctly track 109 of them (91%). CONCLUSION: The GPAC is a reliable valid tool for measuring physical activity with simultaneous measurement of geographic location.
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