Objective: Examine the validity of using high-density electronic ecologic momentary assessment (EMA) to assess physical activity. EMA was further used to explore within- and between-subject variability in adolescent physical activity (PA) patterns.
Methods: Adolescents (n=526, 51% male) participated in EMA waves occurring approximately every 6 months between the 9th and 12th grade. Each wave extended over 4 consecutive days (Thursday to Sunday). Using a Palm III handheld computer, each participant reported his or her primary activity (e.g., exercise, walking, homework) every 30 (+/-10) minutes during waking hours. Heart rate (via Polar heart rate monitor) and activity counts (via wrist accelerometer) were simultaneously assessed during the EMA intervals.
Results: Overall, heart rates and accelerometer counts were greater for diary-reported exercise and walking than for nonphysical activities (p's<0.001). EMA revealed that the typical duration of exercise sessions was longer than walking sessions (p<0.05). Rates of walking and exercise were more consistent between waves (i.e., across high school) than within waves (i.e., across the 4 days of monitoring), most likely due to the significantly higher rates of walking and exercise occurring on weekdays as compared to weekend days (p's<0.001). Average rates of walking were greater for girls than boys, and the reverse was true for exercise. Rates of both walking and exercise declined steadily between 9th and 12th grade (p's<0.001).
Conclusions: Physical activity reported via EMA corresponded to objective activity indicators. EMA yielded information about within-person variability in PA that cannot be obtained readily from traditional self-report instruments. Given its potential for simultaneously assessing important physiologic, psychological, and contextual factors, EMA presents a promising approach to studying adolescent physical activity.