Background: Research measuring physical activity behaviors during infancy is critical for evaluation of early intervention efforts to reduce rapid weight gain. There is little known about the physical activity patterns of infants, due in part to limited evidence for measurement procedures. This study sought to determine the minimal number of days and hours of accelerometry needed to reliably measure daily physical activity in infants using Generalizability (G) theory.
Methods: A total of 23 infants (14 female, 9 male) wore an accelerometer on the right ankle and right wrist for 7 days. Data were manually cleaned to remove activity counts not produced by the infant. G theory analyses were conducted on the average counts per epoch.
Results: Reliable estimates were observed with at least 2 days (G & Φ = .910) and 12 hours (G = .806, Φ = .803) at the ankle, and with at least 3 days (G & Φ = .906) and 15 hours (G = .802, Φ = .800) at the wrist.
Conclusions: These results provide some of the first guidelines for objective physical activity measurement during infancy. Accelerometer monitoring periods of at least 3 days including all daytime hours appear to be sufficient for reliable measurement.
Keywords: accelerometry; instrument psychometrics; measurement; physical activity.