Differential Accuracy Of Physical Activity Self-Report By Body Mass Index

Abstract

Many epidemiologic studies rely on self-reported measures such as the International Physical Activity Questionnaire-Short Form (IPAQ-S) in their assessment of physical activity. Previous work in this population showed moderate correlations between this self-report measure and accelerometer data which varied by gender. What remains unknown is whether reporting varies by body mass index (BMI). PURPOSE: To determine whether agreement between self-reported physical activity and accelerometer measures varies by BMI. METHODS: 135 Blacks residing in low-income housing completed the IPAQ-S and wore an accelerometer for up to 6 days. Both 1- and 10-minute accelerometer bouts were used to define time spent in light, moderate, and vigorous physical activity. Trained research staff measured participant height and weight. BMI was calculated as weight in kilograms divided by height in meters squared. RESULTS: There were 31 people with BMI of 18-24.9, 30 with a BMI of 25-29.9 and 74 with a BMI of 30 or greater. Agreement between the IPAQ-S and accelerometer determined physical activity did vary by BMI. For a 10-minute bout, correlations were 0.38 (BMI <25), 0.57 (BMI 25-29.9) and 0.15 (BMI 30+). For a 1-minute bout, correlations were 0.34, 0.44, and 0.24 respectively. When we classified participants based on attainment of physical activity recommendations, agreement was much better among non-obese persons and significantly lower for 10-minute bouts compared to 1-minute bouts (non-obese: 1-minute kappa = 0.48, 10-minute kappa = 0.023; obese: 1-minute kappa = -0.024, 10-minute kappa = -0.020). CONCLUSION: Correlations between self-reported physical activity and accelerometer data were consistently lowest for obese persons compared to those of lower BMI. When classifying participants according to attainment of physical activity recommendations, there was no agreement between instruments among obese persons for either bout length. Among those with a BMI of 18-29.9 there was moderate agreement for a 1-minute bout, and no agreement for a 10-minute bout.