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ARS Home » Northeast Area » Beltsville, Maryland (BHNRC) » Beltsville Human Nutrition Research Center » Food Components and Health Laboratory » Research » Publications at this Location » Publication #304378
Title: Determinants of variance in the habitual physical activity of overweight adults

Author
item PAUL, DAVID - University Of Idaho
item Kramer, Matthew
item STOTE, KIM - State University Of New York (SUNY)
item Baer, David

Submitted to: Journal of Physical Activity and Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2014
Publication Date: 5/1/2015
Citation: Paul, D., Kramer, M.H., Stote, K., Baer, D.J. 2015. Determinants of variance in the habitual physical activity of overweight adults. Journal of Physical Activity and Health. 12:680-685.

Interpretive Summary: Total physical activity (TPA) is an important component of a healthy lifestyle. In order to study the impact of TPA on health, methods are needed to measure and assess it properly. In order to know how and when to measure TPA, we need to identify and understand sources of within- and between-subject variances, and the influence of sex, body composition, and age. One important issue is to determine the number of days of data collection and the number of subjects necessary to estimate total physical activity (TPA) and moderate-to-vigorous physical activity (MVPA). Seventy-one adults wore accelerometers for seven day intervals over six consecutive months. Body fat and sex influenced TPA and MVPA. The sources of subject-related variation for TPA and MVPA were in descending order, within the individual, between the individuals, and from week-to-week. Based on within-individual variances, the error associated with estimating TPA and MVPA by only collecting one to seven days of data ranged from 12.4 to 28.1% for TPA and 28.0 to 63.3% for MVPA. Based on between-individual variances, detecting a 10% difference between two groups at a power of 90% requires approximately 175 and 625 subjects per group for TPA and MVPA, respectively. Estimates of MVPA are more variable than TPA in overweight adults, therefore more days of data are required to estimate MVPA and larger sample sizes to detect treatment differences for MVPA. These data provide a basis for estimating samples sizes and collecting data on TPA and are of interest to health professionals and scientists involved in assessing physical activity patterns.

Technical Abstract: The number of days of data and number of subjects necessary to estimate total physical activity (TPA) and moderate-to-vigorous physical activity (MVPA) requires an understanding of within-and between-subject variances, and the influence of sex, body composition, and age. Seventy-one adults wore accelerometers for seven day intervals over six consecutive months. Body fat and sex influenced TPA and MVPA. The sources of subject-related variation for TPA and MVPA were within-subject (62.6 and 66.1%), between-subject (31.0 and 30.6 %), and week-to-week (6.4 and 3.4%). Based on within-subject variances, the error associated with estimating TPA and MVPA by only collecting one to seven days of data ranged from 12.4 to 28.1% for TPA and 28.0 to 63.3% for MVPA. Based on between-subject variances, detecting a 10% difference between two groups at a power of 90% requires approximately 175 and 625 subjects per group for TPA and MVPA, respectively. Estimates of MVPA are more variable than TPA in overweight adults, therefore more days of data are required to estimate MVPA and larger sample sizes to detect treatment differences for MVPA. Log-transforming data reduces the need for additional days of data collection, thereby improving chances of detecting treatment effects.