Meeting U.S. Healthy People 2010 Levels of Physical Activity: Agreement of 2 Measures Across 2 Years

Background

Measuring the way people vary across time in meeting recommended levels of physical activity should be a fundamental component of public health surveillance. However, we were unaware of prospective cohort studies that had examined this in a population base using convergent measures.

Purpose

We examined agreement between two validated measures used to estimate periodic change in the rate of meeting U.S. Healthy People 2010 guidelines for participation in moderate or vigorous physical activity.

Methods

A cohort (N = 497) from a random, multiethnic sample of adults living in Hawaii was assessed every 6-months for 2 years starting spring 2004. Latent transition analysis classified people as meeting or not meeting the guidelines. Intra-class kappa statistics and multinomial logistic regression analysis were used to evaluate agreement.

Results

Agreement for classifying stable classes of people who met or did not meet the guideline each time was substantial for vigorous activity (kappa ∼ 0.65–0.70) but fair-to-moderate for moderate activity (kappa ∼ 0.38–0.48). Agreement was poorer for classifying people who transitioned between meeting and not meeting the vigorous guideline (kappa ∼ 0.45) or the moderate guideline (kappa ∼ 0.21–0.29).

Conclusions

Rates of meeting the guidelines varied across time and were estimated differently by the two measures, especially for moderate activity. This finding illustrates an understudied problem for public health promotion. Accurate classification of change within people is necessary for determining exposure in outcome studies, personal determinants of sufficient activity, and for evaluating whether interventions are successful in sustaining increases in rates of meeting physical activity guidelines.

Introduction

Despite its importance for public health (1), the physical activity of many U.S. adults is below levels regarded as sufficient for health promotion (2). Although three of four adults say they engaged in some form of leisure-time physical activity during the past month (3), less than half participated regularly at recommended levels (4) of moderate physical activity for at least 30 minutes 5 or more days per week or vigorous physical activity for at least 20 minutes 3 or more days per week 5, 6. Insufficient physical activity is more likely among women, minorities, and those having low socioeconomic status (7).

Surveillance systems that estimate point-prevalence of physical activity in successive years monitor group trends in a population 8, 9. However, they do not provide measures of change within people or how patterns of change differ between people, which are prerequisites to quantifying exposure in outcome studies and identifying environmental and interpersonal factors that might be modified to increase the rate of meeting physical activity guidelines (10). Measuring within-person change with the use of a prospective cohort design has not been feasible in national surveys, but it can be more easily achieved in smaller cohorts. As far as we knew, this had not as yet been done in a population of adults.

Consistent with reporting standards (11), we describe here periodic (i.e., every 6 months) change across 2 years in the rates of meeting the U.S. Healthy People 2010 recommendations for regular participation in moderate or vigorous physical activity (6) in a cohort of adults living in Hawaii. Population surveys have mainly been limited to self-reporting by participants, which will vary among methods. Hence, our main purpose was to determine the agreement between two widely used measures of physical activity for classifying whether people met the recommendations. Commonly used, validated measures were used that were feasible for administration by interview in a population-based survey and that provided estimates of weekly time spent in moderate and vigorous physical activity during the past 7 days (12) or in a week (13).