A Timely Meeting: Objective Measurement of Physical Activity

Abstract

Accurate and reliable assessment of physical activity remains an important challenge for epidemiologists, exercise scientists, clinicians, and behavioral researchers. The desire to explain and stem the dramatic increase in overweight and obesity prevalence observed among youths and adults has focused attention on the need for better measures of physical activity. These measures fall into three general categories: direct observation, subjective reports, and portable monitors, such as accelerometers. These monitors measure body movement in terms of acceleration, which can then be used to estimate physical activity intensity. As recently as 1999, accelerometry was still considered to be in the developmental stage. In October of that year, the Cooper Institute hosted a meeting titled “Measurement of Physical Activity.” One of the conclusions from the meeting was that objective motion sensors were not practical for large-scale studies because of high cost, uncertain reliability, and difficulties in the interpretation of data (1). Since then, the scientific community has responded vigorously to the need for improvements in this field with technological developments and novel applications of existing and new technology. The published scientific literature provides a demonstration of the magnitude of this response. Figure 1 shows the trend in scientific articles related to accelerometry. The data were obtained by searching the Scopus bibliographic database (www.scopus.org, accessed on June 23, 2005) for original research articles or reviews that included the terms “accelerometer or accelerometry” and “physical activity” in the title, key words, or abstract. Between 1981 and 1996, it was rare to find more than 10 articles per year. From 1997 on, the increase was dramatic, with almost 90 articles per year in 2003 and 2004. These articles encompass reports on accelerometer device development, calibration and validation studies, and behavioral research studies that use the devices. These data suggest that accelerometry technology and its applications have progressed significantly.FIGURE 1—Trends in accelerometer articles.Although much of the growth in accelerometry use involves research applications, investigators also have begun to explore the use of the devices in population surveillance. As this supplement goes to press, objectively measured physical activity data from the 2003–2004 cycle of the National Health and Nutrition Examination Survey (NHANES) are about to be released for public access by researchers. These data were collected on approximately 7000 survey participants ages 6 yr and older who were asked to wear an accelerometer during their waking hours for seven consecutive days. The collection of accelerometry data in NHANES represents the largest implementation of objective physical activity monitoring to date. Despite evidence of increasing use of objective methods of physical activity assessment, further development is still needed. Reliable accelerometry devices are available, but their costs are still high (though not prohibitive). In addition, interpretation of the resulting data remains a challenge because of gaps and inconsistencies in the calibration and validation literature and unresolved issues of statistical analysis of the large quantity of data produced by accelerometers. These remaining gaps are the focus of papers in this supplement, which represents the culmination of a process that began in 2003. In December of that year, Dr. Dianne Ward, Professor and Director of the Intervention and Policy Division, Department of Nutrition at the University of North Carolina, proposed a conference to address questions that researchers encounter in their attempts to monitor physical activity using accelerometers. Support for the conference was provided to the University of North Carolina by Get Kids in Action, a partnership between the University and the Gatorade Company. The National Cancer Institute funded publication of the proceedings. A committee, which included Drs. Oded Bar-Or, Diane Catellier, Kong Chen, Kelly Evenson, Patty Freedson, Janet Fulton, Charles Matthews, Robert McMurray, Richard Troiano, and Stewart Trost was constituted to plan the conference. The committee agreed that the meeting should have the following objectives: Bring together experts in objective physical activity assessment to examine gaps in the knowledge on the use of accelerometers to assess physical activity. Address questions related to the implementation of accelerometers in research studies, such as the number of days required to represent a person’s activity, the number of hours needed to represent a valid day, and appropriate length of accelerometer epochs to use. Propose optimal methods of data analysis that address such issues as handling missing data, producing summary measures, and minimizing bias due to incomplete data. Present monitor calibration data for adults and children that could assist in establishing c