Daily variations of physical activity in the elderly remain unclear. We thus used a uniaxial accelerometer/pedometer to examine the variability of step counts for 1 yr, determining the minimum number of days observation needed to obtain reliable estimates of annual physical activity.
Subjects were 37 males and 44 females, healthy Japanese, aged 65-83 yr. The pedometer was worn on the waistband throughout 1 yr, accumulating information on the individual's daily step count.
The step count spectrum showed peaks with periods of 2.3, 3.5, and 7.0 d and an aperiodic component that had a greater power at low frequencies (i.e., non-white noise). These characteristics were absent in randomly resequenced data. To ensure that 80% of total variance was attributable to between-subjects variance, 25 and 8 consecutive days of observation were needed in male and female subjects, respectively. To achieve 90% on this same measure of reliability, 105 and 37 consecutive days of observation were required. In contrast, 4 d of randomly timed observations yielded 80% reliability for both men and women, and 11 and 9 d gave 90% reliability in men and women, respectively. If sampling also took account of season and day of the week, the respective observation periods for men and women were reduced to 8 and 4 d (i.e., 2 and 1 consecutive days of sampling every 89 d) for 80% and to 16 and 12 d (i.e., 4 and 3 consecutive days every 89 d) for 90% reliability.
When estimating annual step counts, seasonal and/or random sampling of data allows collection of reliable data during substantially fewer days than needed for consecutive observations.
1Department of Work Stress Control, Japan National Institute of Occupational Safety and Health, Kanagawa, JAPAN; 2Exercise Sciences Research Group, Tokyo Metropolitan Institute of Gerontology, Tokyo, JAPAN; and 3Faculty of Physical Education and Health, University of Toronto, Ontario, CANADA
Address for correspondence: Yukitoshi Aoyagi, Ph.D., Exercise Sciences Research Group, Tokyo Metropolitan Institute of Gerontology, 35-2 Sakaecho, Itabashi-ku, Tokyo 173-0015, Japan; E-mail: firstname.lastname@example.org.
Submitted for publication May 2007.
Accepted for publication January 2008.