Day-to-day variability in older adults' physical activity: How many days are needed for reliable data? Rowe, D. A.; Kemble, C. D.; Robinson, T. S.; Mahar, M. T. Activity Promotion Laboratory, Department of Exercise and Sport Science, East Carolina University, Greenville, NC, 27858 Rationale Results Summary/Conclusion Background: In recent years, pedometers and accelerometers have gained widespread acceptance in the exercise science community as objective methods for measuring physical activity. For monitoring older adults' physical activity in clinical and research settings, it is important to know how many days of data are needed in order to obtain a reliable estimate of physical activity. Previous studies have investigated day-to-day variability in the physical activity of children and young to middle aged adults (e.g., Robinson, 2005; Rowe at al, 2004; Tudor-Locke et al., 2004). However, there are no published studies in an older adult population. Purpose: The purpose of this study was to determine the day-to-day variability in physical activity of older adults, measured via pedometers and accelerometers. Descriptive statistics: Average Yamax steps ranged from 4,350 ( 3,134) to 5,214 ( 3,627) steps/day, and average minutes of MVPA ranged from 11.7 ( 16.2) to 16.2 ( 23.2) mins/day (this variable was positively skewed). Reliability: Intraclass correlation coefficients for between 2 and 7 consecutive days for the five outcome variables are presented in Table 1. Day-to-day variability for all outcome variables was generally low (i.e., reliability was high). ICCs for 7 days were .96 (Yamax steps, n = 81), .93 (Actigraph steps, n = 62), .93 (Actigraph counts, n = 62), .91 (Actigraph mins MVPA, n = 62), and .86 (Actigraph minBouts MVPA, n = 62). Minimally acceptable reliability (defined as ICC ≥ .80) required only 2 days for all variables (ICC = .90 for Yamax steps, n = 89; ICC = .87 for Actigraph steps, n = 72; ICC = .90 for Actigraph counts, n = 73; and ICC = .82 for Actigraph mins MVPA, n = 73), except Actigraph minBouts MVPA, which required 6 days (ICC = .80, n = 64). No significant differences among days were found for any variable (F-ratios ranged from 0.67 to 1.73, all p > .05). Summary: Day-to-day variability in physical activity was low in this sample of older adults. This was demonstrated via two objective instruments and several outcome variables. The only outcome variable that did not demonstrate low variability/high reliability was daily minutes of MVPA occurring in bouts of at least 10 minutes. Conclusion: Low variability may be due to the primarily retired status of older adults, meaning that weekdays and weekend days are not systematically different. However, until this finding is replicated in other older adult samples, we recommend that at least one weekday and one weekend day be included in data analysis on older adults. It appears from the current study that fewer days of data may be needed to obtain stable estimates of daily physical activity in older adults than other populations. Methods Table 1 Interday reliability (CI95) of physical activity measures in older adults i Measure 2 days 3 days 4 days 5 days 6 days 7 days Yamax Steps .90 (.85-.94) .92 (.89-.94) (.89-.95) .93 (.90-.95) .94 (.92-.96) .96 (.94-.97) Actigraph Steps .87 (.79-.92) .88 (.82-.92) (83-.92) .89 (.85-.93) .91 (.88-.94) (.90-.96) Actigraph Counts (.83-.93) (.84-.93) (.86-.93) Actigraph mins MVPA .82 (.71-.88) .85 (.78-.90) .83 (.75-.89) (.83-.92) Actigraph minBouts MVPA .58 (.33-.74) .77 (.66-.85) .64 (.48-.76) (.68-.85) .80 (.71-.87) .86 (.79-.90) Participants: Adults over 60 years (N = 91; 67% female; 85% retired; mean age = 74  9 years, mean BMI = 26.4  5.3 kg/m2) were recruited from the general population via newspaper advertisements, and direct contact with local agencies such as church groups, community centers, assisted living centers, and recreational fitness groups. Instruments: Demographic information was gathered via a self-report questionnaire on age, gender, height, weight, education level, and retirement status. Physical activity was measured objectively with Yamax SW-200 pedometers (Yamax Corp., Tokyo, Japan) and Actigraph model 7164 accelerometers (Actigraph LLC, Fort Walton Beach, FL). Procedures: Participants wore both instruments for 7 consecutive days, starting on a random day of the week. Following data cleaning and screening, n = 81 and n = 62 participants provided 7 complete days of useable data for the Yamax and Actigraph, respectively. The data for this study were collected over a period of 18 months. All procedures were approved by the University Medical Center Institutional Review Board. Data analysis: Interday reliability was estimated using two-way ANOVA intraclass coefficients (ICC) for between 2 and 7 consecutive days, for the following variables: steps (Yamax); and steps, activity counts, and minutes of MVPA (Actigraph). Minutes of MVPA were defined as > 3 METs, and were estimated via the 3 MET cutpoint of Freedson et al. (1998), i.e., > 1951 cts/min. Additionally, minutes of MVPA occurring in bouts of at least 10 mins (minBouts) were also calculated. one-way repeated measures ANOVA was used to test for mean differences among days for all variables. References Activity Promotion Lab Promoting Active Lifestyles Freedson PS, Melanson E, Sirard J. Calibration of the Computer Science and Applications, Inc. accelerometer. Med Sci Sports Exerc. 1998;30(5):777-781. Robinson TS. Monitoring of physical activity in 40-60 year old adults using activity monitors. Unpublished dissertation, East Carolina University, Greenville, NC; 2005. Rowe, D, Mahar, M, Raedeke, T, Lore, J. Measuring physical activity in children with pedometers: Reliability, reactivity, and replacing missing data. Pediatr. Exerc. Sci. 2004;16:343-354. Tudor-Locke C, Burkett L, Reis J P, Ainsworth BE, Macera CA, Wilson DK. How many days of pedometer monitoring predict weekly physical activity in adults? Prev Med. 2005;40:293-298.