Number of Days of Monitoring Needed with Accelerometers and Pedometers to Obtain Reliable Estimates of Habitual Physical Activity in Adults T. S. Robinson, D. A. Rowe, M. T. Mahar Activity Promotion Laboratory, Department of Exercise and Sport Science, East Carolina University, Greenville, NC, 27858 Abstract The number of days of assessment necessary to obtain reliable estimates of habitual physical activity differs for measurement method and population assessed. Tudor-Locke et al. (2005) suggested that at least three monitoring days with pedometers are necessary to sufficiently estimate free-living physical activity over one week in adults. PURPOSE: The purpose of this study was to establish a minimal monitoring period necessary to reliably assess habitual physical activity in adults (40 to 60 years) with pedometers and accelerometers. METHODS: Seven consecutive days of data were collected on 120 participants (93 females and 27 males; age = 49.0 ± 5.6 years; BMI = 27.9 ± 5.6) who wore a Yamax SW-200 pedometer and an Actigraph accelerometer. Monitors were randomly selected from among 50 pedometers and 52 accelerometers. Average daily steps, average daily activity counts, and daily minutes in moderate to vigorous physical activity (MVPA) were calculated. Cut-points of Freedson et al. (1998) were used to establish MVPA from the Actigraph. Two-way intraclass correlations (R) were used to determine the number of days of monitoring needed to achieve a reliability of .80. Analyses were conducted on participants who had complete data for all 7 days. RESULTS: Complete data for 7 days were obtained from 97 participants for the Yamax and from 59 participants for the Actigraph. Average daily steps were 10,091 ± 4,124 from the Yamax and 11,885 ± 3,409 from the Actigraph (p < .01). Reliability estimates (95% CI) for 7 days of monitoring were .88 (.84 to .91) for Yamax steps, .84 (.77 to .89) for Actigraph steps, .85 (.78 to .90) for Actigraph activity counts, and .82 (.74 to .88) for Actigraph MVPA. The number of days needed to obtain an acceptably reliable measure of activity was 4 days (R = .80) for Yamax steps, 5 days (R = .81) for Actigraph steps, 5 days (R = .81) for Actigraph activity counts, and 6 days (R = .80) for Actigraph minutes of MVPA. CONCLUSION: The number of days of monitoring necessary to obtain acceptably reliable estimates of usual physical activity in adults differs slightly by measurement method and type of outcome variable expressed. A slightly longer measurement schedule is needed for Actigraph accelerometers than for Yamax pedometers. In addition, if results are to be expressed as minutes of MVPA, 6 days may be needed to assess usual physical activity. Results Summary/Conclusion Summary: The number of monitoring days needed to estimate habitual physical activity in a sample of 40 to 60 year old adults was between 4 and 6 days depending on the outcome variable used. The number of days needed to obtain an acceptably reliable measure ( R ≥ .80) of activity was: 4 days (R = .80) for Yamax steps 5 days (R = .81) for Actigraph steps 5 days (R = .81) for Actigraph activity counts 6 days (R = .80) for Actigraph minutes of MVPA Another important aspect of physical activity monitoring is to include both weekdays and weekend days when attempting to estimate usual activity level, which was accomplished using random start days for each participant. Conclusion: The number of days of monitoring necessary to obtain acceptably reliable estimates of usual physical activity in adults differs slightly by measurement method and type of outcome variable expressed. A slightly longer measurement schedule is needed for Actigraph accelerometers than for Yamax pedometers. In addition, if results are to be expressed as minutes of MVPA, 6 days may be needed to assess usual physical activity. Variable Mean SD Age (yrs) 49.0 5.6 Body mass index (kg.m-2) 27.9 5.6 Steps per day (Yamax) 10,295 5,251 Steps per day (Actigraph) 11,492 5,080 Activity counts per day (Actigraph) 334,970 190,163 Minutes of MVPA (Actigraph) 30.5 34.3 Introduction Background: The length of time necessary to assess physical activity to get an accurate estimate of usual physical activity is unknown. Few researchers have examined the specific number of days of monitoring necessary to obtain a reliable estimate of habitual activity. A reliability standard of .80 has been suggested for estimating acceptable monitoring frames. Tudor-Locke et al. (2005) suggested that at least three monitoring days with pedometers are necessary to sufficiently estimate free-living physical activity over one week in adults. Differences among populations and among monitoring instruments makes it necessary for more research to establish the appropriate monitoring period. Purpose: To establish a minimal monitoring period necessary to reliably assess habitual physical activity in adults (40 to 60 years) with pedometers and accelerometers. i Reliability (CI95) for the Number of Monitoring Days Measure 1 days 2 days 3 days 4 days 5 days 6 days 7 days Yamax Steps .51 (.42-.60) .66 (.49-.77) .74 (.64-.82) .80 (.72-.86) .84 (.78-.86) .86 (.81-.90) .88 (.84-.91) Actigraph Steps .42 (.32-.55) (.42-.80) .70 (.53-.81) (.61-.84) .81 (.72-.88) .83 (.75-.89) (.77-.89) Actigraph Activity Counts .45 (.34-.57) .54 (.23-.73) .63 (.44-.77) (.62-.84) (.73-.88) (.76-.89) .85 (.78-.90) Actigraph Minutes of MVPA .39 (.29-.52) .69 (.48-.81) .67 (.49-.79) .71 (.57-.81) .78 (.67-.85) (.70-.87) .82 (.74-.88) Methods Participants: 120 participants aged 40 to 60 years (mean age = 49.0 ± 5.6 years) 78% female, 85% White 84% some college education or higher, 98% were not retired Instruments: Demographic information on were gathered via a self-report questionnaire. 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. 7 days of complete data was obtained for the Yamax on 97 participants and for the Actigraph on 59 participants. Data Analysis: Outcomes examined: Steps per day Activity counts per day Minutes of moderate to vigorous physical activity (MVPA) per day. Cut-points of Freedson et al. (1998) were used to establish MVPA from the Actigraph [all counts > 1951 cts.min-1]. Two-way intraclass correlations (R) were used to determine the number of days of monitoring needed to achieve a reliability of .80. Analyses were conducted on participants who had complete data for all 7 days. Note: The Spearman-Brown prophecy formula was used to estimate reliability for 1 day from reliability of 7 days. Activity Promotion Lab Promoting Active Lifestyles References Freedson PS, Melanson E, Sirard J. Calibration of the Computer Science and Applications, Inc. accelerometer. Med Sci Sports Exerc. 1998;30(5):777-781. 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.