Assessing physical activity in persons with rheumatoid arthritis using accelerometry.

PURPOSE: To investigate empirically if the nonwear threshold and the "valid day" definition for accelerometer data from the general adult US population are appropriate for accelerometer data from persons with rheumatoid arthritis (RA).
METHODS: This study analyzed data from 107 persons with RA participating in the baseline (2006-2008) accelerometer assessment from two studies with common inclusion/exclusion criteria. We examined candidate nonwear thresholds ranging from 20 to 300 min of zero activity count. The effect of the selected nonwear threshold is examined in regard to 1) mean daily activity counts, 2) activity counts per wear hour, 3) mean daily minutes of moderate to vigorous physical activity (MVPA) according to count thresholds that occur in 10-min bouts, and 4) MVPA bout minutes per wear hour. The effect of ranging the definition of a valid day of accelerometer data from 8 h of wear time to 12 h on data retention was also examined.
RESULTS: In 737 d of accelerometer data analyzed, the average daily wear hours increased with length of nonwear threshold of allowed continuous zero activity count minutes. The mean number of nonzero activity count minutes increased with the chosen nonwear threshold until it stabilized at 478 min.d of activity, which corresponded to the 90-min nonwear threshold. Choosing this threshold and requiring at least 10 h of wear time to constitute a valid day were associated with 92.8% of days of collected data defined as "valid."
CONCLUSIONS: Data supported increasing the allowed nonwear threshold in this RA subpopulation from 60 to 90 min, while retaining the 10-h day as the measure of the "valid day."