Determining activity monitor wear time: an influential decision rule.

BACKGROUND: Accurately measuring physical activity (PA) with activity monitors requires sufficient monitor wear time which can be difficult to assess. Monitor sensitivity to movement and population characteristics (eg, children vs. adults) may dictate the duration of monitor inactivity indicative of nonwear. A standardized method for determining appropriate decision rules to identify wear time is needed.
METHODS: Several decision rules based on minimum durations of monitor inactivity (ie, 60, 90, 120, 150 minutes) to identify nonwear were applied to Stepwatch Activity Monitor data from 1064 adult bariatric surgical candidates. The frequency, pattern, and duration of resulting nonwear and wear periods were examined. Generalized Estimating Equations tested the effect of these decision rules on PA measures.
RESULTS: A 60-minute duration resulted in unreasonably large percentages of subjects with unlikely wear patterns [eg, ≥3 nonwear periods in a day (29.9%); ≥2 wear periods of less than an hour in a day (28.7%)]; 120 minutes appeared most reasonable. Wear time decision rules impacted PA measures.
CONCLUSIONS: The methods described in this paper can be used to determine appropriate instrument and population specific wear time decision rules. Recognizing monitor wear time is estimated, PA measures least affected by wear time are preferable.