Academic journal article Research Quarterly for Exercise and Sport

Assessment of Differing Definitions of Accelerometer Nonwear Time

Academic journal article Research Quarterly for Exercise and Sport

Assessment of Differing Definitions of Accelerometer Nonwear Time

Article excerpt

Measuring physical activity with objective tools, such as accelerometers, is becoming more common (Troiano, 2005). Accelerometers measure acceleration multiple times within a given frequency and summarize this as a count over a pre-specified time period or epoch. The resultant count represents acceleration over the epoch length. Accelerometers eliminate biases associated with self-reporting measures of physical activity.

With the increasingly widespread use of accelerometers, standardization of how the data are collected and reported across studies is needed. In 2005, Masse and colleagues identified five methodological issues regarding reducing accelerometer data to derive summary measures: identifying accelerometer wearing time; defining minimal wear time for a valid day; identifying spurious data; computing summary variables and aggregating days of data; and extracting bouts of activity (Masse et al., 2005). Our study focuses on the first issue; specifically, identifying nonwearing time of the accelerometer.

Understanding whether the accelerometer is worn helps researchers assess compliance and determine if the participants' data will contribute to the resulting analyses. Defining wearing and nonwearing time also affects the derivation of summary physical activity measures, such as average counts per hour worn or minutes in sedentary, light, moderate, or vigorous activity (Corder, Brage, & Ekelund, 2007). Defining and assessing nonwearing time of the accelerometer is not currently standardized across research studies.

Participants typically wear accelerometers during waking hours and remove the devices for showering or swimming, although some devices are now waterproof. When an accelerometer is not worn, it will typically register as zero counts over the epoch. Yet when participants are motionless (e.g., sleeping or sitting) while wearing the accelerometer, it also registers zero counts over the epoch. This can occur for minutes at a time and is common among those who are sedentary throughout the day.

Some researchers recommended that participants keep a log to track when they are not wearing the monitors (Esliger et al., 2005; Trost, McIver, & Pate, 2005). The challenge with this approach is that log keeping introduces a self-reporting aspect of data collection, which can hamper compliance. Tracking logs were used in the National Health and Nutrition Examination Survey (NHANES) pilot study but subsequently dropped in the main study in 20034)4 (Troiano, 2006; Troiano et al., 2008).

A common alternative to log keeping is to deduce the most reasonable nonwearing time based on the accelerometer data. Researchers studying youth and adults using the ActiGraph accelerometer (Manufacturing Technology Inc., Pensacola, FL), for example, defined nonwearing times (i.e., when consecutive zeros are recorded) as 10 min (Brage et al., 2004; Ekelund et al., 2004; Riddoch et al., 2004), 15 min (Rousham, Clarke, & Gross, 2005), 20 min (Jilcott et al., 2007; Savitz et al., 2006; Treuth et al., 2004), or 60 min (Matthews et al., 2008; Troiano et al., 2008). Masse and colleagues explored the effect of differing definitions of nonwearing time on results, comparing 20 and 60 min of consecutive zeros as indicators of nonwearing times among a sample of 40-70-year-old African American and Hispanic women (Masse et al., 2005). However, this study also varied other accelerometer data reduction decisions, making the interpretation of the effects of varying nonwearing times difficult. We used data from the third Pregnancy, Infection, and Nutrition (PIN3) Postpartum Study to describe the effect of redefining accelerometer nonwear time on an aggregated sample of participants.

Method

Study Description

The PIN3 Study was a prospective investigation of preterm deliveries conducted at selected prenatal clinics in central North Carolina. The researchers reviewed new prenatal patients' medical charts and recruited women at their second prenatal visit before 20 weeks of gestation. …

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