Search

The primary aim of this pilot study was to assess how well the combination of global positioning system (GPS) and accelerometer data predicted different activity modes.Ten adults (seven male, three female; 23-51 yr) simultaneously wore a GPS unit and accelerometer during bouts of walking, jogging/running, bicycling, inline skating, or driving an automobile. Discriminant function analysis was used to identify a parsimonious combination of variables derived from accelerometer counts and steps and GPS speed that best classified mode. A total of 29 bouts were used to develop this classification criterion. This criterion was validated using two datasets generated from the complete collection of minute-by-minute values from all bouts.Model development with "calibration" data showed that two accelerometer variables alone (median counts and steps) resulted in 26 of 29 bouts (90%) being correctly classified. Prediction of activity mode using counts and steps in a minute-by-minute "validation" dataset (N = 200) was 86.5%. Using three variables from the accelerometer and GPS (median counts, steps and speed) resulted in correct classification in 27 of 29 activity bouts in the "calibration" data (93%). In the "validation" dataset comprising 200 min, the combination of accelerometer counts and steps and GPS speed were able to correctly classify 91% of the observations. Walking and bicycling minutes were correctly classified most frequently (96%). In another "validation" dataset consisting of activity bouts, this combination of variables resulted in correct classification in 42 of 43 bouts (98%).This pilot study provides evidence that the addition of GPS to accelerometer monitoring improves physical activity mode classification to a small degree. Larger studies among free-living individuals and with an expanded range of activities are needed to replicate the current findings and further determine the merits of using GPS with accelerometers for mode identification.

Purpose: To assess test-retest reliability and validity of the Youth Risk Behavior Survey (YRBS) items for moderate and vigorous physical activity in middle school students. Methods: Students (N = 125; 12.7 ± 0.6 yr) wore Actigraph accelerometers for 6.1 ± 1.0 d and twice completed surveys that included YRBS moderate and vigorous physical activity items. Accelerometer counts were transformed into minutes of moderate (3-6 METs) and vigorous (> 6 METs) physical activity. Days per week meeting moderate and vigorous physical activity recommendations were estimated using four summary methods. Reliability was assessed using intraclass correlation coefficients (ICC) from the two surveys. Validity was assessed as percent concordance, kappa coefficients, and sensitivity and specificity using binary YRBS and Actigraph outcomes. Results: Test-retest ICC for the moderate and vigorous physical activity items were 0.51 and 0.46, respectively. Twenty-two percent of students met the recommended level of moderate physical activity (= 30 min·d-1, = 5 d·wk-1) according to self-reports, whereas 90.4 and 66.4% met the recommendation according to accumulated accelerometer minutes and 5-min-bout criteria, respectively. Concordance between YRBS and Actigraph moderate physical activity measures was highest using accumulated accelerometer minutes. Sensitivity of the moderate YRBS item ranged from 0.19 to 0.23 for four comparisons, and specificity was 0.74-0.92. More than two thirds of students reported vigorous physical activity at recommended levels (= 20 min·d-1, = 3 d·wk-1), whereas the highest prevalence according to Actigraph monitoring was 22.4%. Sensitivity of the YRBS vigorous item was high (0.75-0.92) compared with the four Actigraph measures; specificity was low (0.23-0.26). Conclusion: YRBS questions underestimate the proportion of students attaining recommended levels of moderate physical activity and overestimate the proportion meeting vigorous recommendations. Use of accelerometry for physical activity surveillance seems to be indicated. At the minimum, new questions demonstrating greater validity are needed.