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n South African Journal of Sports Medicine - Monitor placement, sources of variance and reliability of free-living physical activity : a pilot investigation : original research article
Objectives. We investigated the effect of monitor placement on the sources of variance and reliability of objectively measured free-living physical activity (PA).
Design. A convenience sample (N=7; 3 females, 4 males) of rural, adult, black South Africans was recruited from the plantation section of a local lumber mill. PA was assessed using two uni-axial accelerometers placed on the right hip (RH) and left hip (LH), over 3 weekdays. PA indices were total counts, average counts, inactivity (0 - 499 counts.min-1), moderate-1 activity (500 - 1 951 counts.min-1) and moderate-2+vigorous activity (≥1 952 counts.min-1).
Results. Accelerometer output did not differ across trials for either hip placement (p>0.2). There were no significant differences between RH-LH for any accelerometer variable (p>0.1) and effect sizes were small (0.02 - 0.15). Monitor position did not contribute any variance to accelerometer variables. Variance due to monitor unit contributed <2% to raw and derived accelerometer variables. Coefficients of variation derived from the standard deviation of RH-LH transformed differences ranged from 2.7% to 10.5%, except for moderate-1 and moderate-2+vigorous variables (16.0 - 72.8%). Intraclass correlation coefficients (ICC, RH-LH) were >0.90, except for moderate-1 time accumulated in bouts ≥10 min (ICC=0.83). Agreement between RH-LH for achieving CDC / ACSM PA guidelines was significant (kappa=0.79, p=0.002)
Conclusions. Firstly there was no statistically significant difference between RH-LH for any accelerometer variable. Secondly, accelerometer units accounted for little of the variance in accelerometer variables. Thirdly, greater variability in monitor placement was apparent for moderate-1 and moderate-2+vigorous variables.
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