Effect of BMI on pedometers in early adolescents under free-living conditions

Pedometers may provide valuable information regarding the ambulatory patterns of adolescents, but the effects of body mass index (BMI) on pedometer accuracy in this population are unknown.The purpose of this study was twofold: 1) to determine whether the New Lifestyles NL-2000 (NL) and the Digi-Walker SW-200 (DW) (New Lifestyles, Inc., Lees Summit, MO) yield similar step counts as compared with an ankle-mounted criterion, StepWatch 3, when worn by early adolescents in a free-living environment and 2) to study whether BMI percentile affects the accuracy of waist-mounted pedometers in adolescents.

Seventy-four early adolescents (13.0 ± 1.1 yr) wore the devices during one weekday. The study population included 33 normal weight, 21 overweight, and 20 obese participants. Two-way repeated-measures ANOVA was used to determine whether the BMI and the device were related to the number of steps per day and percentage of actual steps.

The NL and DW recorded fewer steps than the StepWatch 3 in each BMI category (P < 0.05). In the obese group, the DW underestimated the steps more than the NL (P < 0.001). For the normal weight, overweight, and obese groups, the NL counted 89.1%, 89.1%, and 91.6% of the steps, respectively, whereas the DW counted 86.7%, 84.6%, and 72.7%, respectively.

Researchers must be cognizant of the limitations of waist-mounted pedometers and carefully choose a device that suits the needs of their investigations. Because of the inaccuracies of the DW when measuring steps in obese adolescents, careful consideration must be given before choosing this device for research and interventions. The NL is a better device for assessment of adolescents' steps than the DW, especially for those who are obese.