Accuracy of Physical Activity Monitors in Persons with Class III Obesity

Background Small, wearable monitors are widely used to assess physical activity (PA) in obesity treatment programs ranging from lifestyle interventions to post-bariatric surgical programs. Although wearable monitors can overcome the recall biases often associated with self-reports, the accuracy of these devices may be impacted by anthropometric measures, mode of PA, and wear location. Thus, it is important to examine the accuracy of objective PA monitors during commonly performed activities such as walking.

Methods Fifteen individuals with class III obesity completed a self-paced 6-minute walk while wearing the StepWatch 3 (SW3), Omron, Digiwalker (DW), SenseWear Pro 2 Armband (SWA), and Fitbit objective PA monitors. Simultaneously, energy expenditure (EE) was measured using a portable indirect calorimeter. Height, weight, hip circumference, and waist circumference were also measured. Monitor values for step counts and Calories were compared to hand tally counts and indirect calorimetry (IC), respectively.

Results Step-counting percent errors (PE) were not significantly different among the SW3 (PE=0.56%), Omron (PE=5.53%), and Fitbit (PE=4.33%). The DW significantly undercounted steps by 28% (p=0.037). The SWA overestimated EE by 71.6% (p=0.003), while the Fitbit’s 10% overestimate did not differ significantly from IC (p=0.114).

Conclusion Objective monitors are useful for step counting and estimating energy expenditure, but consideration should be given to device accuracy when selecting evaluative tools for the bariatric population.