The accuracy of physical activity (PA) monitors to discriminate between PA, sedentary behavior, and non-wear in extremely obese (EO) adolescents is unknown.
Twenty-five subjects (9 male/16 female; age=16.5±2.0 y; BMI=51±8 kg/m2) wore three activity monitors (StepWatch [SAM], Actical [AC], Actiheart [AH]) during a 400 meter walk test (400MWT), two standardized PA bouts of varying duration, and one sedentary bout.
For the 400MWT, percent error between observed and monitor recorded steps was 5.5±7.1% and 82.1±38.6% for the SAM and AC steps, respectively (observed vs. SAM steps: −17.2±22.2 steps; observed vs. AC steps: −264.5±124.8 steps). All activity monitors were able to differentiate between PA and sedentary bouts but only SAM steps and AH heart rate were significantly different between sedentary behavior and non-wear (p<0.001). For all monitors, sedentary behavior was characterized by bouts of zero steps/counts punctuated by intermittent activity steps/counts; non-wear was represented almost exclusively by zero steps/counts.
Of all monitors tested, the SAM was most accurate in terms of counting steps and differentiating levels of PA, and thus, most appropriate for EO adolescents. The ability to accurately characterize PA intensity in EO adolescents critically depends on activity monitor selection.