International Journal of Exercise Science 6(3) : 217-229, 2013. The purpose was to determine the accuracy of the GT3X+ and Actiheart monitors for estimating energy expenditure (EE) and steps. Additionally, to investigate agreement between waist- and wrist-mounted GT3X+ EE outputs. Nineteen participants (mean age=30) completed three treadmill walking trials at self-selected slow, medium, and fast speeds while wearing two GT3X+ (waist and wrist) and an Actiheart. Activity monitor EE was compared to indirect calorimetry criterion EE using Pearson correlations and ANOVAs. A Bland-Altman plot was used to investigate agreement between GT3X+ waist- and wrist-determined EE. GT3X+ determined steps were compared to researcher-counted steps using ANOVAs. EE estimates from all monitors correlated highly with the criterion (r ranged from .72 to .82). However, the GT3X+ (waist and wrist) underestimated EE during slow walking and overestimated EE during fast walking. There were no differences among GT3X+ (waist and wrist) estimates of EE and the criterion during the medium trial. Actiheart estimated EE was not significantly different from measured EE during all trials. The Bland-Altman plot indicated that at EE rates above 4 kcal·min-1, the GT3X+ worn on the wrist underestimated EE compared to when it was worn on the waist. There were no differences between GT3X+ waist-determined steps and researcher-counted steps for all trials. GT3X+ EE correlates highly with measured EE, but has poor absolute agreement during slow and fast walking. GT3X+ step estimates are accurate across the continuum of walking speeds when waist (but not wrist) mounted. Wrist-mounted device outputs are not comparable to waist-mounted outputs. The Actiheart accurately estimates EE
