Objective: To compare activity counts from the ActiGraph GT3X to those from the ActiGraph GT1M during treadmill walking/running. A secondary aim was to develop tri-axial vector magnitude (VM3) cut-points to classify physical activity (PA) intensity.

Methods: Fifty participants wore the GT3X and the GT1M on the non-dominant hip and exercised at 4 treadmill speeds (4.8, 6.4, 9.7, and 12 km h(-1)). Vertical (VT) and antero-posterior (AP) activity counts (counts min(-1)) as well as the vector magnitudes of the two axes (VM2) from both monitors were tested for significant differences using two-way ANOVA's. Bland-Altman plots were used to assess agreement between activity counts from the GT3X and GT1M. Linear regression analysis between VM3 countsmin(-1) and oxygen consumption data was conducted to develop VM3 cut-points for moderate, hard and very hard PA.

Results: There were no significant inter-monitor differences in VT activity counts at any speed. AP and VM2 activity counts from the GT1M were significantly higher (p<0.01) than those from the GT3X at 4.8, 9.7 and 12 km h(-1). High inter-monitor agreement was found for VT activity counts but not for AP and VM2 activity counts. VM3 cut-points for moderate, hard, and very hard PA intensities were 2690-6166, 6167-9642, >9642 counts min(-1).

Conclusion: Due to the lack of congruence between the AP and VM2 activity counts from the GT1M and the GT3X, comparisons of data obtained with these two monitors should be avoided when using more than just the VT axis. VM3 cut-points may be used to classify PA in future studies.