Background: Activity trackers that can measure running dynamics have recently become commercially available. If these devices are to provide real-time feedback on running dynamics and be used in in-field gait re-training or injury prevention, they have to be reliable and valid. However, no in-field evidence of the reliability and validity of these devices exist. Purpose: The purpose of this study was to test the reliability
and validity of the Garmin Forerunner 735XT paired with the HRM-Run (GFR) running dynamics: Vertical Oscillation (VO), Ground Contact Time (GCT), Step Length (SL), and Cadence during in-field running compared to full body kinematics measured with inertial measurement units. Methods: 24 recreationally active subjects ran on a straight path while three-dimensional kinematic data was collected with Xsens MVN Link and running dynamics was collected from the GFR. Two minutes of data were collected across three different running speeds (10, 12, 14 km/h) and two baseline trials. Intraclass Correlation Coefficient (ICC) and Bland-Altman analysis with 95% limits of agreement were used to assess reliability and validity. Results: The reliability showed good agreement between baseline trials with ICC(2,k) values ranging from 0.968 to 0.987. For validity the ICC(2,k) revealed a good agreement between GFR and full body kinematic measures for VO, SL, and Cadence ranging from 0.769 to 0.970, while moderate agreement was found for GCT (0.568). Bland-Altman analysis revealed that GFR overestimated VO and underestimated GCT. Conclusion: The GFR is a reliable and valid tool for measuring running dynamics in-field. However, GCT and VO must be interpreted with caution.