| Summary: | Twenty-one experienced runners completed three treadmill running sessions on different days. Each session consisted of three consecutive 2 min trials at self-selected speeds (RPE = 3, 5, and 7). An eight-camera marker-less motion capture system and instrumented pressure treadmill (TM) collected data over the final ~25 s at each speed. Lower extremity joint angles (ankle, knee, and hip) and segmental angles (pelvis and trunk) were computed for each trial with foot contact and toe off being kinematically determined. Spatiotemporal metrics (ground contact time, step length, and cadence) were measured via TM and compared to their kinematically derived counterparts. All spatiotemporal metrics demonstrated excellent agreement (ICCs > 0.98). Both intra-trial and inter-session variability, averaged across the entire running cycle, for all lower extremity joint angles in all planes were low (intra-trial: sagittal = 2.0°, frontal = 1.2°, and transverse = 1.9°; inter-session: sagittal = 1.4°, frontal = 0.8°, and transverse = 1.3°). Discrete measures of lower extremity joint and segmental angles were evaluated for inter-session reliability at foot contact, toe off, and peak value during the stance phase. On average, discrete measures demonstrated good reliability (ICC<sub>sagittal</sub> = 0.85, ICC<sub>frontal</sub> = 0.83, and ICC<sub>transverse</sub> = 0.77) with average standard error of measurement < 1°. Marker-less motion capture reliably measured treadmill running kinematics in a group of runners demonstrating heterogenous foot strike patterns (13 rearfoot strike and 8 non-rearfoot strike) across a range of speeds (2.67–4.44 m/s).
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