Background: This study evaluated the reliability and reproducibility of biomechanical variables from a foot-mounted inertial measurement unit (IMU; Stryd®) and a trunk-mounted triaxial accelerometer (TTA; RunEASI®) during treadmill running. Methods: Thirteen trained male runners (age: 27.6 ± 8.2 years; body mass: 64.4 ± 6.1 kg; height: 174.2 ± 6.4 cm) completed four treadmill sessions separated by seven days, each with four 5-min trials at 13 km·h−1. Reliability and reproducibility were assessed using intraclass correlation coefficients (ICCs), standard error of measurement (SEM), smallest worthwhile change (SWC) and coefficient of variation (CV). Results: Both devices showed excellent intra-session reliability (ICC ≥ 0.90). Inter-session reproducibility was high for the IMU in running power, step frequency, and ground contact time (ICC = 0.906–0.992; CV = 0.31–1.50%); the TTA showed excellent reproducibility for the spatiotemporal variables—step frequency, ground contact time, and flight ratio (ICC = 0.931–0.956), whereas impact-related variables showed only good reproducibility (ICC = 0.823–0.871; CV up to 7.76%). Mean step frequency values were similar between devices, but trial-to-trial agreement was limited (r = 0.447; R2 = 0.20; Bland–Altman bias = 0.70 steps·min−1, LoA: −15.08 to 16.47), and ground contact time showed systematic discrepancies, with the TTA underestimating values (bias = 16.86 ms, LoA: −45.64 to 11.92). Conclusions: Both sensors provide consistent measurements within their own systems; however, differences in sensing location produce different absolute values, so they should not be used interchangeably and interpretation must remain device-specific.
Alda-Blanco et al. (2026) studied this question.