Does site matter? Impact of inertial measurement unit placement on the validity and reliability of stride variables during running

Background: Inertial measurement units (IMUs) are used for running gait analysis in a variety of sports. These sensors have been attached at various locations to capture stride data. However, it is unclear if different placement sites affect the derived outcome measures. Objective: The aim of this systematic review and meta-analysis was to investigate the impact of placement on the validity and reliability of IMU-derived measures of running gait. Methods: Online databases SPORTDiscus with Full Text, CINAHL Complete, MEDLINE (Ebscohost), EMBASE (Ovid) and Scopus were searched from the earliest record to 6 August 2020. Articles were included if they 1) used an IMU during running 2) reported spatiotemporal variables, peak ground reaction force (GRF) or vertical stiffness and 3) assessed validity or reliability. Meta-analyses were performed for a pooled validity estimate when 1) studies reported means and standard deviation for variables derived from the IMU and criterion 2) used the same IMU placement and 3) determined validity at a comparable running velocity (≤1 m∙s^-1 difference). Results: Thirty-nine articles were included, where placement varied between the foot, tibia, hip, sacrum, lumbar spine (LS), torso and thoracic spine (TS). Initial contact, toe-off, contact time (CT), flight time (FT), step time, stride time, swing time, step frequency (SF), step length (SL), stride length, peak vertical and resultant ground reaction force (GRF) and vertical stiffness were analysed. Four variables (CT, FT, SF and SL) were meta-analysed, where CT was compared between foot, tibia and LS placements and SF was compared between foot and LS. Foot placement data was meta-analysed for FT and SL. All data are mean difference (MD [95%CI]). No significant difference was observed for any site compared to the criterion for CT (foot: -11.47 ms [-45.68, 22.74], p=0.43; tibia: 22.34 ms [-18.59, 63.27], p=0.18; LS: -48.74 ms [-120.33, 22.85], p=0.12), FT (foot: 11.93 ms [-8.88, 32.74], p=0.13), SF (foot: 0.45 step∙min^-1 [-1.75, 2.66], p=0.47; LS: -3.45 step∙min^-1 [-16.28, 9.39], p=0.37) and SL (foot: 0.21 cm [-1.76, 2.18], p=0.69). Reliable derivations of CT (coefficient of variation [CV] <9.9%), FT (CV <11.6%) and SF (CV <4.4%) were shown using foot- and LS-worn IMUs, while the CV was <7.8% for foot-determined stride time, SL and stride length. Vertical GRF was reliable from the LS (CV=4.2%) and TS (CV=3.3%) using a spring-mass model, while vertical stiffness was moderately (r=0.66) and nearly perfectly (r=0.98) correlated with criterion measures from the TS. Conclusion: Placement of IMUs on the foot, tibia and LS are suitable to derive valid and reliable stride data, suggesting measurement site may not be a critical factor. However, evidence regarding the ability to accurately detect stride events from the TS is unclear and this warrants further investigation.