## 1 Introduction  
Biomechanical interventions can be used to assist in the management of chronic conditions, and interest has emerged around gait retraining interventions for people with medial knee osteoarthritis (KOA) [1]. The most researched intervention with a demonstrated ability to reduce indices of knee load is walking with an altered foot progression angle (FPA). FPA is defined as the angle of the longitudinal axis of the foot relative to the line of the body's over-ground progression during gait [2]. Changing the FPA to either an internally oriented position (toe-in) or an externally oriented position (toe-out) has been shown to reduce pain in KOA patients after a six-week gait retraining program [2, 3].  
For gait retraining interventions to be clinically implemented, valid and reliable measurement tools need to be available in a clinical setting. Currently, FPA is measured in a variety of ways. The most comprehensive method is 3D motion capture using markers on the foot tracked by infrared cameras [4]. This is commonly considered as the gold standard for the tracking of human movement [5]. However, these motion capture systems are costly, require a fixed laboratory, are sensitive to optical occlusion, and require a time-consuming analysis to yield results. Alternatively pressure sensitive walkways such as the GAITRite(rrankin, TN, USA), the Strideway(tm) (Tekscan, Boston,MA, USA), and the Zeno(tm) Walkway (ProtoKinetics, Havertown, PA, USA) can collect and analyze spatiotemporal parameters from foot pressure data, including the FPA [6]. Wearable inertial measurement units (IMU) include an accelerometer, a magnetometer, and a gyroscope. A wide range of IMUs, each featuring their own distinctive characteristics, are commercially available from companies such as APDM (Portland, OR, USA), Xsens Technologies B.V. (Enschede, The Netherlands), Technaid S.L. (Madrid, Spain), IMeasureU (Auckland, New Zealand), and Noraxon (Scottsdale, AZ, USA). IMUs are insensitive to occlusion and are not limited to a specified lab or examination room. The challenges of using IMUs may include sensitivity to magnetic disturbances, power use (the sensors are wireless), and their dependence on complex proprietary sensor fusion algorithms and correction software. When placed on the lower extremities, IMUs can measure spatiotemporal data, including the FPA [7]. Therefore, IMUs seem particularly well suited for measuring the FPA in a wide range of clinical practices that aim to apply gait retraining in patients with KOA.  
Although motion capture is considered the gold standard, a pressure sensitive walkway (PSW) designed to measure foot imprints is considered to perform equally well [8]. Yielding valid FPA measurements using IMUs is dependent on both the proper attachment of the IMU to the foot (for anatomical calibration) and accurate signal processing of the sensor signals. To assess whether IMUs are a viable option for the attainment of FPA measurements, the validity and reliability of these measurements need to be established. The purpose of this study is, therefore, to determine the reliability and validity of FPA measurements based on IMUs, by comparing IMU performance in FPA analyses to the highly accurate and established PSW methodology, using the gait strategies that are used for gait retraining in KOA.