Estimation of the External Knee Adduction Moment Using Inertial Measurement Unit Sensors on the Shank and Lower Back: A Pilot Study

<b>Background:</b> The external knee adduction moment (KAM) is an important biomechanical parameter that reflects the load on the medial tibiofemoral compartment during gait. The KAM is typically evaluated using three-dimensional motion analysis (3DMA) systems. The present study aimed to evaluate and validate the waveform similarity between the KAM estimated using only two inertial measurement units (IMUs) sensors, attached to the shank and lower back (IMU-KAM), as a simpler method and that obtained from a 3DMA system (3DMA-KAM) under different step rate conditions. <b>Methods:</b> Three healthy adult men were included. The gait task involved walking in a straight line over a distance of approximately 10 m at three step rate conditions: 115, 100, and 85 steps/min. Data were collected using a 3DMA system, force plates, and IMUs. The primary outcome measures included the KAM waveforms for 3DMA-KAM and IMU-KAM during the early and late phases of the single-limb support (Early-SLS phase and Late-SLS phase, respectively). The coefficient of multiple correlation (CMC) was used to evaluate the waveform pattern similarity. <b>Results:</b> IMU-KAM demonstrated high similarity to 3DMA-KAM waveforms in the Early-SLS phase under 115 and 100 steps/min, with CMC values ranging from 0.66 to 0.99. However, no clear similarity was observed in the Late-SLS phase. <b>Conclusions:</b> In the Preferred and Reduced conditions, wherein the walking rate exceeded 100 steps/min, the KAM waveform pattern during the Early-SLS phase was accurately estimated using IMU sensors attached to the shank and lower back. The findings of this study suggest the potential of simplified gait analysis using IMUs for evaluating knee joint biomechanics and provide foundational data for future clinical applications.