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This paper presents a system of inertial measurement units, each consisting of an accelerometer, gyroscope and magnetometer. They are characterized by a small size, wireless transmission, and open architecture, with the purpose of either integration into lower limb exoskeletons or general human movement analysis. Kalman filtering and the factored quaternion algorithm are used to track the orientation of each unit, and angles of the human joints are calculated from multiple units. After calibration, the system was tested with a wooden leg mockup and an actual human. The Optotrak optical measurement system was used as a reference. Differences between the inertial measurement system and the Optotrak were less than 2 degrees for the wooden leg and less than 5 degrees for the human leg, suggesting that the system represents a promising possibility for wearable movement tracking and analysis.