This paper presents the design and implementation of a cost-effective and small inertial measurement unit (IMU) for application on leg smart orthotics and prosthetics. IMU design based on biomechanical considerations for lower leg devices is presented. Methods for calculation of a number of biomechanical parameters related to gait based on the unit are discussed, including calibration and offset correction procedures. An approach for electronic knee joint control of orthosis during cyclic walking based on IMU signals is discussed. Finally, experiments are conducted for a subject walking on a flat surface wearing a mechanically driven orthosis with the proposed sensor. Experimental results demonstrate the IMU suitability and feasibility for real-time embedded control of wearable assistive devices for walking restoration and monitoring.