Abstract: Disclosed is a wearable inertial sensors (WIS) system and methods for real-time simultaneous tri-planar motion capture of the upper extremity (UE). The sensors simultaneously capture in the frontal, sagittal, and horizontal planes UE range of motion (ROM), which is critical to assess an individual's movement limitations and determine appropriate rehabilitative treatments. Off-the-shelf sensors and microcontrollers are used to develop the WIS system, which wirelessly streams real-time joint orientation for UE ROM measurement. Key developments include: (i) two novel approaches, using earth's gravity (EG approach) and magnetic field (EGM approach) as references, to correct misalignments in the orientation between the sensor and its housing to minimize measurement errors; (ii) implementation of the joint coordinate system (JCS)-based method for tri-planar ROM measurements for clinical use; and (iii) an in-situ guided mounting technique for accurate sensor placement and alignment on human body.