A method and apparatus for monitoring the head and torso orientation of a subject during physical motion. The method and apparatus employ at least two inertial measurement units (IMUs) being temporally synchronized with each other and each measuring three degrees of freedom of angular velocity, acceleration, and optionally magnetic field. A first IMU being affixed to the head of the subject and the second IMU being affixed to the torso of the subject. The orientation of the head, the orientation of the torso, and optionally the relative orientation of the head and torso are calculated based on the data from the IMU's and optionally magnetometer data. The resultant output data is transmitted to a host device (computer, tablet, smart phone) and can be used for monitoring, training, coaching, worker safety and health, and/or rehabilitation.