A system including a plurality sensor modules, each having an inertial measurement unit and attached to a portion of a user (e.g., upper arm, hand, and/or head) to measure the current orientation of the corresponding portion of the user. A computing device coupled to the sensor modules is configured to identify a type of the current pose of the user, compute a front facing direction of the torso of the user from the sensor measurements, select a torso leaning model based on the pose type, project onto a vertical plane the lengthwise directions of the arms of the user as tracked by the sensor modules attached to the upper arms of the user, and calculate a torso leaning angle from the projected lengthwise directions of the arms.