A system including: a sensor module having an inertial measurement unit and attached to an arm of a user. The sensor module is initially calibrated to measure its orientation relative an initial reference pose. To recalibrate the sensor module, the arm of the user is moved to obtain a measurement of an orientation of the arm at a calibration pose relative to the reference pose. The arm is in a horizontal plan in both the calibration pose and the reference pose. Preferably, both arms are straight in the horizontal plane; and the hands meet each other at the calibration pose. The arm module may have twisted around the arm of the user from the time of the initial calibration and the time of recalibration. A twist of the arm module around the arm of the user is computed from the orientation measurement at the reference pose for recalibration.