Abstract: A method of calibrating a pair of body mounted sensors, the method comprising the steps of: (a) in a baseline position of a joint to be measured, determining a first offset between a measured joint angle and an angle between pair of sensors, one mounted on each side of the joint to be measured, so as to calibrate the sensors; (b) after at least one of the sensors has been removed and reapplied, placing the joint back into the baseline position such that the sensors are in a second configuration relative to each other; and (c) determining a second offset between the measured knee angle and an angle between the pair of sensors in the second configuration in order to recalibrate the sensors such that, in each of the first and second configurations, the same joint angle for the baseline position is reported.