In order to eliminate the need of tiresome and complex calibration procedures for posture-detecting devices, means are provided for determining an orientation of a body-mounted or implanted device (1) relative to the body (2), the device (1) having an orientation detection unit, wherein an uncontrolled output of the orientation detection unit over a period of time together with one or more reference conditions defined in a body reference system (xb, yb, zb) are used for determining the relative orientation of the device and hence for calibration.
