A spine or body movement sensing device 100 comprises a string of sensor segments 10 each comprising an orientation sensor, for example an accelerometer 14, magnetometer 12 or gyroscope 16. Mechanical couplings 50 may provide minimum separation between segments, giving a neutral position from which the couplings may be compressible or extendible, or pivot and rotate, the neutral position corresponding to the patients vertebrae spacing. One segment may be a master segment 20 having a power supply 18, a local physical interface 32 for communication with the other segments via electrical couplings 55 and a wireless interface 34 for communication with a controller 150. Also disclosed are methods of attaching sensor strings to a body； methods of determining orientation using two sensor signals (including determining an initial orientation and subsequent change)； and a sensor configured to determine orientation via magnetometer when stationary and via accelerometer or gyroscope during movement.
