A method for determining the flexion or extension of a joint of a human or animal subject, comprising: applying a plurality of strain gauges to the joint in a known configuration; applying a first inertial measurement unit, IMU, to each strain gauge; receiving strain data from each of the strain gauges; receiving motion data from each of the IMUs; and calculating the flexion or extension of the joint in dependence on the received strain data, motion data and the configuration of the strain gauges.