There is provided a wearable item configured to be placed at least partially against a skin of a person； and an optically sensitive detector mounted to the wearable item and configured to detect optical signals reflected from the skin of the person, wherein the detected optical signals represent a relative motion between the wearable item and the skin of the person and wherein the optically sensitive detector is mounted to the wearable item such that there is a predetermined space between the optically sensitive detector and the skin of the person.
