1. A method of compressing data output from an acceleration measurement means configured to be transported, carried or worn by a user, comprising the steps of: measuring acceleration values indicative of the movement of the user at a first frequency; generating values representative of the measured acceleration values at a second frequency, which is lower than the first frequency, said step of generating comprising: defining a plurality of time windows, each time window containing a plurality of measured acceleration values; and applying a transformation to the measured acceleration values within each time window to generate a plurality of transformed values; and, for each time window, storing at least one of said plurality of transformed values and/or one or more parameters associated therewith.