The present inventions, in one aspect, is an activity monitoring system comprising a fixture having size/shape adapted to couple to a location on the user's body and a particular signature; and a portable monitoring device adapted to detect the fixture's particular signature. The monitoring device includes a housing that is adapted to engage the fixture; activity sensors, disposed in the housing, to detect activity of the user and to generate data which is representative of the activity of the user; and processing circuitry, disposed in the housing, to calculate an activity-related quantity of the user using the data which is representative of the activity of the user, wherein the processing circuitry: determines the monitoring device is engaging the fixture by detecting the fixture's particular signature, and, in response thereto, calculates the activity-related quantity using data from a set of the activity sensors.