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.
