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； an activity sensor, 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, wherein the processing circuitry determines the monitoring device is engaging the fixture by detecting the fixture's particular signature and calculates the activity-related quantity.
