A method implemented in a system which comprises a lightweight personal wearable monitoring device, supplied by a battery, comprising an accelerometer, a processing unit, a display, a remote server, said method comprising the steps: /a/ collecting, from a plurality of individuals caused to practice various physical activities from a set of predefined activities, acceleration data from sensing devices placed on each of said individuals, /b/ defining N small data-size specific metrics, computed from acceleration signals, which allow to define a global activity classifier, /c/ acquiring, at a first monitoring device worn by a first user, acceleration signals from the accelerometer of the first monitoring device, /d/ calculating, at the first monitoring device, over each lapsed time unit T1, specific metrics values from the sensed signals, to form a series of specific metrics values, /e/ send them to the processing unit, /f/ allocate an activity type for each time unit together with corresponding specific metrics values of the received series, to form a time chart of activity types presumably performed by the first user over a second period T2, /g/ display the time chart of activity types presumably performed by the first user on the display and allow the first user to confirm or correct partly the type of activity performed over the second period, and allow correction by the first user.