A portable apparatus including an interface configured to input first motion data of a user from a foot-worn first self-contained activity determining unit is disclosed. The portable apparatus also includes a second self-contained activity determining unit configured to determine second motion data of the user. Furthermore, the portable apparatus includes a processing unit configured to determine and store a user-specific dependency between the first motion data and the second motion data, and determine a current speed of the user or a current traveled distance of the user based on current second motion data and the user-specific dependency.