According to an example aspect of the present invention, there is provided an apparatus comprising memory configured to store a user-specific calibration metric, and at least one processing core, configured to determine an activity type identifier of an activity a user is engaged in, and to determine a user-specific intensity level of the activity, wherein determining the user-specific intensity level is based at least partly on the identifier of the activity type, the user-specific calibration metric and data obtained from a kinematic or speed sensor, and to obtain the user-specific calibration metric by causing the apparatus to participate in a calibration procedure, the calibration procedure including communicating heart rate data of the user with a server.
