An activity measuring device (1) is used for recording the activity of a test subject, where the device (1) is attachable so as to be carried on the body of the test subject. According to the invention it is planned that the activity measuring device is located on a buckle (2) of a belt (3) which can be fitted to the body of the test subject, and that it has acceleration sensors (10, 10′) for detection of acceleration in all three spatial directions at right angles to each other. Furthermore, the activity measuring device (1) telemeters the recorded measurement data via a radio link to a central server (7), by virtue of the fact that the measurement data recorded by the activity measuring device (1) can be transmitted by means of a connecting cable (5) to a mobile telephone (4) via a device interface of the mobile telephone (4), which is arranged so as to be carried on the belt, and can be sent from there to the external server (7) via a radio link.