A system for monitoring the physical activity of a user comprises a portable medium and a remote server having access to a database containing data received from the portable medium. The portable medium has a sensor for detecting the movement of the user, a display, a memory, an interface for exchanging data with a gym machine, a processor for deriving in real time a dimensionless parameter representing the physical activity of the user wearing the portable medium, and wireless communication means configured to establish a connection between the portable medium and a mobile telephone which is set up to connect to the remote server to transfer data to the remote server in real time. The remote server is programmed to transmit, in real time, motivational indications as a function of the data transmitted by the portable medium for display on the mobile telephone.