A system for monitoring the physical activity of a user, comprising: - a portable medium which can be worn by the user and which is equipped with: a sensor designed to detect a signal relating to the movement of the user; a power supply; a display unit; a data storage memory; an interface which can be coupled to a gym machine and which is configured to allow data to be exchanged between the memory and the gym machine; a processor programmed for deriving in real time, at least from the signal detected by the sensor, a dimensionless parameter value representing the physical activity of the user wearing the portable medium and for displaying, on the display unit, an indication, as a function of the derived value of the dimensionless parameter and of a reference value of the dimensionless parameter constituting a preset target for the amount of physical activity to be performed by the user in a predetermined time interval; - a remote server having access to a database containing data received from the portable medium, characterized in that the portable medium comprises wireless communication means configured to establish a short-range connection between the portable medium and a mobile telephone, the mobile telephone being equipped with a display unit and being set up to establish an Internet connection and to connect up to the remote server through the connection in order to transfer data in real time to the remote server through the Internet connection, the remote server being programmed to transmit, in real time through the Internet connection, motivational indications as a function of the data transmitted by the portable medium to the remote server, so that the motivational indications are displayed on the display unit of the mobile telephone. - a portable medium which can be worn by the user and which is equipped with: a sensor designed to detect a signal relating to the movement of the user; a power supply; a display unit; a data storage memory; an interface which can be coupled to a gym machine and which is configured to allow data to be exchanged between the memory and the gym machine; a processor programmed for deriving in real time, at least from the signal detected by the sensor, a dimensionless parameter value representing the physical activity of the user wearing the portable medium and for displaying, on the display unit, an indication, as a function of the derived value of the dimensionless parameter and of a reference value of the dimensionless parameter constituting a preset target for the amount of physical activity to be performed by the user in a predetermined time interval; a sensor designed to detect a signal relating to the movement of the user; a power supply; a display unit; a data storage memory; an interface which can be coupled to a gym machine and which is configured to allow data to be exchanged between the memory and the gym machine; a processor programmed for deriving in real time, at least from the signal detected by the sensor, a dimensionless parameter value representing the physical activity of the user wearing the portable medium and for displaying, on the display unit, an indication, as a function of the derived value of the dimensionless parameter and of a reference value of the dimensionless parameter constituting a preset target for the amount of physical activity to be performed by the user in a predetermined time interval; - a remote server having access to a database containing data received from the portable medium,