A user-specific heart rate monitor comprising: characterizing means for determining from the user's electrocardiogram a reference value of a heart rate variable characterizing the heart rate; determining means for determining at least one environmental parameter value proportional to air density using measurement of a physical variable proportional to air density; associating means, connected to the characterizing means and to the determining means, for associating the reference value of the heart rate variable with at least one environmental parameter value; and a register, connected to the associating means, for recording in the register at least one environmental parameter value and the reference value of the heart rate variable associated with the at least one environmental parameter value; characterized in that: the characterizing means are arranged to determine from the user's electrocardiogram an instantaneous value of the heart rate variable characterizing an instantaneous heart rate; the determining means are configured to determine an instantaneous environmental parameter value; the determining means are configured to record the instantaneous environmental parameter value into a register; the register is configured to restore the reference value of the heart rate variable to be associated with the instantaneous environmental parameter value by using the environmental parameter values and the reference value of the heart rate variable associated with each environmental parameter value recorded in the register; and the heart rate monitor comprises first display means that are configured to display the reference value of the heart rate variable to be associated with the instantaneous environmental parameter value and the instantaneous value of the heart rate variable in relation to the reference value of the heart rate variable associated with the instantaneous environmental parameter value. characterizing means for determining from the user's electrocardiogram a reference value of a heart rate variable characterizing the heart rate; determining means for determining at least one environmental parameter value proportional to air density using measurement of a physical variable proportional to air density; associating means, connected to the characterizing means and to the determining means, for associating the reference value of the heart rate variable with at least one environmental parameter value; and a register, connected to the associating means, for recording in the register at least one environmental parameter value and the reference value of the heart rate variable associated with the at least one environmental parameter value; the characterizing means are arranged to determine from the user's electrocardiogram an instantaneous value of the heart rate variable characterizing an instantaneous heart rate; the determining means are configured to determine an instantaneous environmental parameter value; the determining means are configured to record the instantaneous environmental parameter value into a register; the register is configured to restore the reference value of the heart rate variable to be associated with the instantaneous environmental parameter value by using the environmental parameter values and the reference value of the heart rate variable associated with each environmental parameter value recorded in the register; and the heart rate monitor comprises first display means that are configured to display the reference value of the heart rate variable to be associated with the instantaneous environmental parameter value and the instantaneous value of the heart rate variable in relation to the reference value of the heart rate variable associated with the instantaneous environmental parameter value.