1. A method of confirming a performer of a fitness exercise, the method comprising the following steps: measuring heart rate information associated with the performer of the fitness exercise as regards the fitness exercise; forming an identifier value in one or more classes on the basis of one or more parameters of the measured heart rate information by means of a mathematical model comprising dependence information between said parameter of the heart rate information and said class; confirming the performer of the fitness exercise by comparing in said one or more classes the identifier value with a reference value formed for the performer of the fitness exercise in the class.