A method, apparatus, and a computer program for monitoring a fitness exercise are described. A plurality of heart-rate variability values and a plurality of exertion parameter values are measured during an exercise measured. The heart rate variability values correlate with the exertion parameter values through a human physiological mechanism, and the exertion parameter values characterize the physical exertion of the exercise. A mathematical correspondence is then constructed from the plurality of measured heart rate variability values and associated exertion parameter values. The mathematical correspondence describes correlation between the heart rate variability values and the exertion parameter values and the user's physiological state during the exercise. Then, the physical exertion of the exercise is monitored by applying the mathematical correspondence.
