This invention relates to a method and a system for determining the maximum heart rate, called HRmax of a user of in a freely performed physical exercise and using an apparatus with software and memory means (1311, 1312).In the method values are continuously calculated (step 14) for several variables and updating them in memory using said software and memory means, said variables including:a variable depicting current internal intensity using recorded values of heart rate (HR), and at least one of following two according to a previously provided intensity model.The method includes further steps of:selecting a range for heart rate (HR_i) nearby the last recorded maximum heart rate (HRmax)seeking a minimum value of an objective function (f()) (step 18) depicting the difference between relative intensity according to external workload and internal intensity across the range using said intensity model having heart rate HR_i as a running variable therein:and obtaining a maximum heart rate estimate (HRmax_est) at HR_i giving said minimum value,validating and storing the obtained value of maximum heart rate estimate (HRmax_est) or rejecting it based on a chosen criterion (step 20) .
