The invention relates to a device (100) for monitoring the physical activity of a living being comprising: a data input module (110) configured to receive information about the living being's heart beat rate value (111), motion intensity (121) and anthropometric characteristics (131)； an activity recognition and storage module (140) configured to detect, from information received about the living being's motion intensity (121), the living being's activity (141) and to store information about said living being's heart beat rate value (111) and said motion intensity (121) associated with that detected activity (141)； a heart beat rate analysis module (150) configured to determine, from a plurality of heart beat rate values (111) associated with each detected activity (141), statistics of the distribution of heart beat rate values for each activity or a subset of activities； and a fitness estimation module (160) configured to calculate, using the information from the heart beat rate analysis module (150) and the anthropometric characteristics (131), a cardiorespiratory fitness level (170) of the living being. The invention also relates to a method for monitoring the physical activity of a living being.
