what are the heart rate zones?
['Heart rate zone 1: 50–60% of HRmax. This is the very low intensity zone. ... ', 'Heart rate zone 2: 60–70% of HRmax. ... ', 'Heart rate zone 3: 70–80% of HRmax. ... ', 'Heart rate zone 4: 80–90% of HRmax. ... ', 'Heart rate zone 5: 90–100% of HRmax.']