THR zone is the number of beats per minute your heart should be beating during aerobic exercise. For most healthy people, this is in the range of 50% to 80% of your maximal heart rate (MHR). Maximal heart rate is the highest heart rate a person can attain. MHR is a number related to your age. As we grow older, our hearts will start to beat slowly. To estimate your MHR heart rate subtract your age from the number 220.

For example, if you are 40 years old, 220 -40 is 180. Your MHR is 180 beats per minute. At a low intensity workout, (working out at 50%) your heart rate would be around 90 bpm and at a higher intensity (80%) your heart rate would be 144 bpm. So for your age, the THR would be between 90 to 144 bpm. However it is important to note that certain medications can lower your heart rate. An example would be beta blockers used for cardiovascular disorders.