Sport science has shown that as we exercise harder, the heart rate increases in direct proportion to the speed. Thus, due to the straight line relationship between training intensity (speed) and heart rate, we can use heart rates as a means of determining training intensity.
Once maximum heart rate has been determined, the following table can be used to establish heart rate training zones:
These heart rate zones are scientifically based guidelines but they are only guidelines. Too many endurance athletes become slaves to a heart rate monitor or a heart rate that they saw in one of the many heart rate training books available. Many of these books assume each person has a maximum heart rate of 220-age, or that the zone 2 training zone can be determined by taking your age from 180. The only real way to determine your maximum heart rate is in a laboratory or using incremental tests.
When using these heart rate zones and a heart rate monitor, it is also important to remember that heart rates will be higher when exercising in hot and / or humid conditions. This is due to the fact that when you train in the heat, you may dehydrate slightly through sweat loss. This lowers your blood volume that results in the heart having to pump more quickly and harder to get the same amount of blood and oxygen to the working muscles. Secondly, when training in the heat, blood is diverted to the skin to help off load the heat generated in the muscles. Again, the heart has to work harder to keep the amount of blood pumping to the muscles to give them the oxygen they require to maintain speed.
Research suggests that heart rates increase by 1.4% for each degree above 21 degrees Celsius. For example, at a constant pace, a heart rate of 140 at 21 degrees will become 160 at 31 degrees.
2018, Endurance Training - Heart Rate Zones, Masters Athelete, retrieved 11 March 2018, <http://www.mastersathlete.com.au/sidebar/endurance/training/heart-rate-zones/>