Mike and I discussed the key factors that affect heart rate during running, which are explained in his excellent article published in the Journal of Sport Sciences in 1998. Although some of the information is a bit technical, understanding these factors will allow you to use your heart rate monitor more effectively to optimize your training.

Heart rates tend to be lower in the morning. 

The difference in heart rate between running in the morning and afternoon is typically about 5 to 6 beats per minute, but can be as great as 10 beats per minute. Your maximal heart rate is also several beats per minute lower in the morning. This means that if you set your heart rate zones based on your morning heart rates, and train in the afternoon, then you will train a bit less intensely than planned. Similarly, if you use afternoon or evening heart rates to determine your training zones, and then train in the morning, you will train somewhat harder than planned.

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Heart rate increases at high temperatures. 

Your heart rate is higher when running on a hot day. As the temperature increases from 60 degrees to 75 degrees, a runner’s heart rate at a given speed increases by about 2 to 4 beats per minute. When the temperature increases from 75 degrees to 90 degrees, you can expect your heart rate running at a given speed to increase by approximately 10 beats per minute. High humidity magnifies the effect of high temperatures on heart rate.

To gain the same benefits as on a cool day, you should increase your heart rate zones by 2 to 4 beats per minute when the temperature is in the 70’s and the humidity is low. On a high humidity day in the 70’s or a low humidity day in the 80’s, you should increase your zones by approximately 5 to 8 beats per minute to correct for the heat factor. 

In more extreme conditions, such as a high humidity day over 80 degrees, you cannot accurately adjust your heart rate zones for the conditions. On the most brutal summer days, it is wise to adjust your training schedule to avoid high intensity training.

Dehydration causes an increase in heart rate. 

When you become dehydrated, your blood volume decreases and less blood is pumped with each heart beat. Your heart rate at a given running speed, therefore, increases. A 1992 study by S. J. Montain and Ed Coyle, PhD, found that heart rate increases approximately 7 beats per minute for each 1% loss in bodyweight due to dehydration. For example, if you weigh 150 pounds, when you lose 1.5 pounds due to dehydration your heart rate at a given running speed would increase by about 7 beats per minute. Water loss of this magnitude occurs after an hour of running on a mildly warm day. On a hot day, runners typically lose over 2 pounds of water per hour. If you set heart rate training zones when properly hydrated and then become dehydrated during training, your pace will decrease as you become progressively more dehydrated.

Heart rate during running varies by a few beats from day-to-day. 

Several studies have found that heart rate during running at a given pace varies by a few beats per minute from day-to-day. It is not clear why this occurs, but most physiological variables exhibit similar amounts of day-to-day variation. This means that if you monitor your heart rate religiously, you will find that some days it appears you are getting slightly fitter and other days it appears you are getting out of shape, when in fact, your fitness level may not be changing. You should be cautious, therefore, in interpreting the results of any one session of heart rate monitoring. Do not put too much emphasis on small changes of 2 to 3 beats per minute in heart rate found during one run. When you find a systematic reduction in heart rate at a given pace, however, you can be confident that your fitness has improved. Similarly, if you find that your heart rate is consistently higher than expected, you can confidently conclude that something is wrong; i.e. you may be losing fitness or (more likely for most runners) overtrained.

Training heart rate does not predict racing heart rate. 

During competition, your heart rate does not increase logically with your running speed. So many other factors affect your heart rate while racing, that it is not a good indication of how fast/hard you are running. If you measure your heart rate at your desired race pace during training, and use that heart rate to determine how fast to run during a race, then you will run quite a bit slower than planned, because with the excitement of the race, your heart rate will be elevated. You could account for the increase and still use your heart rate to accurately select your race pace if the increase in heart rate due to racing was consistent. Unfortunately, how much higher heart rates are at a given pace during racing compared to training has been found to vary greatly from person to person and from race to race.