According to Professor Mike Lambert at the Sports Science Institute of South Africa (SSISA), some basic points need to be considered when using heart rate to monitor exercise intensity. The first concept to understand is that oxygen consumption is directly proportional to the running speed (intensity). The faster the running speed and the greater the power output, the more oxygen is consumed. This linear relationship continues to the point of exhaustion where oxygen consumption reaches a maximum (VO2 max). Also, heart rate is proportional to oxygen consumption under most conditions (~50-90% functional capacity/VO2 max) suggesting that heart rate may be used as an indirect measure of oxygen consumption and exercise intensity. This relationship however is not perfect, and there are several factors that can affect this heart rate/running speed correlation. It is important to understand the implications of following factors otherwise the measured heart rate may be misinterpreted.

• Changes during the day. Heart rate is at its lowest in the early morning and increases to its peak during the afternoon. The difference between the lowest and highest heart rate is about 10 beats per minute. This applies to both the resting and exercising heart rate.
• Environmental temperature. Heart rate increases with increasing environmental temperature, particularly above 25 °C / 77 ° F. When environmental temperature increases from 25° to 35 °C (77° to 95° F), the heart rate increases by about 10 beats per minute when running at the same speed.
• Dehydration. As a rule of thumb, heart rate increases by about 7 beats per minute for every 1 percent body weight lost during exercise. For example, a 155 lb runner who loses 3 lbs as a result of fluid loss during a long training run or race may experience a heart rate increase of over 14 beats per minute at the same relative intensity.
• Competition. The heart rate/running speed relationship during competition is quite disturbed and heart rate may vary by up to 40 beats per minute at any speed during a race. The reason for this is not quite clear, but this applies to both short, fast events (10 km) and ultra-marathons.
• Heart rate drift. Heart rate increases gradually as the exercise duration increases. After an hour of running in moderate conditions, heart rate may increase by 20 beats per minute, even if there is no fluid loss. Rising humidity and temperature increase the heart rate drift even further.
• Training As fitness levels improve as a result of training, heart rate at any running speed decreases. The change when adapting from a sedentary state to a trained state may be up to 20 beats per minute lower at any submaximal speed. The decrease in heart rate is much less significant (4 – 10 beats per minute) for a highly trained runner, since their window of adaptation is much smaller than an untrained individual. Conversely, with detraining, or during the early onset of overtraining, heart rate at any running speed may increase.

In conclusion then, if the factors that affect heart rate are controlled or accounted for, training-induced changes in heart rate can be measured and interpreted. This information can then be used to make intellectual decisions about intensity and training loads. Wiser training is key to having the competitive edge!

NOTE: You're free to republish this article on your website, in your newsletter, in your e-book or in other publications provided the article is reproduced in its entirety, including this note, author information and a LIVE link to this website.

Copyright 2005 David Petersen BS, CSCS & B.O.S.S. Fitness

Find out about our telephone/online chat sessions