With these facts in mind you may not realize that heart rate can be used as a tool to help you monitor yourself to not become overheated as quickly and to help monitor your fluid (sweat) losses so you can train more effectively and safely in the heat.
Slow down to survive
Training in hot weather will cause your heart rate to rise significantly, as your body sends more of your blood supply to your skin in an attempt to cool your body. This is particularly true after the first 30 minutes of exercise, when your core temperature is starting to climb rapidly.
What this simply means is that you have to slow down sometimes drastically in hot weather to stay within a comfortable exercise zone and not cause your body temperature to rise out of control.
An increase in temperature of about 20 degrees Fahrenheit may cause your heart rate to increase by 10 beats for the same workout as you did on a cooler day.
This increase in heart rate is the result of higher ambient temperatures, coupled with the increase in heart rate as a result of more blood going to your skin and sweating to help with cooling.
By using a heart rate monitor you will be able to hold back your speed or exercise intensity so as to not over stress your body and overheat and too dehydrated, which will cause "hitting the wall" at the end of a hard training session or competition.
The heart rate heat index
John Booth of Australia conducted an experiment in which runners ran on a treadmill at about 9 miles per hour in a warm laboratory (90 degrees F and 60 percent relative humidity). The heart rates of the runners increased from 168 beats per minute to 188 beats per minute after 30 minutes. The body mass of the runners in the study also decreased 2.2 to 4.4 pounds.
The 20-beats-per-minute increase in heart rate of these runners which may have been related to fluid loss can perhaps be explained by Scott Montain and Ed Coyle from the University of Texas.
These scientists showed that for every 1 percent loss in body weight due to dehydration, heart rate increased by about 7 beats per minute.
Their results also showed that adequate fluid replacement during exercise reduced the rise in heart rate considerably. The following table adapted from work conducted by Michael Lambert of the Sport Institute of South Africa shows the magnitude of heart rate increase after fluid loss and can be used as a guide for adjusting heart rate during exercise.
By using a heart rate monitor to accurately monitor your heart rate during exercise, you now have the ability of not only knowing how your cardiovascular system is affected by heat stress and dehydration, but you have a tool that will help you possibly rehydrate more effectively during exercise and keep dehydration at bay.
The following graph shows the effects of fluid loss:
| Change in weight | 110 lbs | 130lbs | 155lbs | 175lbs | 200lbs |
| 1 pound | 7 bpm | 6 bpm | 5 bpm | 4 bpm | 4 bpm |
| 2 pounds | 14 bpm | 12 bpm | 10 bpm | 9 bpm | 8 bpm |
| 3 pounds | 21 bpm | 18 bpm | 15 bpm | 13 bmp | 12 bpm |
| 4.5 pounds | 38 bpm | 23 bpm | 20 bpm | 18 bpm | 16 bpm |
To help combat dehydration, begin drinking even before you start your exercise session. Drink 6 to 8 ounces of water or a sports drink as you are getting out the door or stepping on the court.
During the exercise session, slow down your intensity by monitoring your heart rate and try to drink at least 8 to 12 ounces of fluids every 20 minutes (make sure you sip fluids to avoid stomach discomfort).
If you cannot carry enough fluids in your water bottles if you are on a long bike ride or hike, wear a back or hip-mounted hydration system to ensure you drink enough. Such systems also keep fluids colder, and cool drinks tend to taste better, so you are apt to drink more.
Beating the heat
Training, acclimatization, proper fluid replacement, pacing and by using a heart rate monitor will help you perform your best in summer heat. The bottom line on all of this is to know your body. And by planning ahead you can minimize the effects of heat stress. Remember you can't change the weather, but with a little planning, you can beat the heat.
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