Tolerating the Heat

By Marilyn Trout | 07/09/12
Tolerating the Heat

Photo © Jonathan Devich/

We’ve teamed up with Marilyn Trout, certified USA Cycling Elite Coach to answer Voler Newsletter List members’ training questions. You can view her coach profile at Send your cycling inquiries to Marilyn, and for a limited time, if yours is selected to be answered in our Training column, Voler will send you a $20 gift certificate that can be used towards any purchase from the Voler Store at To submit your inquiry, e-mail her at, and type “Voler Training Question” in the subject line of the e-mail.

The following question is a reprint of a question from David Kulcinkski from September 2009, but still holds true.

Tolerating the Heat

Hi Marilyn,

I'm a 56-year-old female in pretty good shape.  I weight train 3X per week and cycle two to 5 times a week and usually average 15+ mph over 15 - 30 miles. My resting heart rate is around 56 and my max HR is around 180.

I live in Phoenix, AZ where we are blessed with a nearly year-around cycling climate. However our summer temperatures can be brutal to ride in.

This is the first summer that I've actually stayed on my bike beyond May and I'm noticing some "odd" heart rates while cycling. For example, yesterday, I rode just under 25 miles with a group of casual riders. It was what I would consider an easy, casual, conversational ride. For the first 15 or so, my heart rate stayed within what I would consider a normal range for me for the casual nature of the ride.  However, during the last 9+ miles, I noticed that my heart rate increased significantly, and didn't recover in my normal 30 - 40 seconds timeframe. I was well hydrated and fueled. The temperature was approaching 101 degrees by the time the ride was over at 8:00 AM. I've attached a few of my data read-outs from my Garmin... each chart plots the averages for each 5-mile segment.

MY QUESTION:  Can heat cause the heart rate to increase beyond what would be considered "normal" for cooler temps? And what's the best way to train the body to tolerate cycling in the heat?  Any tips would be much appreciated.




Indeed we can see higher heart rates in response to exercise in hotter temperatures. For a more detailed explanation to your first question, check out the Training Tip in the Voler archives, “Strange Heart Rate Data.” A number of readers have also experienced “higher than normal” heart rate readings during this “hotter than usual” summer in some areas and have posted their comments at the end of this particular article. Presently, I would like to focus on how you can acclimatize to the heat so you can enjoy good health and perform your best on the bike.

Living in Arizona, you definitely know what hot temperatures are. No doubt your strategy for being well-hydrated and fueled would have been sufficient for riding in “cooler” conditions, however, 101 degrees falls into another category altogether. As you mentioned, this is your first summer cycling through the Arizona summer, so your strategy probably needs to be tweaked to accommodate these adverse heat conditions. There are a number of factors that can complicate your “tweaking”: fitness, intensity and distance of ride, fatigue whether from life or training, hydration and fueling levels before and during the ride, electrolyte losses and replacement and heat acclimatization to name a few.

Here are some facts…

Sweat rates, Electrolyte Losses and Heat

  • Perspiration is the process the body goes through to prevent its core temperature from rising. As the body perspires, it loses water and electrolytic minerals. If this continues, the cooling mechanism can become less efficient, cardiac and motor-signaling efficiency may be effected and the risk of muscle cramping increases due to these changing conditions.
  • The average sweat rate is between 0.8 to 1.4 liters per hour during exercise with the average fluid absorption rates ranging from 0.8 to 1.2 liters per hour. Although the difference between the sweat rate and fluid absorption are small, some athletes sweat at higher rates per hour than their fluid absorption rate. These rates depend upon ambient temperature, humidity, genetics, fitness, exercise intensity, weight and heat acclimatization. As little as 1 percent loss of body fluids can negatively impact endurance performance. For a 130-pound cyclist, that equates to about 1.3 lbs. of fluid loss.
  • Athletes should consume fluids containing water and electrolytes (sodium, chloride, potassium, and magnesium) according to their personal sweat rate. With such a variance in sweat rates, it is important to discover what is best for you personally. At the start line, all the training hours a rider has done will have been done in vain if fluid replacement is not sufficient. According to a study done by Dr. Tim Noakes, it is "the amount of sodium and potassium in the body that determines the water balance, not the other way around." Electrolyte balance affects your hydration balance. Balance is a good thing. Just as consuming too much water and no electrolytes during extended exercise is not good, so to consuming too many electrolytes with too little fluid is not good and can cause your body to retain fluid rather than releasing it for cooling purposes.
  • Sweat rate and sodium-loss rates change with heat acclimatization. Sweat contains about 0.8 to 2.0 grams (800 to 2000 milligrams) of sodium chloride per liter of sweat in a heat-acclimatized individual and 3.0 to 4.0 grams in a non-acclimatized individual (grams of sodium chloride per liter of sweat.)
  • Acclimatization to heat can take about 10-14 days. A cyclist that is unfit and unacclimatized to the heat will lose almost twice the sodium, twice the potassium and one-and-a-half times the chloride in their sweat compared to a fit and acclimatized cyclist. Fitness and heat acclimatization reduces electrolyte losses.

Electrolyte Contents of Sweat and the Affects of Fitness and Heat Acclimatization

Sweat of Unacclimatized,
unfit subject
Swear of fit but
unacclimatized subject
Sweat of fit and
acclimatized subject
Sodium (NA+)
Potassium (K+)
Magnesium (Mg+)
Chloride (Cl-)
  • All values in grams per liter.
  • Table adapted from Table 4.2 Electrolyte Contents of Sweat and Blood and the Affects of Fitness and Heat Acclimatization, "Fourth Edition, Lore of Running", Tim Noakes M.D., pp. 214

Simple Tips to Tolerate the Heat

  • Weigh yourself before and after your ride. Ideally, if the reading on the scale stays the same, you have supplemented your fluid needs just fine. If you have experienced a weight loss, you have lost more fluid than you have replaced during the exercise. No rocket science here. Although being a little dehydrated can be tolerated, remember that losing 1-2 percent can have a negative impact on your performance and if weight loss is over 2 percent, you may see a drop in performance up to 20 percent. You can’t go wrong starting out with at least one water bottle of fluid per hour. Fine-tuning can be done from there.
  • Make note of your sweat rate in different weather and cycling conditions. As I mentioned previously, your sweat and hydration rates will change throughout the year due to many factors so what’s good for hot temps may not be sufficient for REALLY hot. Taking an extra bottle can come in handy and if need be you can put it back in the fridge at the end of the ride. A bigger price has to be paid if you don’t have it.
  • At first glance, it may seem reasonable that your average speed for a ride is a good measurement of the actual work done, however, your cycling situation is too variable for this to be a reliable gage. There is a difference between the measurement of external work produced on the bike and the amount of energy needed to perform that work. Unfortunately, most of the energy expended is considered “waste” heat, with only 20-25 percent of converted energy being used to propel the bike in most trained cyclists. Wind direction, riding alone or in a group and terrain are some of the conditions that can impact speed and work produced. It would be better to link your sweat rate with the work done by means of heart rate or average power output rather than speed.
  • Although the weather is changing to cooler temperatures for most of us as we move into autumn, you will still have plenty of opportunity in Arizona to hone your ability to tolerate the heat.

Eat, drink and be safe,

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