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running in the heat and humidity
Pace Adjustments for
Heat and Humidity*3
Temperature Adjustment Adjustment for Humidity > 60%
55°F-60°F +5 sec/mi +10 sec/mi
60°F-65°F +15 sec/mi +25 sec/mi
65°F-70°F +30 sec/mi +45 sec/mi
70°F-75°F +40 sec/mi +1:05 min/mi
75°F-80°F +1:10 min/mi +1:45 min/mi
80°F-85°F +2:00 min/mi +3:00 min/mi
85°F+ Not Recommended
Summary
* Running in temperatures above 55°F or 60°F raises your core temperature .
* The body diverts some of the blood formerly used to fuel muscles to the skin capillaries for cooling.
* With less blood and oxygen reaching the working muscles, you are forced to slow down.
* Several factors can affect how much heat your body generates and is able to dissipate (heat balance). These include exercise intensity, body size, medications, caffeine, sweat evaporation, humidity, air temperature, wind speed, cloud cover, clothing, acclimitization, and running economy.

This is a topic of great interest to me because living in Texas and training in the summer means you have to learn to deal with hot and humid conditions (morning lows in the 80's and humidity over 70%). I have always been susceptible to heat illness during running, so I've done a great deal of research in order to better prepare myself for running during our Texas Summers.

First, some background on why hot weather running may be difficult and can be dangerous if not approached correctly. "As much as 70% of the total chemical energy using during muscular contraction is released as heat rather than as athletic endeavor."*1 Your body has to dissipate that heat either through sweat or evaporation or both.

"When you exercise strenuously in even moderate heat (above 60°F; above 55°F for beginning runners), you raise your core body temperature. This triggers a release of blood into the capillaries of your skin to cool you down, which then reduces the blood supply available to your exercising muscles. This basically means that you will have less blood and oxygen delivered to the power source that moves you forward--and less blood to move out the waste products from these work sites. As the waste builds up in the muscle, you will slow down."*2

Factors Which Affect Heat Balance:

Intensity of Exercise
The faster you run, the more heat you will generate. So, under warmer and more humid conditions, you are going to want to slow down. Galloway's Book on Running (1984) has a chart called "Adjusting race pace for heat".

Estimated Temperature at FinishSlower than Goal Pace
55°F -60°F1%
60°F-65°F3%
65°F-70°F5%
70°F-75°F7%
75°F-80°F12%
80°F-85°F20%
Above 85°FForget it...

He attaches a note: "This chart is based upon my own experience in the heat and talking to other runners. It has no scientific verification, but I think you get the general idea."

The chart*3 in the information block above also indicates pace adjustments for heat and humidity was was provided courtesy of a training class at RunOnTexas.

Body Size
The larger you are, the more effort it takes to move your skeleton, and the more heat your body generates. A larger body mass also acts as an insulator, and you definitely don't want a lot of insulation during the warmer months.

Medications
Always check with your doctor first before adjusting, taking, or discontinuing medication! Antihistamines, anti-depressants, anti-inflammatories (e.g. Aleve), cold medicines, cholesterol and blood pressure medications and several other medications can interfere with the body's heat balance system. For example, did you know that NSAIDs like Advil, Motrin, Aleve, ibuprofen can intefere with a hormone that helps the body to retain the electrolyte sodium?

Caffeine
Drinking coffee, tea, or other items containing caffeine before a warm weather run can also cause you to generate more heat, not to mention increase your heart rate.

Sweat Evaporation
Sweating itself does not cause heat loss. It is the evaporation of the sweat into the atmosphere that causes heat to be lost. For this reason, if the air is too humid and there is not much wind, very little sweat will evaporate. If the humidity is high, the air cannot absorb more water, and the sweat does not vaporize and simply drips from the skin, thereby not providing much cooling effect.

Humidity
For the reason discussed above, if the humidity is high, the air can't absorb more water and sweat cannot evaporate and produce a cooling effect. During low facing winds, sweating is the most important method of heat loss during exercise. For that reason, it's a good idea to check the humidity, temperature, and wind before going on a run. Intellicast.com has an hourly forecast for any zipcode and shows all of these values.

Air Temperature
Because your body generates a lot of heat during exercise, it makes sense that if the surrounding air and objects are cooler, it's going to be easier to dissipate the heat. "Convection is the transfer of heat energy into the surrounding air. Any nearby object whose surface temperature is lower than the skin temperature (for example, trees or the road surface), will attract this heat...In contrast, when surrounding object are hotter than the body - for example hot tar roads - heat is transferred by convection from those objects to the runner's body."*1

Wind Speed
Running into a moderate to high wind allows you to lose heat by convection. If the wind is coming from behind you at the same speed you are running, it prevents heat dissipation from convection. This is due to the fact that both speeds are the same and in the same direction, which essentially is the same as running in a windless environment. Both cycling and running produce an effective wind speed but this may not be enough to provide sufficient heat loss during high temperatures and high humidity.

Cloud Cover
The body can absorb additional heat from not only hot roads, but also the sun. Therefore, it's easier for the runner to dissipate heat when there is a good cloud cover or the sun hasn't risen or set yet. However, running in the dark poses its own set of hazards.

Clothing
Lighter colors reflect the sun's rays and darker colors absorb them. Therefore, I would steer away from wearing black or dark colors during the warmer months. In addition, clothing can trap a thin layer of air next to the skin, which heats to body temperature and acts as an insulator. This is especially true of non-porous, tight, or form fitting clothing. Because you can only undress so far without breaking the law :), the best thing to do is wear light, breathable CoolMax™ garments. The lightest, coolest singlets I have found are from RaceReady. They are available in a variety of colors and only around $22. As far as shorts, the lightest, coolest ones I have found are the Brooks Women's Hvac Shorts. They actually have pinholes in them so the air isn't trapped against your skin. T-shirts are one of the worst things (in my opinion) that you can wear for running in warm weather. A t-shirt can hold up to 4 times its weight in sweat. So that 8 ounce t-shirt suddenly weighs 2 lbs if you've been sweating heavily. So not only are you not able to dissipate heat, you're actually generating more heat by having to run with more weight!

Hats
You can dissipate a substantial amount of heat through your head. In warm weather running, I have found that even the mesh running caps hold in too much heat. Yet, I still wanted to protect my face and eyes from the sun. A visor is the answer. It lets the heat dissipate off the top of your head, but provides some face/eye protection. In addition, many of them are lined with sweatbands, preventing the sweat from dripping into your eyes. I also like the fact that when we get a pop-up thunderstorm (so common here in DFW), that the visor usually keeps the rain out of my eyes. I never realized how much heat is lost from the head until I saw another runner come inside after a cold run. He took off his hat and there was steam coming from his head!!!

Acclimitization
According to the experts (Armstrong and Maresh 1991) , heat acclimitization occurs after 7 to 14 days, although improvements can occur for up to 30 days. They recommend training in the heat for gradually increasing amounts of time (30 minutes to 100 minutes) for 10 to 14 days. "Heat acclimitization adaptations can also be lost in 10 days." -- Patty and Warren Finke, Team Oregon. Therefore, if you start getting acclimated to the heat, then take 10 days of running on a treadmill inside, you may have to start all over again getting acclimated to the heat.

Sponging or Wetting the Body
"As the skin temperature rises, it causes blood to pool in the veins of the arms and legs. The veins soon become filled with a large volume of blood essentially lost from circulation and only can be returned to circulation if the skin temperature is lowered...Thus, the benefits of skin wetting during exercise probably relate to its ability to increase return of blood from the limb veins to central circulation."*1 By returning the blood to central circulation your heart can take advantage of the oxygen in this blood and maintain a higher stroke volume.

Running Economy
With good running economy, you can expend less energy (and generate less heat) for the same pace and distance.
3 Major Types of Heat Illness

Heat Cramps
  • Cause: Electrolyte deficiency or imbalance due to exercising in hot weather.
  • Symptoms: Sharp, stabbing pain in a muscle, especially in the leg muscles, although it can occur in the diaphragm, resulting in a very painful side stitch.
  • Treatment: Stop running, drink fluids containing electrolytes, cool the body with wet towels, and get to a cool area immediately. Seek medical attention if necessary.

Heat Exhaustion
  • Cause: Body unable to dissipate enough heat generated during exercise in warm, humid conditions and also loss of electrolytes.
  • Symptoms:Moderate rise in body temperature, dizziness, nausea, vomiting, headache, weakness, lack of coordination, heat cramps, heavy sweating accompanied by moist and cold skin, "goose bumps", rising heart rate, fatigue that causes you to slow down.
  • Treatment: Stop running, drink fluids containing electrolytes, cool the body with wet towels, lie down with feet elevated a few inches above heart, and get to a cool area immediately. Seeking prompt medical attention is also highly recommended. Untreated heat exhaustion can rapidly progress to heatstroke.

Heatstroke
  • Cause: Body's thermoregulation fails. It cannot dissipate the heat generated during exercise due to one or more of the factors mentioned above and core body temperature rises to 105°F or higher. Untreated heat exhaustion can rapidly progress to heatstroke.
  • Symptoms:May or may not include symptoms of heat exhaustion. Symptoms include lethargy, extreme weakness, confusion, odd behavior, disorientation, unconsciousness, cessation of sweating, hot, dry skin. Can lead to convulsions, seizures, coma, and death.
  • Treatment: This is a medical emergency that requires immediate medical attention. Call 911 and/or the emergency response team immediately. Move runner to a cool area, rub body with ice, or immerse in cold water.


*1 Tim Noakes, MD (Lore of Running)
*2 Coach Jeff Galloway
*3RunOnTexas