Revision as of 08:11, 4 September 2013

Running in the heat is far more stressful and can be dangerous.

Running in the heat is much harder than running in cooler temperatures, reducing performance and creating serious health risks. Moderate exercise in mild temperatures, such as 60f/15c, will reduce performance, but the perception is likely to be fatigue rather than overheating. More extreme exercise or temperatures can create the risk of heat illnesses which can be dangerous.

1 Heat and Fatigue

It is generally accepted that exercise at a constant effort will be voluntarily terminated once the core body temperature reaches about 40c/104f^[1]. However, if subjects are allowed to vary their pace freely, they will preemptively reduce their exercise intensity to prevent their core temperature form reaching the critical point^[1]. One mechanism for this reduction is that fewer muscle fibers are recruited in hot conditions, even before the core body temperature starts to rise^[2]. Elevated core temperature reduces the voluntary maximum force generated, though it is unimpaired when electrical stimulation is used, indicating the central nervous system is responsible^[3] . When exercising at a constant Rating of Perceived Exertion, subjects steadily reduced their power output at 59f/15c, 77f/25c or 95f/35c even though their perception of their temperature comfort remained constant^[4].

1.1 Mechanisms

Exercise in the heat burns more carbohydrate than in cool conditions^[5] . However, it is not clear to me if this is because of a direct change in the use of carbohydrate, or if the hot conditions reduce V̇O₂max and therefore the exercise intensity is effectively increased. Exercise in the heat also burns more muscle Protein^[6][7], and the waste products from this Protein metabolism could result in mental fatigue due to a rise in GABA^[5].

2 Hyperthermia - Heat can kill

Running in the heat can kill, as the body's protective mechanisms that normally cause us to slow down can be ineffective. There are a number of risk factors for serious heat injury

1. Being overweight, as fat can act as an insulator.
2. Being untrained, as fitness gives some heat adaptation over the unfit.
3. Lack of heat acclimatization, as this acclimatization gives some protection from the heat.
4. Thinking you can run faster than you can in the conditions.
5. Having suffered from heat illness before.
6. Ignoring warning signs (see below).

3 Staying Alive in the Heat - The Warning Signs

The best advice seems to be to take things cautiously if you are not used to running in the heat. Pushing yourself harder than normal in familiar heat, or attempting to run normally in heat you are not used to is dangerous. Traveling to a warmer area for a race is especially risky. Look out for the following warning signs, and if you have any doubts, slow down or stop and cool off.

• Nausea or vomiting. These symptoms can occur before true heatstroke, as running makes digestion harder.
• Weakness. An unusual muscular weakness could be due to low blood sugar, but elevated core temperature also creates weakness.
• Headache. This can also be caused by dehydration, or low blood sugar. Having had headaches from each of the three causes, I have found the type of headache is different. My limited experience is that a headache cased by heat is particularly painful and intense.
• Dizziness or confusion. This is a serious symptom that suggests either extremely low blood sugar or heatstroke.
• Flushed/Hot Skin. I've found that an early warning sign is the feeling that my skin is burning, especially my face.
• Panting. Another symptom I've found of overheating is that my Breathing becomes labored beyond what is reasonable for the exercise intensity.
• Core Temperature. The only sure test is to check your core temperature using something like an in-ear thermometer. Using a mouth thermometer may not be accurate if you've been Breathing hard.

If you have any doubts, stop and check your temperature. Avoid High Intensity Interval Training in the heat; the intense work can spike your core temperature too high too quickly for you to recover. It's possible that a runner that suffers heat stroke may exhibit none, or only one of these warning signs.

4 Tips for Running in the Heat

• Get used to running in the heat slowly. See Heat Acclimation Training for more details
• Check the temperature and humidity before you run. Knowing what to expect can help you adjust your pace. Remember that humidity has a big impact on your ability to cool off.
• Run in the cooler parts of the day. Early morning and later in the evening work well, late afternoon is often the worst.
• Stay Hydrated. The entry on Practical Hydration covers hydration and electrolytes. Remember that electrolytes are as important as water for staying hydrated. Hyponatremia can kill!
• Cold drinks can also help lower your body temperature, but don't drink too much without electrolytes (see above tip on hydration)
• Wear white. It may not be your favorite color, but it is the coolest (see below).
• Under Armor Heat Gear Top in white can keep you cooler than bare skin
• Wear a (white) hat. In very hot, sunny conditions, a 'legionaries cap' can help protect your neck and face. See Running Hats for more details.
• Adjust your pace for the heat. See Impact of Heat on Marathon Performance for some guidelines.
• Protect yourself from the UV. Either wear sunscreen, or cover up. Remember that skin cancer is a nasty way to die.
• Wear sunglasses with UV protection. I love my Oakley Half Jackets ;}
• Wearing a Jimbo Bandana works wonders and it can be used for Precooling.
• Pouring water over yourself can help slightly
• Watch out for blisters as your feet are more likely to be wet than in cooler conditions.
• Run in the shade of possible.
• Plan ahead; carry more drink with you than you expect to need.
• For speed work, prefer shorter intervals and be very cautious.
• Take Walking Breaks to cool off if you are having problems
• Watch out for the warning signs listed above. I find nausea is the first sign I have of overheating and headache is my first sign of dehydration.
• Carry a cell phone. If you get it wrong, you can call for help. (This is true in many situations.)
• Changing your running route so that you are doing several shorter loops or out-and-backs. You can then cache some drinks for each section rather than carrying enough for the whole run.
• Ice bandanas can also be used around the wrists. (Jim P)
• Ice under the hat. (Jim P)
• If you are doing loops, keep a spare shirt in a cooler full of ice, and switch every loop. (Jim P)
• Even holding some ice in your hand, or holding a cold water bottle can help^[8].
• Some antidepressant medications have been linked to heatstroke.

5 The effect of clothing color

A study^[9] comparing clothing color in hot conditions (38C) and strong sun showed that black clothes result in 2.5x the gain heat from the sun compared with white clothes. Tan clothing of a military uniform gained 1.7x more than white clothes and just shorts (semi-nude) gained 2.2x more. Therefore, it's important to wear white clothes in hot sunny conditions.

6 Tight or lose clothing?

The study^[9] of clothing color used black and white versions of the traditional Bedouin clothing, which has two layers of material and allows air to flow freely between them, creating a chimney like effect. This clothing mitigated most of the extra heat absorbed from the black clothing, as the hotter air was able to escape. This suggests that lose clothing may be an advantage, but only if the air can freely circulate. Also, the study used stationary people, so the benefit of loose clothing may not transfer to exercising athletes.

7 See Also

• Running calculators
  • Running Heat Model
  • Perceived Temperature For Runners
  • Heat limited running pace
  • Optimum Running Temperature
• Heat Acclimation Training
• Impact of Heat on Marathon Performance

8 References

1. ↑ ^{1.0 1.1} R. Tucker, Thermoregulation, fatigue and exercise modality., Med Sport Sci, volume 53, pages 26-38, 2008, doi 10.1159/000151548, PMID 19208997
2. ↑ R. Tucker, L. Rauch, YX. Harley, TD. Noakes, Impaired exercise performance in the heat is associated with an anticipatory reduction in skeletal muscle recruitment., Pflugers Arch, volume 448, issue 4, pages 422-30, Jul 2004, doi 10.1007/s00424-004-1267-4, PMID 15138825
3. ↑ Nybo, Lars. "Hyperthermia and fatigue." Journal of Applied Physiology 104.3 (2008): 871-878.
4. ↑ R. Tucker, The rate of heat storage mediates an anticipatory reduction in exercise intensity during cycling at a fixed rating of perceived exertion, The Journal of Physiology, volume 574, issue 3, 2006, pages 905–915, ISSN 0022-3751, doi 10.1113/jphysiol.2005.101733
5. ↑ ^{5.0 5.1} T. Mündel, Exercise heat stress and metabolism., Med Sport Sci, volume 53, pages 121-9, 2008, doi 10.1159/000151554, PMID 19209003
6. ↑ RJ. Snow, MA. Febbraio, MF. Carey, M. Hargreaves, Heat stress increases ammonia accumulation during exercise in humans., Exp Physiol, volume 78, issue 6, pages 847-50, Nov 1993, PMID 8311952
7. ↑ FE. Marino, Z. Mbambo, E. Kortekaas, G. Wilson, MI. Lambert, TD. Noakes, SC. Dennis, Influence of ambient temperature on plasma ammonia and lactate accumulation during prolonged submaximal and self-paced running., Eur J Appl Physiol, volume 86, issue 1, pages 71-8, Nov 2001, PMID 11820326
8. ↑ AR. Hsu, TA. Hagobian, KA. Jacobs, H. Attallah, AL. Friedlander, Effects of heat removal through the hand on metabolism and performance during cycling exercise in the heat., Can J Appl Physiol, volume 30, issue 1, pages 87-104, Feb 2005, PMID 15855685
9. ↑ ^{9.0 9.1} Amiram Shkolnik, C. Richard Taylor, Virginia Finch, Arieh Borut, Why do Bedouins wear black robes in hot deserts?, Nature, volume 283, issue 5745, 1980, pages 373–375, ISSN 0028-0836, doi 10.1038/283373a0