Dr. Pribut on Heat Stress

Running In The Heat: 2000 Version
Dr. Stephen M. Pribut's Sport Pages

You've got a 15 miler scheduled .... how bad does It feel today?

Check out the table and javascript heat index that follows to see what it feels like at 82 degrees with 60 percent humidity. Probably not very good. The best defense against heat injury is hydration. So drink up. Drink before your long run, drink up before you go to sleep, and drink up when you wake up. In general you should drink at least 8 glasses of water per day under normal circumstances. Always drink before you run and drink about 8 oz. every 20 minutes while you run. Water is just fine for runs of up to an hour, but you may find that sports drinks will help both your recovery and your performance for runs over one hour. Most popular sport drinks have a low level of electrolytes and also contain carbohydrates (both simple and more complex polymers) to help speed up glycogen replacement. Please don't count coffee, beer or caffeinated or alcohol containing drinks in your daily tally of fluids. They act as diuretics and will cause an overall fluid loss. Caffeine does seem to enhance performance, so you may not want to skip it, but make sure you get adequate fluids and don't use beer to fuel your run.

Immediately following exercise the muscles are able to replenish glycogen stores at a much higher level than a few hours later,

Try a protein-carb mix to more rapidly replenish your glycogen stores after a long run. But don't forget your overall fluid replacement needs should be met with a sports drink or water.

so a sports drink or a fluid source such as water with another source of carbohydrates for glycogen replenishment would be fine. Don't forget that if you are using Sport Gels or Bars for carbohydrates drink 4 oz to 8 oz of water with them to aid your digestion and assist in their absorption. There is some evidence that a protein-carbohydrate drink may assist in glycogen replacement better than carbohydrates alone. This also assists in preventing the body from eating or catabolizing its muscle for energy and will help your muscles to repair themselves by having amino acids available for protein synthesis. One way to get a protein-carb mix would be to mix whey protein (which weight lifters love) and fruit juice (or orange juice). Some whey protein comes in flavors such as praline, vanilla, chocolate, but the taste would be called an acquired taste. However, if you can down those yucky gels, you can probably get this stuff down also and it could help build lean and strong muscles and is definitely good for a recovery from your long run. You can also mix it with low fat milk and a chocolate milk mix for added flavor and carbs. The choclate milk mix I use has less than 5% fat and no cholesterol. It does help it go down and after a workout or long run actually tastes good. But don't forget that water and sports replacement drinks are the best way to replenish your fluid deficit. This protein-carb mix is to help replenish your glycogen loss and speed your muscular recovery from a hard or long workout.

To help you stay hydrated during this long, hot summer of running you might consider stopping at every water fountain you pass and taking a drink. Don't forget to give yourself a few weeks to acclimate to the heat. This is best done by running 3 to 4 miles

Try combining treadmill running and outside running to get more distance on the really hot days.

in the heat, gradually building up your mileage before you try for speed. Also try to just plain miss the heat by running early in the morning or late at night. Remember though, early in the morning you mayexperience more humidity. And of course you can consider using a treadmill on the worst days. I have on occasion mixed a warm up and interval repeats on the treadmill with an outside longer and slower run. This way you can get a workout and a few more miles in a cooler environment.

And now on to the facts.

Heat Production:

During exercise, heat is produced in direct relationship to the amount of energy expended. In running, this results in speed being the primary determinant of body heat production. Elite runners would run at an energy level expenditure of 1500 kcal/hr while a novice runner would be likely to run at an energy level expenditure rate of 500 kcal/hr. Body metabolism as a means of energy production is estimated to be only 25 percent efficient. That leaves 75 percent of the energy used going solely to the production of heat. That is great for cold weather, but not of much help to hot weather runners.

The radiant energy of the sun may be responsible for up to 150 kcal/hr on a clear and cloudless day. Heat production alone would thereby give an average runner a heat burden of approximately 1000 to 1200 kcal/hr. If there were no means of heat dissipation the core temperature would rise by approximately 23 degrees. In other words, if there were no functioning thermoregulation, the body temperature would rise approximately 1 degree centigrade for every 5 to 8 minutes of running.

Heat Loss:

An individual who is heat acclimated may perspire almost twice as much as an unacclimated individual.

As we have already mentioned, perspiration and evaporation of perspiration, is the primary means for the body to cool during exercise. Skin blood flow increases significantly during exercise. Blood flowing near the surface results in cooling by both conduction and convection. There appears to be a 25 to 40 percent lower increase in Skin Blood Flow, however, in the older athlete. Sweat glands become active as body core temperature rises. One liter of sweat is generated during the expenditure of about 500 kcal. Studies have shown that training increases both sweating and skin blood flow.

An individual who is heat acclimated may perspire almost twice as much as an unacclimatized individual. One of the effects of acclimatization is to allow an individual to begin perspiration earlier in the course of exercise, this allows for a quick, effective and efficient beginning to heat dissipation and alleviation of early heat buildup. Each gram of perspiration that evaporates cools the body by 0.6 kcal. Acclimatized individuals may produce up to 30 g/minute of perspiration. This would allow for considerable cooling.

As we noted, the environment has a major impact on heat loss. Humidity, playing the largest role, but temperature, of course, also a major factor. These both impact upon heat loss via conduction and convection besides the impact on evaporation of perspiration. The worst possible conditions would be a hot, humid, windless day, following a cool spell.

Heat Induced Illness

Heat Exhaustion

Heat exhaustion is caused by dehydration. The symptoms include chills, lightheadedness, dizziness, headache, and nausea. The body temperature is usually between 100 - 102 degrees and profuse sweating is evident.

Treatment should be to get the individual to a cool shaded area and administer fluids either by mouth, if conscious or IV if the individual is unconscious. Seek medical advise.

Heat Stroke

Heat stroke may be fatal

Heat stroke is caused by a sudden failure of the thermoregulatory system of the body. Heat stroke may be fatal. Some consider it to lie on a continuum with heat exhaustion. It initially appears similar to heat exhaustion, but may rapidly progress to manifest more serious neurological symptoms: disorientation, loss of consciousness and seizures (status epilecticus). The body temperature may be higher than 104 degrees Fahrenheit. Sweating is often absent, but the skin may be quite moist from earlier perspiration. The core temperature must be brought down immediately. In fun runs, perspiration is usually noted, and you do not need the absence of perspiration or an rectal temperature to make the diagnosis. There is usually a sinus tachycardia present, with the pulse noted to be faster than 160. The blood pressure may be low. Kidney damage (acute nephropathy) occurs in about 35 per cent of cases. Rhabdomyolysis (muscle breakdown) occurs and the myoglobulinuria (excretion of muscle breakdown products) contributes to the kidney injury. Liver damage is also evident when liver enzymes are measured following heatstroke.

These are the individuals who get "packed in ice". Rapid medical attention is vital.

Avoiding Heat Stress Injury

Hydration: Drink plenty of fluid 30 - 45 minutes before exercise and then a cupful every 10 - 15 minutes while exercising. After exercise drink more fluid than you think you need. Especially if you are over 40. Your thirst mechanism, does not appear to be as effective a gauge of your water needs as you age. Drinking fluid, while exercising and after completion will help speed your recovery. Sport replacement drinks are superior to water at longer distances and times (over 60 - 90 minutes). The electrolytes and carbohydrates in them will also help speed your recovery from the stress of fluid loss and your long distance run. Somehow they do seem to taste great in the heat and the good taste encourages you to drink more and replace your fluids.
Acclimatization: Gradually build up your tolerance for running in warmer weather.
Stay Fit and don't overestimate your level of fitness: Individuals with a higher VO2 Max are more tolerant of heat tolerance than those with a lower level of fitness.
Watch your health: Make sure you are aware of both medical conditions that you have and medications that can affect your tolerance of exercise in the heat. Medical conditions affecting your heat tolerance include diabetes, high blood pressure, anorexia nervosa, bulimia, obesity and fever.
Dress Cool: Wear light weight shorts and a singlet rather than a tea shirt, to permit evaporation of perspiration.

HEAT INDEX
DERIVED FROM TEMPERATURE AND RELATIVE HUMIDITY

The Heat Index Table

Relative Environmental temperature (F)
Humidity(%) 70 75 80 85 90 95 100 105 110 115 120

0% 64 69 73 78 83 87 91 95 99 103 107

10% 65 70 75 80 85 90 95 100 105 111 116

20% 66 72 77 82 87 93 99 105 112 120 130

30% 67 73 78 84 90 96 104 113 123 135 148

40% 68 74 79 86 93 101 110 123 137 151

50% 69 75 81 88 96 107 120 135 150

60% 70 76 82 90 100 114 132 149

70% 70 77 85 93 106 124 144

80% 71 78 86 97 113 136

90% 71 79 88 102 122

100% 72 80 91 108

At an apparent temperature of:

90-104 Heat cramps or heat exhaustion possible

105-130 Heat cramps or heat exhaustion likely,
heatstroke possible

130-more Heatstroke highly likely

Note: Combined heat index of heat and humidity, what it "feels like"
Exposure to full sunshine can increase considerably.

`Eggs are cooking on the sidewalk.....`

In case nothing jumps out at you in this jumble of numbers, let's look at a typical example for Washington, D.C. Let's say you get up a bit late on a Saturday morning, after a Friday night of "renting" beer. A touch dehydrated, but unaware of this, you decide to go out for a run around 11:30 A.M. You head for Rock Creek Park, in the shade. It's hot but what the heck, you planned to run about 7 to 8 miles. Off you go. It's humid, sticky, hot, one to two miles later you feel as if you'd run 5 miles already. You wise up, turn around and have completed between 3 - 4 miles, feel as if you'd done 10 without training. Later you find out the temperature was 102 degrees.
With the humidity at greater than 70%, 100 degrees would feel like 144 degrees. When the humidity is 90%, 90 degrees will feel like 113 degrees. Hmmmmm, maybe it is not such a good idea to run then!

WBGT (Wet Bulb Globe Temperature) Index & Thermal Stress

The above table is a simplification of the usually accepted means of determining the risk of environmental heat. As can be seen from the table, humidity plays a great role in the risks associated with heat and exercise. Clearly, humidity will decrease the evaporation of perspiration, and thereby inhibit one of the primary means by which the body can cool itself. This is what led the development of the WBGT Index method of determining heat illness risk. This is also known as the wet bulb globe temperature. This method depends upon temperature measured with 1) A dry thermometer to measure ambient temperature, 2) Wet Bulb Temperature, and 3) Black globe temperature to determine radiant heat. The wet bulb temperature is measured by placing a wet wick over a thermometer bulb. This reading will be affected significantly by both humidity and wind velocity. The relative humidity is the ratio of the wet bulb to dry bulb temperatures. If the humidity is high, evaporation and consequent cooling will not take place and the reading will be close to that of the ambient temperature. Likewise, higher wind velocity will increase evaporation and lower the wet bulb reading. The wet bulb reading can never exceed the ambient temperature.

Neat Formula

But the table is easier to understand.....

WBGT = 0.7 x Twb + 0.2 x Tg + 0.1 xTdb

Twb = Wet bulb temperature
Tg = Black globe temperature
Tdb = Dry globe temperature

From this formula, it is evident that the humidity is valued as the major determinant of heat stress, as it is weighted at 70% of the value. Below a WBGT value of 64 degrees (18 C) the risk of heat injury is small. Above a WBGT value of 82 degrees (28 C) races should not be undertaken.

Heat Regulation

Body core temperature is the result of the balance between those processes that create body heat and those that dissipate heat. Winslow, Herrington and Gagge in 1937 used the following formula to approximate how the body's thermoregulatory system worked:

S= M + R + CD + Cv - E

S = stored heat
M = metabolic heat production
R = amount heat gained or lost by radiation
Cd = conductive heat lost or gained
Cv = convective heat lost or gained
E = evaporative heat loss

From this formula it is evident that heat balance in the body depends upon the amount of heat produced by muscle activity and metabolism and the amount of heat that is gained or lost either by the effect of the environment and evaporation of perspiration. Note that perspiration that drips from your body does not appreciably contribute to evaporative heat loss. Air flow either through the effect of wind or your own body motion does aid the cooling of the body. The sudden loss of this air flow is one of the reasons why you will start sweating more when you stop running.

Stay

The heat index formula gives good results for temperatures above 70 degrees Fahrenheit. The formula is as follows: t = temperature in degrees Fahrenheit, r = %relative humidity (-42.379+2.04901523*t+10.14333127*r-.22475541*t*r-(6.83783e-3)*t^2- (5.481717e-2)*r^2+(1.22874e-3)*t^2*r+(8.5282e-4)*t*r^2-(1.99e-6)*t^2*r^2)

This formula was posted to rec.running by Phil Margolies and he reports it is more accurate than the Javascript above.

For the heatedly obssessed Phil Margolies also posted the following Excel formula:
You can paste the following forumula on an Excel spreadsheet. If you put the temperature in cell A1 and the relative humidity in cell B1 and paste this formula in cell C1: =-42.379+2.04901523*(A1)+10.14333127*(B1)-0.22475541*(A1)*(B1)-0.00683783*(A1*A1)-0.05481717*(B1*B1)+0.00122874*(A1*A1)*B1+0.00085282*(A1)*(B1*B1)-0.00000199*(A1*A1)*(B1*B1)