Performance may be influenced tremendously by temperature. As air temperature rises, the combination of environmental heat and increased body heat from exercise may result in bad effects ranging from decreased performance to death. Extreme cold, while usually not life threatening, can cause excessive body heat loss making good performance difficult.
The human body is able to maintain a fairly constant temperature under varying environmental conditions. To do this, it must be able to gain or lose heat. The core temperature is regulated to remain relatively constant, but the temperature of the shell, the skin and the tissues directly beneath it, varies directly with environmental conditions. The hypothalamus in the brain controls the body temperature and calls into play either heat loss or heat production mechanisms. Regulation comes in response to changes in the skin or blood temperature.
Normal metabolism in the body produces heat. Increased heat production can come from higher metabolic rates, disease, shivering or exercise. During exercise, the increased metabolic rate and energy production both generate heat. Most of the heat gain is due to the lack of efficiency of the body. It converts only 20-25% of energy produced into work; the rest is dissipated as heat.
Heat loss is governed by the following physical means.
In a cold or cool environment, conduction and convection, along with some evaporation of sweat, can maintain the heat balance. As the temperature rises, evaporation of sweat becomes the main way of controlling the rise in core temperature. Evaporation can keep the body's exercising temperature in the normal range of 102-105 F under normal environmental circumstances.
Four environmental factors can interact to change the effectiveness of evaporation mechanism. They are air temperature, relative humidity, air movement, and radiation. Caution should be advised when the temperature is above 80 F or when the relative humidity exceeds 50-60%. A small breeze will help keep the body temperature near normal by helping to evaporate sweat. Radiant heat absorbed from the sun by the body will add to the heat load.
Running unwisely under environmental heat stress may lead to a variety of heat illnesses which can be life threatening. These illnesses are caused by three factors: increased core temperature, loss of body fluids, and loss of electrolytes. While running in the heat, monitor your condition for signs of weakness, dizziness, nausea, disorientation, cessation of sweating and piloerection, the standing up of body hairs. If these signs occur, stop running and start the appropriate treatment. They could be symptoms of any of the major heat illnesses described below.
There are ways to reduce hazards when running in the heat and/or humidity, most are common sense:
Acclimatization is the process of adapting your body to be able to run more efficiently under hot environmental conditions. When it is hot the blood goes to the skin for cooling the body as well as to the working muscles. This increases the workload of the heart and the exercising heart rate. Intensity of exercise will need to be reduced when running in the heat and when acclimatizing for proper adaptation.
The body makes several adjustments during the heat acclimatization process. The circulatory adaptations to acclimatization provide better transport of heat from the core to the skin. There is better distribution of the blood to regulate temperature. This frees a greater portion of the heart output for the working muscles. Sweating mechanisms undergo complementary changes. Sweating starts at a lower body temperature and the capacity for sweating nearly doubles. The sweat becomes more dilute, contains less salt, and is more evenly distributed over the skin. Major changes occur during the first week of heat exposure and are mostly complete after 10 days.
The ways to acclimatize are:
Optimal performance depends on proper hydration. Dehydration or excessive loss of body water reduces the amount of time you can exercise as well as necessitating slowing down. Changes that take place at the cellular level adversely effect muscle contraction. Water losses of 2% or more of body weight impair circulatory function and create heat imbalance.
Sweat is comprised mainly of water and sodium and chloride ions. These ions are known as electrolytes. Other electrolytes are also present in small amounts. Studies of electrolyte balance during and after exercise have shown increases in the electrolytes in the blood, but these changes are probably due to water loss and muscle use.
Electrolyte deficiency probably does not occur during marathon running. It is during the recovery period after prolonged sweating when the content of electrolytes in the blood has been shown to be lower. Electrolytes should be replaced following the run. Studies of marathon running have shown that the most important factor is to replace body water lost during the run. Small amounts of glucose taken throughout the run may be helpful as well.
If running in the heat for several consecutive days, try to replace fluids and eat a balanced diet. Add salt to foods and select foods high in potassium such as bananas and citrus fruits. Salt tablets are unnecessary and may be harmful when not taken with adequate water.
There has been no evidence to show that glucose-electrolyte solutions help replenish body water better than plain water. Electrolytes do not need replacing during exercise. After exercise, replenish as noted above. Glucose might be useful during exercise, but the concentration of the solution is very important and differs depending on the temperature. Too high a sugar concentration will retard absorption of water and too high electrolyte concentration tends to lead to intestinal cramps. If you plan to take any of these substances during a race, experiment during training before doing so. During high intensity prolonged exercise in the heat fluid replacement drinks:
You should drink 6-8 ounces of fluid every 15-20 minutes during exercise. You can also hyperhydrate by drinking 2-4 cups of cold fluid 15-30 minutes before exercise.
Cold is usually not as hazardous for the runner as is heat. With exercise metabolism, the body is able to maintain a constant core temperature in air temperatures as low as - 22F. This is regulated by internal mechanisms and not necessarily by the heat produced from exercise. Shivering can be seen during exercise when the core temperature is low. Under this stress, oxygen consumption is higher than when doing the same amount of exercise in warm weather.
Common sense tells you to be comfortable while running; this is also true in cold weather. Both body fat and clothing act as heat conserving mechanisms. High body fat is not conducive to good performance and is not common in runners, so most must learn to dress warmly. Layers of clothing trap and warm air between them to act as insulation. If clothing becomes wet either through sweating or external sources (rain, snow), it can conduct heat away from the body. Fabrics that are waterproof, but can still breathe are best for external layers. Polypropylene is excellent next to the skin as it wicks away the water and allows a warm air layer to remain. A major part of heat loss is through the head, so wear a hat or ski headband to help keep warm. Gloves are nice as well. You can remove gloves, hat or layers of clothing as you become warmer. Running with bare legs in cold weather is not advised. The red color of the skin shows that a great deal of the blood is detoured to the skin trying to keep the body warm and is not going to the exercising muscles where it is needed most. Cold muscles feel tight and are more susceptible to injury, especially pulls and strains.
Run The Planet thanks Patti & Warren Finke and Team Oregon for the permission to reprint the complete online version of the first edition of the book Marathoning Start to Finish (Hypertext Version 1.02) by Patti & Warren Finke. © 1986, 1996 wY'east Consulting, All Rights reserved.