by Roy Stevenson
With the hot summer months looming up on us, it’s time to review the single most serious threat to a runner’s life – heat. It has a higher fatality rate than sudden death (heart attacks), or being killed by automobiles while running. And it sidelines thousands of runners during their competition with heat illness.
Elite runners have even been known to have heat problems, but it’s the semi-conditioned rank and file runner who is most susceptible to heat injury. Even dehydration can lead to unconsciousness and death if allowed to escalate into heat exhaustion or heat stroke.
Let’s examine the factors that combine to cause heat problems: air temperature, air movement, humidity, exposure to the sun, acclimatization and the intensity or duration of your exercise session or competition. We can generally tolerate temperatures as high as 80 – 90 degrees because we’re able to sweat as much as two liters per hour. In dry air, most of this sweat evaporates, cooling the body as it does so. However, as humidity increases, the already-saturated air absorbs less sweat, and body heat begins to build up. The greater the air movement around us, the greater the cooling effect as air currents enhance evaporation. A headwind helps evaporation but a tailwind actually reduces the airflow over the body, hindering evaporation.
Sunlight acts as an insulating blanket by warming the skin. Direct sunlight causes a rapid rise in body heat by elevating skin temperature; ideally the skin should always be at least two degrees cooler than your core temperature to allow for a cooling heat gradient.
Your workout intensity and the length of your workout contribute to stress produced by heat. We generate heat during exercise and the human body isn’t particularly efficient in this respect – 75 percent of our expended energy is turned into heat. Thus the faster and longer we run, the higher the heat load placed on our body.
Acclimatization, or our previous recent exposure to heat, is also a major factor in determining our susceptibility or resistance to heat illness. Through training we partially (but never completely) adapt our thermoregulatory mechanism. In addition, people respond very differently to heat, so adjustments to exercising in heat should be made on an individual basis. Thus heat will always be a limiting factor to our endurance performance.
Even fit runners can only tolerate a narrow range of internal core temperatures. The good news is that a fit runner can tolerate a higher core temperature than an inactive person, so heat problems usually arise when endurance athletes are inadequately conditioned for a race or pushing beyond their limits.
It’s usually a combination of two or more of the above factors that increase your risk of heat injury. The most formidable combination is simultaneously elevated heat and humidity. An air temperature of 60 degrees plus 95 percent humidity is more dangerous than a “dry” 85 degrees. In this situation, heat is carried from the body core to the skin, where evaporating sweat cools the blood before its return to the body’s core. But when your skin absorbs heat faster than evaporation can cool it, you run into problems. The hypothalamus – the body’s thermostat – detects this discrepancy and responds by dilating the blood vessels in the skin to be cooled. It also makes the heart pump faster to shunt more blood to the surface, causing your sweat glands to produce more sweat.
By this time, a vicious competition for blood has ensued between the brain (which needs 25 percent of heart output to function) and the working muscles, which need more blood but are getting less and less. It’s here the inexperienced or foolhardy runner makes a mistake. Instead of slowing down, they keep pushing themselves. By continuing to push, they worsen their current state of dehydration. And with this increased sweat loss, the plasma becomes thicker and more viscous, causing the heart to pump harder. These combined events place an extra burden on the cardiovascular and thermoregulatory systems, so it’s no coincidence that runners with undetected cardiac problems “choose” hot races to collapse in.
Continuing to sweat without taking in adequate fluids amplifies these demands on the circulatory system, which are by now becoming intolerable. At this stage the runner is a prime candidate for heat exhaustion, and if they ignore the signs, heat stroke. Usually performance has declined by this stage, pressuring the overcompetitive runner to pick up their pace. Then the competition for blood becomes unbearable and the circulatory system and hypothalamus shut down. Blood pressure drops. Unconsciousness. Possible death.
How to Prevent Heat Injury
Generally, males handle heat less efficiently then females, as do larger people who have less cooling surface per pound of body weight than slim people. Food digestion interferes with the blood flow to the working muscles, so avoid large meals before a race or extended training session in the heat. Wear light-colored clothing that breathes well (cotton) and repels the sun’s rays, and put some thought into the Sun Protection Factor (SPF) of your clothing. Workout and race clothing should be opaque, so that you cannot see light through it when you hold it up to the sun.
On hot or humid days don’t start your race too fast for your current level of fitness, and don’t push beyond your limits under these conditions. Know your current state of fitness and be adequately conditioned for your race – if you aren’t, don’t compete. Be wary of races organized by local charities – they may be directed by amateurs who have no idea of the dangers of heat stroke, and may have inadequate precautions for these eventualities, such as not enough water at aid stations.
Drink lots of cold water before, during and after your training and racing efforts. Be aware of hyponatraemia, learn about and balance your water consumption with your sweat rate, as practiced on your training runs. Keep your body wet. The temporary relief is well worth it. Putting ice under their caps is an old trick used by seasoned ultramarathoners. On hot, muggy days don’t try to stick to your planned training distance. Be prepared to cut back if conditions are dangerous. There may be days when you bag the outside training gig, and work out in your local fitness club. Another precaution is to exercise with a partner and keep an eye on each other.
For good acclimatization, run at least three days a week in conditions similar to those you’ll race in. Early morning and evening training sessions will not fully prepare you for the midday heat. You’ll need to allow 10-14 days of slowly progressive exercise to adjust to the heat.
The benefits of acclimatization are less sweating at a given workload, and less elimination of electrolytes in your sweat. Some research has shown the chloride in table salt can inhibit your body’s ability to deal with heat stress, so high levels of salt intake during training may not be necessary, or may even be counterproductive. You’ll get adequate salt intake from your meals, even without salting them.
More recently, sports drinks with sodium have been found to help endurance athletes retain water in their system, so these sports drinks are advised. Electrolyte replacement drinks are highly-advanced these days, and lots of research has gone into their efficacy. However, watch out for the imposters that are simply loaded with sugar and no better for you than soft drinks. One problem that runners may encounter with electrolyte drinks is that they’re too concentrated, making the athlete feel nauseated. If this is the case, dilute the drinks 100 percent or more to make them palatable.
Lastly, there is nothing macho or intelligent about shunning water during your racing or training efforts. This practice is detrimental to performance and can lead to heat injury. Recognition of the signs, symptoms and treatments of heat exhaustion and heat stroke is half the battle in dealing with heat injury. Knowing them could save your life, or a fellow runner’s life. It’s a good idea to be familiar with these guidelines to prevent unnecessary hyperthermia.
Roy Stevenson has a master’s degree in coaching and exercise physiology from Ohio University. He’s coached hundreds of serious and recreational runners and triathletes in the Seattle area. His articles on running and triathlon training have been published in over thirty regional, national and international running and triathlon magazines in the U.S., U.K., South Africa, and Australia. Roy teaches exercise science at Seattle University in the Puget Sound, Washington