by Shawn McDonald
The human body has the ability to adapt to working and exercising in a hot environment given adequate exposure to heat as well as proper hydration. In the article this month, we will review some of details about heat generation and loss, how to gain and maintain full heat acclimatization, and some tips on racing in the heat. We’ll also review findings from recent research into heat illnesses, hydration needs, how heat affects running performance and factors that determine individual susceptibility to heat.
Upon exposure to heat on a regular basis during exercise, the human body makes a number of changes that allow for greater aerobic performance (duration or intensity) in heat. Most studies have observed significant changes in sweat rate and composition as well as performance when athletes complete workouts for 60 to 90 minutes in moderate heat (heat index 80 – 95F) at least every other day during an acclimation period lasting two to three weeks. Acclimation can be maintained by completion of one or two warmer weather (midday) workouts per week. Fluid and salt intakes should be such that they replace the bulk of water and sodium lost via sweat during any exercise. Athletes should estimate their sweat rates on a regular basis in a variety of conditions (temperature, humidity, wind) as the rate will change as they become acclimated to heat.
Sweat rates can be measured in the lab by use of sensors on the skin or on a larger scale by determining weight change during a workout combined with measurement of fluid intake. At home, you can measure your sweat rate by calculating the difference between your weight before and after a run and dividing by two to get the sweat rate (in quarts per hour). Multiply this number by 0.95 to convert to units of liters per hour. Sweat rates for most runners vary from 0.8 to 2.2 liters per hour. Sweat rates rise with increasing temperature/humidity, increased running pace, increased body mass, and after a period of heat acclimatization. High heat and humidity also increase sweat rates which can lead to dehydration and lower performance over the course of a run or race.
Lighter runners have a thermal advantage
Exercisers lose heat through three main mechanisms: radiation to the surrounding air, convection – which heats a thin layer of air above the skin, and evaporation (mainly of sweat). Heat is generated as a result of metabolism during exercising by the burning of fuel to power muscle contraction and release. In running, about 25 percent of the fuel metabolized actually powers the athlete forward and 75 percent of burned fuel is cast off as heat. Both the rate of heat production and sweat rate are positively correlated (proportional) to body mass (Marino et al., 2000). The Marino studies also showed that heat storage was strongly correlated with body mass at 93 degrees F and moderately correlated at 80 degrees F. To maintain heat balance (production versus loss) a 100 pound athlete could run a marathon at 5:07 per mile pace while a 165 pound athlete could only run at 7:55 minutes/mile pace. The conclusion reached was that lighter runners have a substantial thermal advantage when running in conditions at which heat dissipation processes have reached their limit.
Head winds and tail winds
Ambient conditions greatly influence how much heat the runner can lose on a given day. Convection heat loss is proportional to the difference between skin temperature (typically 93F) and the air temperature. A head wind also greatly increases convection by removing heated air right near the skin and replacing it with air at the ambient temperature. Likewise, a tail wind at or near the same speed as the runner’s pace leaves air trapped near the skin and substantially decreases convection. Sweat evaporation is higher at lower air humidity and when running into a headwind, and lower in humid conditions or when running downwind.
Heat affects the work capacity of the body via an increased core body temperature and a lowered blood volume in a dehydrated state. Both the temperature and volume conditions degrade the cooling processes in the body and fuel/oxygen delivery to muscles, leading to lowered running performance. In a study of the role of dehydration in cardiovascular function (Gonzales-Alonso et al., 1997), a dehydration of four percent of body weight due to cycling in 93F heat for 100-120 minutes led to a seven to eight percent decrease in stroke volume of the heart and an adapting increase in heart rate to maintain cardiac output. The combined state of dehydration and hyperthermia led to cardiac instability (reduced cardiac output, lowered mean arterial blood pressure by 5 mm Hg, and increased systemic vascular resistance by 10%). Such a condition would give a serious performance hit and possibly dizziness or loss of consciousness. Restoration of blood volume after dehydration restored stroke volume to control levels when exercising at lower temperature.
In a similar study of cardiac function during heat exercise (Gonzales-Alonso et al., 2000), each one percent loss in body weight due to dehydration gave a reduction in stroke volume by almost five percent in the heat or two and one-half percent in the cold. Thus, even slight dehydration can lead to noticeable degradation in running performance, especially in hot conditions. Heart rate was much higher at 30 minutes of exercise time at 20C when dehydrated versus fully hydrated and rectal temperatures were much higher in dehydrated subjects than fully hydrated athletes after 60 minutes of exercise. Thus, the combination of dehydration and increased body core temperature leads to lowered aerobic work performance; stroke volume decreases, heart rate increases, total cardiac output drops, more blood flows to the skin to allow for cooling, and less blood is available to deliver fuel and oxygen to working muscles.
Individual runners will have different susceptibility to heat, dependent upon a number of factors. Heat susceptibility is higher for athletes having a greater stress response, greater body mass, faster running pace, or when in a dehydrated state. Older athletes, those taking diuretics before exercise, and athletes who are not acclimated to heat are more likely to suffer heat effects. As mentioned above, lighter runners have lower metabolic heat production at a given running speed than heavier runners (Dennis and Noakes, 1999).
Women may in general have a slight advantage over men when exercising in the heat due to a small size. A lower rate of heat generation (proportional to body mass) more than offsets a smaller skin surface area (where cooling occurs via the evaporation of sweat). Changes in physiology during the menstrual cycle of women do not have a great influence on heat tolerance. Rectal temperatures, heart rate, skin temperature, rating of perceived exertion, and pre/post exercise body weight following cycling exercise for 60 minutes at 60 percent maximum work load were the same in the follicular (days five – eight after the start of menstrual bleeding) and luteal (days 22-25) phases of the menstrual cycle (Garcia et al., 2006). Sweat rate was higher by ten percent and post-exercise urine volume was reduced in the luteal phase (about 100 mL less). Thus, women who hydrate well during exercise offset the increase in basal body temperature during the luteal phase by an increase in their sweat rates and by lowered urine volume.
Acclimated yet dehydrated
Hot weather can drastically reducing running performance, even for athletes who are heat acclimated. They often do not consume enough fluids during a long training run or race, leading to dehydration and degraded performance. Many runners underestimate both their fluid needs and sweat rates and would benefit from analyzing their sweat rate periodically throughout the year and using a regular schedule of drinking water and sports drink during hot weather runs.
Fluid uptake is usually higher when the fluid being consumed is cold. In a clinical trail, heart rate was lower by five bpm when drinking cold fluid and the time to exhaustion was greater in the cold fluid trials (62 versus 55 minutes). The authors of the study surmised that the cold fluid acted as a heat sink to minimize the rise in core body temperature. Heat stress can also be mitigated by adequate intake of sodium (400 -1200 mg/hour depending on your level of heat acclimation) to keep plasma sodium concentration in the appropriate range.. Core temperature can be lowered by immersion of limbs in a bucket of cool water or a stream, by resting in the shade every few miles during a long training run, and by backing off your effort on uphills to lower heat generation.