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Exercise in the Heat (part 1) - Thermoregulation

Yuri Elkaim, BPHE, CK

 

As the summer and hot weather approaches, it is important to be aware of the risks that come with exercising in the heat.  Exercising in hot climates can lead to severe dehydration leading to events such as heat cramps, heat exhaustion, and worst of all heat stroke.  As such, this article introduces the mechanisms of heat stress injuries and outlines the importance of thermoregulation, as well as, several guidelines that you can follow to best prepare your body for exercising in the heat to prevent any negative effects of heat stress.

Thermoregulation

Thermoregulation refers to the processes that your body undergoes to maintain its optimal core temperature.  Continuous exercise in a hot/humid environment poses a particularly stressful challenge to the maintenance of normal body temperature and fluid homeostasis (balance).  Since your body temperature needs to be maintained at 37°C (98.6°F), the presence of environmental heat stress increases the requirements for sweating and circulatory responses to remove excess heat from the body.  In addition, the act of exercising (muscular contraction) increases your metabolic rate above resting levels and increases the rate at which heat must be dissipated from you body to prevent your body temperature from rising to dangerous levels.  Thus, the act of thermoregulation is essential in order for the body to function properly.

How your body loses heat

The predominant way your body loses heat is by sweating.  More specifically, heat is lost by the evaporation of sweat.  Since sweat evaporation is influenced by the amount of moisture in the air, your body’s ability to thermoregulate will differ depending on the environment.  For example, hot and dry air receives vaporized sweat readily, whereas hot and humid air receives little sweat evaporation because it is already heavily laden with moisture.  Because sweat can’t readily evaporate in hot and humid environments, your body begins to rely increasingly on non-evaporative dry heat loss via increased skin blood flow.  This explains why your skin becomes red and flushed when you exercise in hot-wet conditions.

Performance in a Hot Environment

It is well known that performance during both prolonged, submaximal exercise (ie. marathons, triathlons, 3-5 set tennis matches, etc…) and intermittent, high-intensity exercise (ie. soccer, rugby) is impaired in a hot/humid environment.  It is best to consider the influences of elevated body temperature (hyperthermia) and dehydration as the critical factors, rather heat and humidity per se.

Hyperthermia

The 3 most important causes of hyperthermia include high exercise intensity, high air temperature, and high relative humidity.  There are 3 ways in which such hyperthermia can impair physical performance:

  1. Reduced muscular endurance.  This is due to the fact that hyperthermia creates competition for blood flow between the working muscles and the skin.  That is, as the body temperature rises with exercise, more blood moves away from the working muscles toward the skin to assist in cooling the body.
  2. Increased glycogen depletion.  A high body temperature shifts metabolism from primarily aerobic to the anaerobic form.  As a result, muscle and liver glycogen are consumed at a faster rate along with a paralleled increase in lactic acid production.
  3. Increased cardiovascular strain.  Hyperthermia causes the dilation of the blood vessels in the skin and pooling of blood in the limbs.  This reduces the volume of blood that returns to the heart, reduces cardiac output, and increases circulatory strain.  These effects are perceived as increased fatigue because the capacity to deliver oxygen to the muscles is reduced.

Dehydration

Although mild to moderate exercise typically results in whole body sweat losses of 0.8 – 1.4 L/h, the highest sweat rate ever recorded for an athlete was 3.7 L/h during the marathon at the 1984 Summer Olympic Games.   Since 1 L of sweat of water loss is equivalent to 1 kg of body weight it is easy to see why many athletes who exercise in hot climates routinely experience a 2-8% loss in body weight during training and competition.  Thus, the importance of proper hydration before, during, and after exercise in such environments cannot be undermined.

Research has shown that it is unlikely that small reductions in body weight due to dehydration     (-1% to -5%) alter muscular strength.  However, sustained or repeated exercise that lasts longer than 30 seconds can be seriously impaired when moderate to severe dehydration exists (-6% or more).  As previously mentioned, this is most likely caused by the shunting of blood away from the working muscles, waste removal, and heat dissipation.  Dehydration also has detrimental effects on endurance performance (ie. running, cycling) as is seen when examining the effects of hydration levels on maximal oxygen uptake (VO2max) and endurance capacity (ie. exercise time to exhaustion).  It has been shown that increasing dehydration interacts with air temperature to reduce both VO2max and endurance capacity.  For instance, a 2% and 4% decrease in body weight during exercise in a hot environment leads to a 10% and 27% decrease in VO2max and 22% and 48% decrease in endurance capacity, respectively.   These declines coincide with the observation that plasma volume (blood) decreases significantly during prolonged exercise in hot environments to the point that such exercise is significantly impaired to do a phenomenon known as “cardiovascular drift”.

Combating Hyperthermia and Dehydration

Because the body adapts specifically to how it is trained, it is important to prepare your body in the way it will be stressed during competition.  In other words, to best cope with exercise in the heat, it is imperative to train in the heat so that your body can acclimatize.  Regular exercise in hot environments results in a series of physiological changes designed to minimize the effects of heat stress.  The primary adaptations that occur with heat acclimatization include:

  • Increased plasma volume
  • Earlier onset of sweating
  • Higher sweat rate
  • Reduced sodium-chloride loss in sweat
  • Reduced skin blood flow
  • Increased heat shock proteins in tissues

All of these changes have the overall effect of lowering the heart rate and core temperature during submaximal exercise.  Ensuing, your body is more capable of sustaining performance in hot environments.   It is important to remember that complete heat acclimatization requires up to 14 days. 

Guidelines for Heat Acclimatization

  1. You can facilitate heat acclimatization by exercising in the heat at intensities greater than 50% VO2max.  Ideally, your total exercise-heat exposure time each day should be 90-100 minutes, but you will want to gradually build up to duration over the 14 days.
  1. Exercise with a partner during the initial days of heat acclimatization training and on very hot days.
  1. Rather then compromising the intensity of your workouts in hot-humid weather, it is preferable to that your high-intensity training be conducted in the cooler air of early morning to allow your body to experience rigorous training.
  1. Monitor your body temperature during or immediately after heat acclimatization training to ensure that is within safe limits (less than 39°C or 102°F).  You can do so with the use of a glass rectal thermometer.

Rehydration Strategies

In a hot environment, most fluid is lost as sweat, which may exceed 10 L if heat exposure lasts for an entire day.  Such exposure can result in events such as heat cramps, heat exhaustion or even heat stroke (covered more in part 2).  The easiest way to remain aware of your hydration level is to measure your body weight before and after exercise.  Since 1 L of sweat is equivalent to 1 kg of body weight, any weight loss should be replaced by consuming 1 L of fluid for each kilogram of body weight that was lost.

Symptoms of dehydration may include:

  • dry mouth
  • thirst
  • irritability
  • general discomfort
  • headache
  • apathy
  • weakness
  • dizziness
  • cramps
  • chills
  • vomiting
  • nausea
  • head or neck heat sensations
  • excessive fatigue and/or decreased performance

 

The following are recommendations made by the American College of Sports Medicine to help prevent the onset of dehydration and to optimize performance in hot environments.

  • At least 2 h prior to exercise, you should consume 500 ml of fluid.  This promotes proper hydration and allows time for the kidneys to excrete excess water, if any exists.
  • During exercise, you should begin consuming fluids at a rate equal to your sweat rate.  In general, this amounts to 150-200 ml every 15-20 min for most individuals.
  • Fluids should be cool (15-22°C), palatable, and readily available.

 

Stay tuned for Part 2 where you’ll find out more on hydration protocols and how to prevent dehydration…

 

© 2006 Yuri Elkaim, BPHE, CK. 

 


References

 

Gonzalez-Alonso, J et al. (1999).  Metabolic and thermodynamic responses to dehydration-induced reductions in muscle blood flow in humans.  Journal of Physiology, 520: 577-89.

Sawka, M. & Wenger, C. (1988).  Physiological responses to acute exercise-heat stress.  In Human performance physiology and environmental medicine at terrestrial extremes, edited by Pandolf, K. et al, 97-151.  Indianapolis: Benchmark Press.

Rowell, L. (1988). Human circulation regulation during physical stress.  New York: Oxford University Press.

Armstrong, L. et al (1986).  Preparing Alberto Salazar for the heat of the 1984 Summer Olympic marathon.  The Physician and Sportsmedicine, 14: 73-81.

Horswill, C.  (1991).  Does rapid weight loss by dehydration adversely affect high power performance?  Gatorade Sports Science Exchange, 4: 1-4.

Armstrong, L. (1991).  Keeping your cool in Barcelona, a detailed report.  Colorado Springs: US Olympic Committee, Sport Sciences Division, 1-29.

American College of Sports Medicine (1996). Position stand. Exercise and fluid replacement. Medicine and Science in Sports and Exercise, 28(1): i-vii.

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