Summer in Michigan can be one the best times of the year. The lakes, barbeques, and hot temperatures can make us all but forget the frigid winters and blowing snow. As much fun as summer is, though, it can present some very real health concerns for those of us exercising outdoors.
With fall sports starting to gear up for their preseason practices and runners getting prepared for their fall races, I thought it would be a good time to talk about how to stay safe and have productive workouts when exercising in the heat.
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Challenges of Exercise in the Heat
Exercising in hot/humid conditions presents a real challenge for the body on a couple of different levels. First, exercising increases your metabolism as your body burns fuel to perform the activity. A byproduct of metabolism is heat. This internal heat, combined with the external heat from the environment, can greatly increase core temperature. The body likes to regulate temperature in a fairly narrow range of 5-7° or so, and once body temperature exceeds 104°, life-threatening symptoms can occur (remember, normal body temperature is around 98°). The body works hard to maintain this narrow temperature range through regulatory cooling mechanisms. One of the best mechanisms is sweating. In this process the body sends more blood to vessels nearest to the surface of the skin (thus away from the core) to dissipate the heat through sweating and evaporation of that sweat. This transfers heat away from our bodies to the atmosphere. The challenge with exercise is that there is a competition between blood flow to muscles (to deliver nutrients and remove wastes) and the skin for cooling. This competition makes the cooling process even more difficult.
Consequences of Cooling the Body
There are a number of associated consequences with the cooling process mentioned above. First, heart rate increases in an effort to pump blood faster, trying to keep up with the demands of both the muscles’ need for energy and the body’s need to send blood to the skin for cooling.
Increased sweating for cooling reduces body water content, specifically the water content of our blood stream (also known as plasma). With even moderate water loss in plasma, the blood thickens and is harder to pump. This stresses the cardiovascular system further.
Lastly, as more and more water leaves the body in an attempt to aid in cooling, there is less fluid in the blood stream to carry water to the surface of the skin, progressively making this cooling process less effective.
As a result of all this, exercise performance declines rapidly, in large part because the body is more concerned with the stress of cooling itself rather than the stress of exercise.
The good news is in the vast majority of cases, this entire cooling process can happen without any problem. However, in certain weather conditions (and for certain types of people) this cooling process may be ineffective at reducing body temperature, leading to hyperthermia (hyper = high, -thermia = state of heat). Hyperthermia can have severe implications if not recognized and treated properly.
Hyperthermia Should Be Taken Seriously
Also known as heat illness (or in more severe cases heat stroke), symptoms of hyperthermia indicate significant problems occurring in the body. These symptoms include: dehydration, fatigue, flushing, thirst, excess sweating, fainting, inability to sweat normally, lightheadedness, low blood pressure, nervous system dysfunction, cramping, stiff muscles or muscle spasms, nausea and/or vomiting. Symptoms such as these that occur as a result of exercising in hot, humid conditions should be taken seriously as they can be very harmful (and in extreme cases, fatal).
Mild cases of heat illness can be treated by giving cold fluids and moving to a cooler (preferably air conditioned) environment. Moderated and advanced cases, however, require clinical-level care (such as IV fluid, cold immersion, and continual monitoring). When in doubt, if you think someone is suffering from heat illness, take them to the emergency room immediately for advanced care and treatment.
Impaired Exercise Performance
Far before full-blown heat illness occurs, exercise performance is impaired. This has a lot to do with the strain on the cardiovascular system mentioned above. Reduced blood volume, increased heart rate, and more blood driven to the skin for cooling (rather than the muscles for energy) will all impair exercise performance.
Additionally, temperature sensors in the brain recognize the increase in core temperature and act to depress nervous system activity in an effort to keep the body cool (this is a very interesting theory of fatigue, beyond the scope of this article, called the Central Governor Theory). Finally, the mental stress of exercises in the heat can diminish pain tolerance, negatively affect mood, and diminish motivation - all of which impair exercise performance.
Because heat illness can have such devastating effects on the body, exercise performance can be impaired quickly. Loss of 1-2% of body weight in water can begin to impair aerobic capacity, 2-3% loss can begin to diminish endurance performance, and 4-6% losses in body water can impair strength and cause cramps.
Given the fact that it’s not uncommon to lose upwards of 2 pounds of water when exercising in normal environmental conditions (like in the gym), it is easy to see how quickly you can get into the ranges mentioned above when exercising in the heat. In fact, during high intensity exercise, upwards of 4+ pounds of water weight can be lost. For a 150 pound person, that represents over 2.5% of your body weight lost in water!
What Increases Risk of Heat Illness
Certain populations are more predisposed to heat illness:
- Sedentary Individuals
- Overweight/Obese
- People who aren’t used (or acclimatized) to exercising in the heat
- People over the age of 60
- People who are taking certain medications (for blood pressure, colds, depression, and a number of other health issues)
Although individual predisposition can certainly increase the risk of heat illness, the biggest risk factor is obviously the environment. Conditions (both heat and humidity) have to present themselves in order to greatly increase the risk of heat illness. Heat’s contribution is fairly obvious; however humidity plays a big role as well. More humid air is more saturated with moisture, which makes evaporative cooling from sweat more difficult.
Because of the relationship heat and humidity play in heat illness, The Heat Index (a measure of the intersection of heat and humidity) is a useful tool to determine when conditions are right for heat-related illness. See the chart to the (right, left, I’ve attached it) for how heat and humidity converge to increase risk of heat illness.
Preventing Heat Illness
There are number of steps that can be taken to prevent heat illness; below is a list of the best strategies:
Enjoy exercising outside this last part of summer and into the warmer fall. Just keep in mind the topics and tips mentioned in this article so you stay safe while doing so! Looking for a fun way to burn some calories outdoors? Ever heard of a "Pop-Up" Workout?
Michael Stack is an exercise physiologist and founder of Applied Fitness Solutions (AFS). With locations in Ann Arbor, Plymouth, and Rochester Hills, AFS affordably fuses lifestyle coaching with guided group exercise classes.