By Brian B. Parr, Ph.D., ACSM Certified Clinical Exercise Physiologist, Associate Professor
It’s that time of year again! Summer weather can make outdoor exercise especially challenging. Even if you take precautions to avoid serious heat-related injuries, exercise on a hot, humid day can adversely affect your performance. At the very least, your workout will feel harder.
It’s not the heat, it’s the humidity. The humidity is a huge factor. If you have tremendously high temperatures and high humidity, you will be sweating but the sweat won’t be drying on the skin. That’s why it’s not just heat but the combination of heat and humidity that matters.
When you exercise, you produce a lot of heat though muscle activity. To avoid hyperthermia (the condition of having a body temperature greatly above normal), that heat must be dissipated. Blood flow to the skin is increased to help you lose heat to the air around you, a mechanism called radiation. At the same time, you start to sweat. As the sweat evaporates, it takes heat away from your body. On a cool, low humidity day you can easily lose heat by radiation and evaporation, maintaining your body temperature close to normal.
On a hot, humid day it is much harder to lose heat and evaporate sweat from your skin. In fact, on a 100 degree day with very high humidity you may actually gain heat from the air, rather than losing it! But your body responds the same, directing more blood flow to the skin and increasing sweat rate. These physiological responses can impact your performance.
Continuing to direct blood flow to the skin may mean that less blood goes to the muscle. Over time, an excessive sweat rate can lead to dehydration, which can also reduce blood flow to muscle. This means that less oxygen as well as carbohydrate and fat fuels get to the muscle. Since your muscles require oxygen and fuel to produce energy to power your muscles, this can lead to fatigue – an inability of the muscles to maintain force production, which you notice as a drop in running or cycling speed.
Dehydration also means that there is less fluid to lose as sweat, impairing your ability to lose heat by evaporation. Since you are still generating heat from muscle activity but not losing nearly as much, this causes hyperthermia. This is why fluid replacement is so important during exercise, especially in a hot, humid environment.
Even if you do stay hydrated, rising body temperature can still cause feelings of fatigue. Since all muscle activity is regulated in the brain, increasing body (and brain) temperature can inhibit the activation of muscle. Additionally, when you’re hot, exercise seems harder and you may feel less motivated. Both of these result in lower muscular force production, making you move slower.
The impact of extreme sustained heat on the body The systems in the human body that enable it to adapt to heat become overwhelmed. When a person is exposed to heat for a very long time, the first thing that shuts down is the ability to sweat. We know that when perspiration is dried by the air there is a cooling effect on the body. Once a person stops perspiring, in very short order they can move from heat exhaustion to heatstroke.
You can prevent many of these heat-related problems by taking some common-sense precautions: Make sure you drink plenty of fluids before, during, and after exercise, wear clothing that will help keep you cool, limit the duration of your workout and take it easy on really hot, humid days. Brian B. Parr, Ph.D., is an ACSM Certified Clinical Exercise Physiologist and Associate Professor in the Department of Exercise and Sports Science at the University of South Carolina Aiken.
Brian B. Parr, Ph.D., is an ACSM Certified Clinical Exercise Physiologist and Associate Professor in the Department of Exercise and Sports Science at the University of South Carolina Aiken.
Both heat exhaustion and heatstroke are caused by exercising or playing in a hot, humid environment where the body becomes dehydrated. However, they show different combinations of symptoms.
• Heat exhaustion is usually accompanied by a fever no higher than 104 degrees Fahrenheit, excessive thirst, nausea, fainting, cool and clammy skin, weakness, muscle aches, heavy sweating, slow heartbeat and dizziness.
• Heatstroke may develop following heat exhaustion if the condition is not treated. It occurs when the body’s temperature rises and the cooling system stops working. This potentially life-threatening condition is characterized by nausea, vomiting, headache, dizziness, fatigue, rapid heart rate, hot and dry skin, shortness of breath and decreased urination.
Any person who is exhibiting the signs of heatstroke should immediately seek medical attention.