Exercising in warm weather is challenging. Whether you’re exercising
indoors in a room that’s too hot or outside, heat makes the experience more
unpleasant. The real question is how does heat affect your workout? Why are
workouts in the heat more challenging and will the heat cause you to fatigue
How Your Brain Responds to Exercising in the Heat
When muscles contract, your body produces more internal heat during
exercise but it has evolved a number of mechanisms that allow it to control
body temperature and keep it within a relatively narrow range. The “master
regulator” of body temperature is a portion of your brain called the
hypothalamus. One of the many jobs of the hypothalamus is to keep your core
body temperature within a certain range. It’s the anterior part of the
hypothalamus that responds to a rise in core body temperature, while the
posterior hypothalamus regulates heat when temperatures are cold.
When you exercise in a hot environment, receptors on your skin detect an
increase in temperature and send a signal to the hypothalamus. When the
hypothalamus gets the message, it initiates a series of events to help maintain
core body temperature. The way it does this is by activating sweat glands and
dilating blood vessels to help release heat. Blood flow also increases to the
surface of the skin to help release heat.
This feedback system works well, but when you’re working out in a humid
environment or doing high-intensity exercise, the system may not be able to
“keep up.” As you know, sweating releases heat and a humid environment can
interfere with the ability of sweat to evaporate, making it harder for your
skin to release heat. That’s why exercising in a humid environment can be
particularly challenging and even dangerous.
Exercise Performance in Hot Environments: Does Heat Cause
You to Fatigue More Quickly?
Exercising in hot weather poses challenges from a heat control standpoint
but how does it impact performance? Not surprisingly, you become fatigued
earlier when you’re exercising in a hot environment. Why might this be?
One theory is when your hypothalamus senses a temperature rise, after being
alerted by feedback from skin receptors, it sends less nerve stimulation to
your muscles to contract so fewer motor units are activated. At the same time,
your perception of how hard you’re working goes up. There’s more “perceived
effort” and you’re likely to consciously reduce your exercise intensity.
Another theory is that when you exercise in heat, your muscles produce more
free radicals and these free radicals contribute to muscle fatigue. Some
research also shows that glycogen stores are depleted more rapidly when you
work out in a hot environment. When muscle glycogen levels drop, fatigue sets
Likely, it’s a combination of a number of factors – both central feedback
from the hypothalamus and peripheral influences like free radicals that cause
you to fatigue more quickly in the heat.
When Does Temperature Become a Limiting Factor?
Interestingly, research shows that most athletes become exhausted and have
to stop at a core body temperature of just below 40 degrees centigrade. This
temperature is lower in untrained people who exercise in the heat. This may be
a protective mechanism since heat stroke typically occurs at around 42 degrees
centigrade. If you’ve been exercising in heat for several weeks and your body
has become heat acclimated, it’ll take you longer to reach this core body
temperature since your body is better at releasing heat, so you’ll be able to
exercise longer in the heat without being limited by fatigue relative to
someone who isn’t heat acclimated.
How Does Exercising in the Heat Affect Your Heart?
As your body diverts more blood flow to your skin to release heat, the
amount of blood your heart pumps with each beat (stroke volume) goes down. At
the same time, your heart rate goes up to compensate for the decrease in stroke
volume. Since muscles need more blood flow during exercise and so does your
skin, the two are essentially in competition with each other. This puts greater
demand on your heart to meet the needs to both skin and muscle. At some point,
the demands become so high that not enough blood flow reaches the skin and heat
can’t be released as readily. At this point, your body temperature can rise to
the point where heat stroke occurs.
Tips for Exercising in the Heat
Hydrate, hydrate, hydrate. Performance declines once you’re 3% dehydrated.
Water is fine if you’re exercising for less than an hour. An electrolyte-rich
drink is best for longer periods of exercise.
Exercise during the coolest time of day. Avoid working out in a hot
environment that’s also humid whenever possible. First thing in the morning is
usually when it’s coolest.
Wear light-weight clothing that’s light in color that “breathes” so your
body can more readily release heat. Save the tight spandex for cooler days.
Give your body about 2 weeks to acclimate to exercising in the heat. Until
then, reduce the intensity of your workout and take frequent breaks. Once your
body has acclimated to the heat, you’ll begin sweating sooner and more
profusely, making it easier to release heat. Plus, plasma volume, the portion
of your blood made up of water and proteins, increases and this helps your
heart maintain a higher stroke volume. Once you’re acclimated, you also lose
less electrolytes like sodium and potassium when you sweat.
The Bottom Line?
Heat has a definite impact on exercise performance. In general, you’ll
fatigue earlier, especially if you aren’t acclimated. Take it a little easier
when you’re exercising in a hot or humid environment. Push fluids even if
you’re not thirsty and dress lightly. Fortunately, your body has the ability to
acclimate to exercising in the heat and you won’t overheat as quickly once it
For more fit tips, contact our fitness director/certified personal trainer,
Noelle Lusardi, at stepaheadwellnesscenter.com.