When (Not) to Exercise on the Lazy, Hazy Days of Summer


Memorial Day weekend, the unofficial start of summer.  The long, hot, humid dog days are fast approaching, if they haven’t already arrived where you live.  The perfect time to review the dos and don’ts of hot weather exercise.

The Human HVAC System 101: The human body is often compared to an efficient machine. When talking about the body’s natural cooling system, an apt comparison would be a smart phone on a battery saver setting.  A phone on this setting will give priority to the most important functions of the phone, particularly the ones that don’t use up a great deal of battery power, and begin to slow or shut down other less necessary, battery-sucking functions – WiFi, web browsing, Bluetooth – as the phone gets low on power.

Similarly, keeping the core body temperature within a few degrees of normal is a major priority – a life-saving one, as a matter of fact.  The body knows a rise of a mere two or three degrees in core temperature can lead to heat stroke and heat exhaustion – both of which will cause one to become very ill immediately and, if left untreated, can lead to organ failure and even death.  Prior to this point, the body will shut down less necessary functions (a workout, for example) in an attempt to keep the temperature down. This process triggers other illnesses like heat cramps and dehydration, that are signals to the person to stop what she is doing and seek help.

Therefore, the body has a natural system, perspiration, that works to cool the body when internal and external factors are warming the body.  Sweat glands are triggered to release water, which also contains sodium and other chemicals, on the skin’s surface.  It is not the existence of the sweat on the skin that cools the body but, rather, the evaporation of the perspiration.  Anything that impedes the evaporation process puts a strain on the body’s cooling system.  The more the cooling system is taxed, the more priority the body gives to it, while other systems are automatically slowed or turned off.

Hot Weather Workout Theme

Don’t #1: Do NOT choose the hottest time of day to workout: The myth persists: The more I sweat, the better quality my workout, the more calories burned.  There is no correlation between sweat and number of calories burned.

It is true that as the heart rate rises and the longer the heart rate is in an elevated state, the more sweat one is likely to release in response and the more calories burned as compared to at rest.  But, the calorie burn is related to the elevated heart rate or work output, not the generation of sweat.  For example, go for a run on a cold day and you will perspire.  Clearly, there is also significant calorie burn taking place.  But, you probably would sweat just as much lying in direct sunlight on the beach in July.  There is little to no excess calorie burn happening while sunbathing even though perspiration is high.  In fact, the heart rate will slow, purposely slowing the body’s energy production.

Why?  The body’s energy production comes from burning calories – expending energy releases heat as a by-product of burning calories.  Remember the body’s prioritizing of systems.  Whether you decide to sunbathe or go for a run at noon on a 90 degree day, your body is going to find a way to perspire to cool the body while expending the least amount of heat-producing calories as possible.

Yes, it may feel like you’re getting a great workout, but the exhaustion you feel comes from the body diverting its resources away from the cardiovascular system and muscles in order to keep the cooling system from being overrun.  The more hot and humid the conditions, the harder you are working and the longer you are out, the faster you will reach the point of those warning signs – cramping and dehydration.  This is not a productive or high quality workout.  On the contrary, it’s a risky workout with none of the rewards.

Don’t #2: Do NOT overdress: Again, the myth persists: If I wear more clothes, I’ll sweat more and I’ll burn more calories.  Wrong.  You aren’t sweating more, you’re evaporating less.  Remember, the body cools not from the sweat itself but from the evaporation of sweat off the exposed skin.  Having skin covered impedes the evaporation process.  The hotter and more humid the day, the more exposed skin you should have in order to assist the body’s cooling system.

But this isn’t just for outdoor, summertime exercise.  This is also true of indoor workouts.  Outdoor workouts in cold temperatures are the only workouts that require covering arms and legs in their entirety.

Don’t #3: Do NOT believe humidity is your workout buddy: Wet, heavy, still air impedes the evaporation process.  Dry, breezy air is your friend.  Humidity is the enemy.  Put another way, running in 95 degree weather in the desert is risky; running in 85 degree weather in 85% humidity could be deadly.

The best approach is to take both heat and humidity into consideration before deciding to exercise outdoors.  Weather websites and apps will post current temperature and relative humidity.  An easier indicator to use is a third number often posted, sometimes called the “feels like”, “real feel” or the wet bulb globe temperature (WBGT).  WBGT is derived from a formula that takes into consideration temperature, humidity and radiant heat.  See below for the guidelines on exercise using the WBGT.

If you workout indoors in an un-air conditioned environment, it’s essential to use fans blowing directly on the body to assist in the evaporation process when the air temperature and humidity levels are high.

DO #1: For Healthy, Regular Exercisers Only: Take precautionary measures when “Feels Like” temperature (WBGT) exceeds 82 degrees:

  • Hydration: 2 hours prior to exercise drink 17-20 oz of water.  Sip water while exercising – approximately 7-10 oz every 10-20 minutes.  Following exercise, drink 16-24 oz for every one-hour period of your workout.  In this circumstance, it’s okay for a recreational exerciser to have a sports drink post-exercise that contains sodium and electrolytes which are also diminished while working out in heat and humidity.
  • Attire: Wear shorts, sleeveless or short-sleeved top, and short socks made of synthetic, light-colored fabrics that wick away moisture and apply sunscreen to all exposed skin.  Wear UV proof sunglasses and/or brimmed hat or visor.
  • Time of Day: Avoid the hottest time of day when sunlight is direct and shade is in short supply – 11am to 5 pm.  Opt for right after sunrise or just before sunset when the sun is low on the horizon.  Keep an eye on the sky and forecasts, high heat and humidity increases the likelihood of thunderstorms.
  • Intensity: Avoid any high-intensity exercise.  These are the days to do low to moderate intensity, shortened workouts.  Save your best runs, power walks and steepest cycling hills for a cooler day.

DO #2: For Deconditioned or Those with Cardiovascular or Pulmonary Conditions: Avoid all outdoor workouts and indoor exercise in an uncontrolled environment as WBGT nears or exceeds 82 degrees:  Those who don’t exercise regularly, are overweight or who have heart or lung conditions are especially vulnerable in hot, humid conditions.  Opt to exercise indoors in air conditioned environments instead.

DO #3: Avoid Outdoor Workouts and No A/C Indoor Workouts: When “Real Feel” Temperature (WBGT) nears or exceeds 90 degrees: If you are a regular exerciser, particularly if you have the same weekly routine, being forced to change things up courtesy of Mother Nature can actually be a positive thing.  It’s healthy and productive, in terms of fitness gains, to keep your body guessing and introduce unfamiliar workouts on different days.  Here are some suggestions for alternatives to your regular workout when it’s too risky to do the status quo:

  • Get a day pass for a nearby gym
  • Find out the lap swim times for a local pool
  • Sample a studio yoga or Pilates class
  • Do an online cardio-strength training body weight workout in a cool, indoor environment
  • Swap your rest day for the week

Remember, be smart and healthy on those oppressive days and be fit and happier all summer long.






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