Heat and exercise

I would argue that your body will burn slightly less cals when it is hot and humid, the main reason for this is you will not be able to push as hard or go as long, so in the end you end up burning less.

according to my HRM, on the nasty-yucky days, I tend to burn more calories, but I'm not able to do as much. a couple weeks ago, when it was just rain rain rain, I could bust out a 10k in a little over an hour. this week... I can only get 5 miles under my belt in an hour. but my burns are still through the roof. just make sure you stay hydrated. heat headaches are no fun.

tibby531 wrote: »

according to my HRM, on the nasty-yucky days, I tend to burn more calories, but I'm not able to do as much. a couple weeks ago, when it was just rain rain rain, I could bust out a 10k in a little over an hour. this week... I can only get 5 miles under my belt in an hour. but my burns are still through the roof. just make sure you stay hydrated. heat headaches are no fun.

HRMs just measure HR, then plug that number into a calculation. Your HR may be higher in the heat, but not necessarily for the same reason that causes calories burned (oxygen uptake), which does not increase with heat.

markdvsmo wrote: »

It's been oppressively hot and humid here (MO), and I do a good bit of work outside - no choice. I log some of my chores as exercise, because it is... carpentry and heavy garden work type stuff.

What effect does heat have on exercise and how many calories are burned?

Work (including commuting, housework, yard work, and child care) is part of your activity level, and should not be logged as exercise. Work = activity level. Workouts = exercise.

Increase your activity level and stop logging your work as exercise.

As for the heat, all the burns & calorie counts are nothing but estimates. Change your focus from looking for "extra credit" to using trial & error to find the "sweet spot" where you're eating at an appropriate deficit for your size.

tibby531 wrote: »

erickirb wrote: »

HRMs just measure HR, then plug that number into a calculation. Your HR may be higher in the heat, but not necessarily for the same reason that causes calories burned (oxygen uptake), which does not increase with heat.

bennettinfinity wrote: »

Keep in mind that your HRM is just using an algorithm to determine calorie burn... just because your HR is higher than usual when it's hot (all other variables controlled) doesn't mean your burning more calories. Your HR is responding to the heat, not increased exertion.

well darn, and double darn. I thought I was rocking it!

You ARE rockin' it! You're out there getting the miles in... just no bonus calories...

Since HRM's will report a higher heart rate in the heat, therefore your heart is working more, you are doing a little more work. But generally, not that much and you shouldn't log your chores.

markdvsmo wrote: »

Nuke_64 wrote: »

Since HRM's will report a higher heart rate in the heat, therefore your heart is working more, you are doing a little more work. But generally, not that much and you shouldn't log your chores.

I may have used the wrong term in 'chores'... the stuff I log is like this morning, four hours of cutting, drilling and installing sheet metal in the sun on a steep metal roof, with more to come this afternoon. I'm pretty sure baseline doesn't account for that kind of activity.

I do treat it all as a bonus, though, and generally stay below my 1800 cals with or without the added exercise. But strenuous activity is exercise whether you walk away from it with a new roof or a few more miles on the exercise bike's odometer.

If you're doing that type of activity every day, it should be part of you baseline. If not, it's not something I would purposefully log. I think you should only be logging things that you do on a consistent basis. If this is a once every few months kind of thing, it's best to consider it bonus burn. As @editorgrrl said, don't be looking for extra credit - that's not how this works.

Exercising on a hot day in the sun increases your heart rate because some of the blood that should go to your muscles is diverted to your skin for cooling--evaporating sweat cools your skin, therefore cooling your blood diverted there. Since some of the blood has been diverted to your skin for cooling, your heart must beat faster to provide the necessary volume to your exercising muscles (the full volume required by them).

There is another mechanism at work, too. In high humidity, or in the case where your sweat doesn't evaporate quickly enough to keep your core temperature at or below a critical value (102°F, if I recall correctly), your brain/nervous system will force you to slow down enough that your evaporating sweat will keep your core temperature at or below that critical value.

Since HRMs use heart rate and an algorithm to calculate Calories (kcal) burned, the higher heart rate calculates to a greater Calorie burn. The actual Calorie burn is negligibly higher due to increased heart rate but not due to increase in skeletal muscle work [assumes identical workouts, other than increased heart rate due to heat].

For example, on a day that is sunny, hot (>93°F), and moderate relative humidity (~55%), I can run an easy pace for about three miles before that mechanism kicks in and slows my pace ~90"/mi to 120"/mi after that; the same run on days that are milder (<85°F), I usually run negative splits (each mile faster than the previous mile) for 6-8 miles and the HRM shows lower average heart rate, increased average pace (fewer minutes per mile), and lower Calorie burn overall.