Does fatigue affect the rate of calorie burn during exercise?

Lets say over the course of an 8 hour period, I decide to do 100 push ups. Would I burn more calories if I did them all at once and rested for 8 hours, or spaced each push up over 5 or 10 mins and rested in between?

In sheer energy expenditure, the same effort (work) costs the same number of calories regardless of the period over which it was expended. Whether your body has to work harder in some sense to produce that effort in a shorter period I’ll leave to the physiologists.

For that amount of exercise, it makes no difference.

If you want to drastically increase your calorie burning, you have to do a lot more exercise. If you exercise long and hard enough, you can raise your metabolic rate (i.e. burn more calories) for the next several hours after you stop exercising. You have to exercise continuously at about 70% max VO2 to get this effect. You’re at 70% max VO2 when you’re exercising so hard you can’t hold a conversation. At roughly 45 minutes of this, the afterburn effect starts, although going longer will increase the effect. The afterburn lasts over half a day, slowly decreasing over about 14 hours.

But most people probably can’t work out that long at that intensity. They need to work their way up to that. And exercising at lower intensities does not produce the effect at all.

Do you have numbers on the afterburn effect? I’ve read somewhere that it’s really not all that much (under 50 extra burned calories or something.) I’m wondering what the Straight Dope on this is.

The one study that I have data from (and I’ll admit it’s from a newspaper article, since I haven’t bothered to google the actual paper) is that exercising at that rate burns 420 calories in 45 minutes. Another 190 extra calories are burned in the subsequent 14 hours. This study isolated subjects in metabolic chambers for 2 24-hour periods, one a control period. All the subjects were men, if that makes a difference.

If you want to look it up yourself, the journal is Medicine and Science in Sports and Exercise; lead author is Amy Knab. The newspaper article was published in April this year; don’t know when the journal article was published, but it was recent then.

There are several different chemical reactions used to power your muscles. Your body will transition between these depending on the power level. Some are more efficient (those at the lowest rate) and some less so (the highest rate). The middle rate one tends to use more fat, so it is what people trying to lose weight target. Doing the pushups as quickly as possible will burn the most food calories.

cite: Bicycling Science Which I don’t have here to consult, so the above summary might be a bit off, and of course I have forgotten all the chemical names and reactions involved.

I still don’t get it. Could this be correct:

Exercising at a lower intensity (the mid-level) burns more fat, so you lose less weight poundage-wise, but your overall volume/displacement goes down? Or you exercise at the higher intensity and burn (relatively) more calories, but you don’t burn as much fat so your overall physical appearance changes at a slower rate?

What happens after six months? Exercising at a higher rate will have burned several pounds ‘worth’ of calories (assume a fixed, controlled diet) beyond the lower-intensity workouts. If twins exercised at the two different heartrates, what would the difference be? Will one be noticeably thinner than the other?

Can’t tell you how frustrating this is–my Polar tells me that I’m spending about forty to fifty minutes in their ‘zone 3’ (they call it ‘max performance’) and only twenty to thirty in ‘zone 2’ (‘max fitness’) and prompts me to spend more time in zone 2. Why? I’ve never been in shape before, so don’t know what to expect–but now that I’ve been monitoring things for several months and can make it through a workout without dreading it, why would/should I back off?

Let’s first address the actual op.

Same level of intensity, done for the same total amount of time, spread out in multiple sessions over an 8 hour period versus in one session. No significant difference.

OTOH short higher intensity work of less time and same amount of calories burned during exercise will burn more fat over the course of the day due to that “afterburn effect”

The fat burning zone concept is garbage, btw. Rhythmdvl, if you are enjoying working out at what they call “zone 3” there is no reason whatsoever to back off. Here, for your interest, is a recent article discussing the potential that high intensity interval exercise has as part of an exercise plan. Higher intensity will lose you more subcutaneous and abdominal fat, while preserving or increasing muscle mass, and doing more to increase fitness parameters. (That said, my personal opinion, with no cites to back it up, is that mixing it up as much as possible, some higher intensity, some longer lower intensity, the more different sorts of different sorts of exercise the better, is a good general principal. And of course higher intensity work presumes an established fitness base.)

The other angle to take on the op is not looking at calories per se but muscle growth. For that a certain level of fatigue is of use and the 100 push ups done at one time will give a different result than would one push up every 5 minutes until 100 are done.

Is there a similar effect for cardio exercises? The gym I go to (least unacceptable one I could find for various reasons) happens for no apparent reason to have rather poor ventilation by the rowing machines, which appear to have a high calories per hour burn rate. However, because of the aforementioned poor ventilation, after about 10 minutes my sweat has saturated my terrycloth headband and is running down into my eyes. If I take a break, walk to a cooler area and then come back and do another 10 minutes, am I doing myself less good as far as general fitness, endurance and so on than if I struggled through without a break or worked slower for a full 20 minutes? I don’t care a fig about appearance, I just want to not be as fat and to improve overall cardiovascular health.