Human Biochemistry, the basics as relating to dieting.
Some of you have probably seen this before in other fora, and I've mean to post it up here for a while, but hadn't summoned enough round tuits. A reasonable chunk of my audience here are quite aware of this stuff already, but there's also plenty who aren't. If you're one of the former, feel free to drop corrections in if any are spotted. And if ( Read more... )
On to your question... The relatively simple answer is that when you do anaerobic exercise it burns glucose, but in a short-cut fashion, not using oxygen (anaerobic literally means "non air using"). The product of this is lactic acid. Lactic acid is pretty much the thing that makes muscles "tired". When they're full of lactic acid, they can't continue to operate at any kind of reasonable rate, because they have to sit there and wait for the bloodstream to carry the lactic acid away.
That overall signal to your body when your blood goes "hey, I'm full of lactic acid" contributes to a general slowdown of your metabolism all over, as well as just in the "tired" muscles.
The biggest factor, though, is simply that as a result of production of lactic acid, you cannot do anaerobic exercise for any lengthy period of time. Even if you wanted to, your muscles would eventually fill up with actual crystals of lactic acid, and physically seize up, no matter what you want to do with them. Properly aerobic exercise doesn't generate lactic acid, because it burns the glucose molecule "fully", making carbon dioxide and water.
Note that the amount of exercise that is "aerobic" vs "anaerobic" is directly related to how "fit" a person is. "Fitness" means that your blood circulatory systems are in better shape, your lungs are in better shape, your muscles are in better shape to absorb oxygen and glucose from the blood and push CO2 back out to the blood, etc, etc. The whole point of "fitness" is to improve the ability of your body to burn glucose "properly", rather than having to short-cut to lactic acid.
The corollary to that is that depending on how fit you are, you should do a rate of exercise that is in proportion. Your theoretical obese TV couch potato would find walking to the kitchen and back a partially anaerobic exercise, whilst your olympic athletics competitor would find that several laps around a football field at top speed would still be almost entirely aerobic.
The key for any individual is learning where that rate-point is, and exercising at that rate and no faster. If you're unfit or unsure, start with slow walks around the block. Your body will tell you when it's capable of going faster - your muscles will feel bored, and so will your brain. If you push the rate too fast, your muscles will get tired very quickly. That's "the wall" that runners talk about - the point where lactic acid production starts to outstrip the rate at which it can be pulled out from your muscles by your bloodstream.
Erm... that's lots of digression. :-) Anyway. The main point is that the metabolic crash is due to lactic acid production.
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