I’ve recently started an exercise program to burn off some of the holiday fat and I noticed when I do crunches or push-ups or other muscle targeting workouts, my muscles have a burning sensation. Is this from the actual fat around the muscle burning off? Or is it a lack of oxygen or energy?
Welcome to the SDMB!
That’s the buildup of lactic acid that you’re feeling.
http://www.brianmac.demon.co.uk/lactic.htm
Soup_Nazi,
The burning sensation is a result of lactic acid buildup in the muscles. When you exercise extensively, the muscles burn more oxygen than the blood can replace. When this happens, the metabolism in the muscles goes from being aerobic to anaerobic. One of the products of anaerobic respiration is lactic acid.
Two words: lactic acid.
As lactic acid builds up in your muscles, you start to feel them “burn”. If you go beyond a few minutes of this, the acidity of the muscle tissue increases and the muscles start to have difficulty generating meaningful amounts of energy.
If I’m not mistaken, lactic acid is a by-product (I want to say of metabolising glucose in the absence of oxygen, but I’m not 100% sure).
I’m sure someone else will shed more light on the subject, though.
Grrrr…here I thought I had the jump on everybody. Thanks, Phobos.
Thanks alot guys. My question was answered completely in less than 10 minutes! I think I’m gonna like this place!
Thanks again.
har. If I had a nickel for every time that happened to me…I’d have 35 cents. 
So get on over to my thread and make it two for two, if you’re a mythology freak, that is.
The burning sensation is caused by the buildup of lactic acid in the working muscle. What happens is that when muscle is rapidly contracting (non-aerobic exercise), its energy demands are high. So high that there’s no time to completely oxidize (ie convert to water and CO2-the final products of energy metabolism) its fuel supply. Energy is supplied more rapidly, albeit less efficiently, by only partially oxidizing (the oxidation takes place in a stepwise fashion, in different cellular compartments-partial oxidation is simply stopped at an earlier step) of the muscle’s fuel supply. In order to keep the partial oxidation reactions rolling, though, the tissue converts the end product of the partial oxidation pathway, called pyruvate, to lactate. This last reaction regenerates an essential intermediate in the partial oxidation pathway (NAD+ from NADH).
The lactate produced in this manner is carried away by the circulation (but builds up in muscle to great enough quantities to cause the burning sensation) to the liver, where it is converted back to pyruvate.
BTW, in brewer’s yeast, the lactate produced in this manner undergoes one more chemical reaction; it’s converted to ethanol (A nobel creature, the yeast!). And WE are only one enzyme short of being able to carry out this reaction also. Imagine, exercise induced ethanol intoxication. Would give a whole new meaning to the runners’ high
On preview I notice quite a few others beating me to the punch. Damn my hunting and pecking
Hmmm…Am I correct in guessing that Soup is the only Doper ever to have the word “Nazi” in his user name?
Dunno, scratch, but I think it’s probably just a harmless Seinfeld reference.
My name comes from the episode of Seinfeld where the “soup nazi” runs a succesful soup business and can refuse service to anyone, and does to George Castanza. Didn’t mean to offend anyone with it. Peace.
My understanding is that the lactic acid burn is not especially good for your muscles and you should stop when you hit that point. Don’t believe the “no pain, no gain” crap.
But as you get in better shape, especially better aerobic shape, it will take more and more effort to get to that point. Once you get to real good shape, it will be close to impossible to get a lactic acid burn. At least that’s my experience in bicycling.
Don’t expect this all the time, you just got the premium service
dtilque, lactic acid buildup is not necessarily bad. And getting in better aerobic shape will not prevent lactic acid buildup due to anaerobic effort. As you’ve bicycled more, it became more of an aerobic exercise than an anaerobic one. The OP was talking about lactic acid buildup as a result of push-ups and other weight-lifting type exercises. Studies have shown that lactic acid buildup is important to muscle growth. That’s why many body-building magazines have recommended against products designed to neutralize lactic acid. Of course, muscle growth is not absolutely necessary for strength-building or better health, but you have to realize that you’ll experience some discomfort when working out.
IMHO I disagree that lactic acid is the cause for the burn. If you read the link supplied by an early link, it states that lactic acid causes fatigue, not the burn.
Lactic acid is a by-product of the Krebs cycle, which is the energy cycle found in the mitochondria using O2 and glucose, with ATP as the energy source. It takes a while for lactic acid to build up. And it won’t build up if you are getting plenty of O2. When I do the quadriceps exercise on the machine, I feel the burn after 10 reps. I do them slowly, as you are supposed to, and can feel the burn. If I do them fast, I don’t. This is just the opposite of what you would expect if lactic acid was the cause, as by doing them slowly I’m allowing plenty of O2 to be used by muscles, which would prevent lactic acid build-up.
IMHO, that burn comes from the blood surging thru the muscles. I have not read any where, including this thread, where lactic acid causes the burn. I see no reason why it would. Fatigue, yes. Burn no.
barbitu8,
The chemistry of muscle burn is pretty well established. This snippet comes from http://www.exploratorium.edu/sports/faq1.html .
My biochemistry book says virtually the same thing.
Oh ohoh! I know I know! I’m taking Bio this year, I really like it! So basically, it’s all about lactic acid production when your muscles go into oxyge-
OH screw you all.
Lactic acid buildup is a byproduct of GLYCOLYSIS not the KREB’s cycle. As I posted earlier, lactic acid is formed from pyruvate, the last compound produced by the glycolysis pathway. It also happens to be the compound that is used for entry into the Kreb’s cycle (which is probably the source of the confusion).
But you aren’t take into account how much energy your quads are using per unit time, or how long you are taxing them. It sounds like your assumption is that since the amount of work done in the fast vs slow rep examples is equivalent, then your energy expenditures must also be equivalent. However, unlike a rope or cable, muscles must expend considerable energy to hold a weight in a given position (See how long you can maintain an intermediate pushup position). The situation is further exacerbated when you make your muscles contract when they aren’t at an optimal length. Anyway, it is likely that your energy expenditure per unit time is very similar whether you do your squats rapidly or slowly.
The next question to ask is: from where does the oxygen for oxidative metabolism originate? You imply that the source is the circulation and in an ultimate sense you are correct. However, your muscles have a mechanism to stockpile oxygen locally. They make a protein called myoglobin, which is actually related to hemoglobin. The myoglobin acts as a short term oxygen reservoir. The oxygen is rapidly available, but there is a relatively small amount available. Local oxygen storage provides the means to allow short bursts of activity to proceed primarily in an aerobic (oxygen consuming) fashion. Once you’ve used up the locally stored oxygen, delivery by the circulation can easily be too slow to meet demand Thus the lactic acid production (burning) when you slowly squat.
It’s called “Lactic acidosis”.
You’re welcome.