Why do my muscles burn when I exercise?

[Soup_Nazi]

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?

[Phobos]

Welcome to the SDMB!
That's the buildup of lactic acid that you're feeling.
http://www.brianmac.demon.co.uk/lactic.htm

[Ogre]

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.

[UrbanChic]

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.

[Ogre]

Grrrr...here I thought I had the jump on everybody. Thanks, Phobos.

:p

[Soup_Nazi]

Thanks alot guys. My question was answered completely in less than 10 minutes! I think I'm gonna like this place!

Thanks again.

[Phobos]

Originally posted by Ogre
Grrrr...here I thought I had the jump on everybody. Thanks, Phobos.

:p

har. If I had a nickel for every time that happened to me...I'd have 35 cents. :)

[Ogre]

Originally posted by Phobos
Originally posted by Ogre
Grrrr...here I thought I had the jump on everybody. Thanks, Phobos.

:p

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.

[choosybeggar]

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

[scratch1300]

Hmmm....Am I correct in guessing that Soup is the only Doper ever to have the word "Nazi" in his user name?

[Ogre]

Dunno, scratch, but I think it's probably just a harmless Seinfeld reference.

[Soup_Nazi]

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.

[dtilque]

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.

[Speaker for the Dead]

Originally posted by Soup_Nazi
Thanks alot guys. My question was answered completely in less than 10 minutes! I think I'm gonna like this place!

Thanks again.

Don't expect this all the time, you just got the premium service :p

[Aestivalis]

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.

[barbitu8]

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.

[Ogre]

barbitu8,

The chemistry of muscle burn is pretty well established. This snippet comes from http://www.exploratorium.edu/sports/faq1.html .

Some of the pyruvate is absorbed into the muscle cell's mitochondria and converted into useful energy. But during strenuous exercise, the mitochondria can't handle all the pyruvate that's produced. The excess pyruvate becomes lactic acid, a dead end as far as energy production is concerned. As the concentration of lactic acid in the muscle fiber increases, the acidity of the cell changes, causing muscle fatigue and the all-too-familiar "burn."

My biochemistry book says virtually the same thing.

[ElusiveMiser]

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.

[choosybeggar]

Originally posted by barbitu8
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.

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).

Originally posted by barbitu8
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.

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.

[handy]

It's called "Lactic acidosis".

You're welcome.

[choosybeggar]

Originally posted by handy

It's called "Lactic acidosis".

You're welcome.

??????

The only context I've heard lactic acidosis used in is medical. It implies that so much lactic acid is produced, that it changes the blood pH. Common causes are shock, cardiac arrest, acute fluid build-up in the lungs, and carbon monoxide poisoning.

I found a mention of severe exercise producing lactic acidosis, but i don't think that's what were talkin' 'bout

[barbitu8]

Thanks to Soup-, Ogre, and Choosybeggar on this thread. Soup for posting it, Ogre for giving me a link, and Choosy to explain why I get the burn when I do the exercise slowly, holding for a count of 1 or 2 at the end of the eccentric motion. I've always wondered about the burn too, but assumed that it was the blood.

[barbitu8]

I'm back. I've been mulling this over and I'm not convinced inspite of all the posts and links to authority. I have not read anything with documentary evidence of lactic acid causing the burn. I'll tell you why I have my doubts.

I've done many long runs, long races, marathons, and even an ultra and never experienced any muscle burning. I've hit the wall, running out of glycogen, and felt fatigued. But I've also accumulated much lactic acid in my leg muscles, and got "thunder thighs." My thighs just would ache when I tried to lift them. But, you say, those are mostly aerobic stuff and weight lifting is largely anaerobic. However, I've done speed work, intervals, etc., and never felt any burn either.

So I've built up plenty of lactic acid, much more than a set of reps could produce. I've exhausted my myoglobin of oxygen. I've never felt any burn except when I do weights. In addition, I notice that when I feel the burn, I can see the blood flowing through the muscle (by way of its redness).

Explain that to me or, in the alternative, give me a link to some documentary evidence, and I'll be convinced.

HAPPY NEW YEAR.

[choosybeggar]

This looks like a good site: LINK (http://sportsmedicine.about.com/health/sportsmedicine/cs/exercisephysiology/index_2.htm?iam=anaconda) Follow the muscle physiology link on that page.

Check out the bottom third of the page. There's also a very nice graph. It reminded me about Creatine Phosphate, which is a very short term, high yield energy source stored in muscle.

I think it clearly states that long and slow exercise (marathons and such) rely on long and slow energy production (aerobic, using glucose and fatty acids as substrates). There's no lactic acid buildup with aerobic matabolism and that's why there's no burn with this type of workout.

Short and intense (weightlifting) on the other hand, relies on high-output, short tern energy (creatine phosphate-no lactic acid, but also anaerobic glycolysis-lactic acid production).

So far I haven't found something like "we infused lactic acid into a resting muscle and are reporting the sensations of the subjects." That would settle things and who knows it may be out there. i'll keep looking.

[barbitu8]

Originally posted by choosybeggar
I think it clearly states that long and slow exercise (marathons and such) rely on long and slow energy production (aerobic, using glucose and fatty acids as substrates). There's no lactic acid buildup with aerobic matabolism and that's why there's no burn with this type of workout.

Short and intense (weightlifting) on the other hand, relies on high-output, short tern energy (creatine phosphate-no lactic acid, but also anaerobic glycolysis-lactic acid production).


Thanks for the link Choosybeggar. As an aside, let me note that the main link states that exercise intensity will not be reduced during long, slow events since the body can use glycogen, fats, and proteins as substrates. However, a subsidiary link states that since glycogen depletion occurs, exercise intensity will be reduced dramatically. As any one who has run a marathon knows, that is the case. That's "hitting the wall." Fat and protein metabolism is too slow to prevent that.

As for "there's no lactic acid build up with aerobic metabolism," that may be true enough, but there is no exercise which is purely aerobic. Even a slow pace is somewhat anerobic. I have read a lot about marathons and marathon running and I have no doubt that a lactic acid build up does occur in marathons.

[handy]

Dear choosy: from webmd.com:

"Lactic acidosis occurs when lactic acid accumulates in the blood stream
at a faster rate than it can be eliminated. Lactic acid is produced in the
presence of decreased oxygen. The most common cause of increased
lactic acid is intensive exercise in which the body builds up an oxygen
debt."

[choosybeggar]

Handy,

I guess my point was I think of lactic acidosis as a clinical condition; something that when discovered you'd try to do something to treat it. This implies that the magnitude of the acidosis would be such as to produce overy clinical manifestations. I totally agree that exercise will produce lactic acidosis in a formal academic sense (you exercise, you make lactate, it's there, right?) but rarely in a clinical one.

I'm curious what the full-fledged MDs on the board will say, but when I hear acidosis, I think blood pH below 7.35. I wonder what intensity of exercise we'd be talking about to acheive this. My ref (Rose's renal physiology book) cites SEVERE evercise as a cause of lactic acidoisis. My interpretation is that Rose means clinically relevant lactic acidosis.

Anyhow, I think that our main difference is semantic.

[Aestivalis]

barbitu8, perhaps in weight-lifting you are also experiencing micro-tears in the muscle whereas you are not during sprinting. Do you get the burn when you do squats, or just upper body work? It could be that your legs are in such good condition that they are acclimated to lactic acid accumulation. Or it is also likely that you are not experiencing much sustained overload when you are sprinting as opposed to squats or other weight lifting.

[barbitu8]

Muscle tears I can believe as a possibility for the burn, but I doubt it. That's the cause for DOMS (delayed onset muscle soreness.) However, I refuse to believe that lactic acid is the cause, unless it can be proved to me. BTW, I only feel the burn in the quardriceps when I use the quadriceps machine (the sit-down, raise legs with toes under the bar). I sometimes feel them in the hamstrings machine, but I've never felt them any where else.

One other point I should have mentioned. It has been conventional wisdom for lo these many years that a marathoner should jog easily (or swim or do anything with a light level of exertion) the day after the marathon to get the blood flowing, thereby helping to remove the lactic acid build up. I've also read somewhere the different percentages of aerobic metabolism vs. anaerobic metabolism for different levels of effort. For example, all out sprinting is nearly 100% anaerobic, but even an easy jog has some anaerobic component. Further, when I race, even such a long distance as a marathon, I'm putting out over 70% effort. And I race many short distances when I'm putting out 90% effort. There's quite a lactic acid build up there. BUT NO BURN

I have quite a few bookmarks under "Running" and I'm going to search through them for something to uphold my position, that it's the blood.

[choosybeggar]

Originally posted by barbitu8
It has been conventional wisdom for lo these many years that a marathoner should jog easily (or swim or do anything with a light level of exertion) the day after the marathon to get the blood flowing, thereby helping to remove the lactic acid build up.

I can't imagine why the lactic acid would stick around this long. Think of lactic acid as a small molecule that gets taken up by the blood stream by virtue of diffusion. All the lactic acid should be cleared locally on the order of minutes (an hour or two at most) post exercise. Thinking that lactic acid would hang out for a day or so is akin to thinking that the carbon dioxide produced by respiration hangs out for a day or so.

Why light exertion? I dunno. There may be a very good reason to do this, I just can't imagine it to have anything to do with lactic acid removal.

[choosybeggar]

barbitu8,

I found this article. Sounds like it addresses pain causing factors during exercise. I can't get b/c it's not online and our library isn't open. But the abstract says:

Pain perceived during exercise is considered to result from a combination of factors including acids, ions, proteins, and hormones. Although it is commonly believed that lactic acid is responsible for this pain, evidence suggests that it is not the only factor. However, no single factor has been identified.

Exercise-induced muscle pain, soreness, and cramps
J Sports Med Phys Fitness 1994 Sep;34(3):203-16

Way to stick to your guns. I'll try to get the article (unless you have easy access).

[barbitu8]

Thanks very much chooseybeggar. I don't have easy access and I would appreciate reading at least a summary of what it states. I guess lactic acid is a factor then, but it's not the sole factor. The OP thanked everybody for the quick response to his question, but a quick response is not necessary the correct response. I guess it was the thinking that lactic acid is the cause, just like it has been the thinking that easy exercise the day after the marathon rids the body of lactic acid. I always wondered about that too, as I thought the body would convert it back to glucose quite quickly.

Most of the stiffness and pains the day after the marathon is caused by DOMS. I don't know why easy exercise would alleviate that. It may help for reasons we don't know, but it never hurts.

[Rick]

Let me toss in my $.02 here. I'm 49 and about two years ago decided after almost 30 years of sitting on the couch to get back into shape. I decided to buy a mountain bike. When I first got the bike after one block (two if downhill) I felt the burn. boy did my thighs burn. Now two years later I can ride all day and not feel the burn, fatigue yes, burn no. The only way I can get the burn now is to either do interval training, lower my seat or stand and sprint. These things cause me to use my muscles in a different way and I can still get the burn.
Barbitu8, I think that you are in too good a shape, is why you don't feel the burn.

[barbitu8]

My quadriceps, which is undoubtedly my strongest muscles, are the only ones that feel the burn when I do the strength trainining thing (occ. my hamstrings which are probably the 2d strongest). I have never felt a burn when exercising (other than weights), even when I first started to run about 25 years ago. Pain, tightness, stiffness,"doms," fatigue yes, but burn no.

[choosybeggar]

Originally posted by barbitu8
Thanks very much chooseybeggar. I don't have easy access and I would appreciate reading at least a summary of what it states.


barbitu8,

I have to wait for interlibrary loan. Will report back when info arrives.

[choosybeggar]

Here's the straight dope on muscle pain during exercise. According to the article I cited above:

The identification of a substance that singularly causes pain to skeletal muscles as a result of contraction has yet to be made. Probable metabolic byproducts include lactic acid and pyruvic acid. Due to ease of diffusion and prevalence in the bloodstream before and after exercise, lactic acid is considered the likeliest of candidates. However the pathology and symptoms of phosphorylase deficiency, McArdle's disease, introduce uncertainty as to the involvements of lactic and pyruvic acids. The hallmark symptom of McArdle's disease is exercise intolerance such that muscle pain is experienced with relatively little exercise under conditions of free and occluded circulation. Without phosphorylase, glycogen cannot be broken down, and glucose is extremely limited as a metabolic substrate. As such, lactate and pyruvate concentrations were found to decrease during exercise ofr the McArdle's patient rather than increase as for normal individuals. McArdle's disease is also associated with painful muscle cramping that, in contrast to a true muscle cramp discussed subsequently, is electrically silent in nature. Although this evidence does not indicate that the build-up of lactic acid does not cause pain in normal exercising muscle, it strongly suggests that additional factors are likely to be involved.(underline mine)

It is possible that the pain-producing substance is not a metabolic byproduct, but simply a substance released by contracting muscle. Phosphoric acid, adenosine, acetylcholine, phosphocreatine, serotonin, potassium, and histamine have been suggested as potential mediators of pain during exercise. All of the mentioned substances, including lactic and pyruvic acids, are potential stimulators of type IV afferent free nerve endings (pain fibers). In light of the literature presented, it is suggested that there are many substances produced or released in contracting muscle that are diffusible and able to stimulate pain receptors. Identification of one particular substance has not been possible to date.(underline mine) A collection or combination of substances, including acids, ions, proteins and hormones, build up within skeletal muscle during contraction and may cause pain if not cleared away.

I always wonder how an orthodoxy develops in the absence of data. Great call barbitu8 for holding out on lactic acid.

[barbitu8]

Thanks very much, Choosy Beggar. I knew you would come through.