It's the acid that does it and that acid is caused by synthesis of lactic acid taking place during anaerobic exercise, or so the story goes. That happens under extreme conditions when the energy needed by working muscles exceeds the ability to produce it by normal aerobic oxidation. It all sounds so logical ... and so biochemical.
It's all a myth. Lactic acid has nothing to do with acidosis (the buildup of acid in the muscles). In fact, it's not even clear that acidosis is the problem, but let's deal with that another time.
Assuming that acid buildup in muscles is what causes the pain of the long distance runner, where does that acid comes from? In order to answer that question we need a brief lesson on acids.
Acids are molecules that can give up a hydrogen ion (H+), or proton. Hydrochloric acid (HCl) and acetic acid are classic examples. They can both dissociate into H+ and a negatively charged ion; either a chloride ion in the case of hydrochloric acid (Cl-) or an acetate ion (CH3COO-) in the case of acetic acid.
The concentration of hydrogen ions is what makes solutions acidic and we describe that concentration by referring to the pH of the solution where the "H" stands for hydrogen ions. The pH scale is a log scale and it is the negative log of the hydrogen ion concentration (don't ask). Solutions with low pH have very high concentrations of hydrogen ions.
Muscles need biochemical energy to do their work and that energy is supplied by ATP, the common energy currency in the cell. As ATP is used up, it needs to be regenerated and the quickest way to do that is to makes make more ATP using creatine phosphate, a high energy molecule stored in muscle cells. When the creatine phosphate is depleted, muscle cells mobilize their store of glycogen converting it to glucose that is then metabolized by the glycolysis pathway. The end products of this pathway are pyruvate, ATP, and NADH. The ATP produced during glycolysis is used by the muscle cells.
Under normal conditions, pyruvate enters a pathway called the citric acid cycle and this pathway regenerates NAD+ from NADH so that glycolysis can continue. This reaction is coupled to synthesis of more ATP by mitochondria, a process that requires oxygen.
Here's where things get tricky for athletes. Their muscles are often working so hard that the resupply of ATP by glycolysis and the citric acid cycle can't keep up with the oxygen supply, no matter how hard the athletes are breathing. Pyruvate begins to accumulate in the muscle cells because it can't be metabolized quickly enough in the citric acid cycle.
Under these conditions, pyruvate is converted to lactate in order to generate more NAD+ so that glycolysis can continue. This is often referred to as anaerobic metabolism. Look closely at the reaction.
The product of this reaction is lactate, not lactic acid. Lactate is not an acid because it can't give up a proton. The overall reaction doesn't produce acid (H+), it actually consumes it. Muscle cells do not accumulate lactic acid, they accumulate lactate and that's not the same thing.
So what causes acidosis in muscles? It may come, in part, from the reaction that consumes ATP but this can't be the whole story because ATP is rapidly regenerated, using up the hydrogen ion.
Some of the acidity may be indirectly due to a buildup of lactate affecting buffering capacity but this doesn't seem to be a likely cause of acidosis.
The important point is that lactic acid is not produced in muscles so it can't be the source of acidosis. This has been known in the scientific literature for twenty years but it doesn't seem to have entered the biochemistry textbooks until recently. The myth of lactic acid has been debunked in newspapers and science magazines but it's still believed by athletic coaches and trainers and by the athletes themselves. It doesn't really matter since training is able to overcome the limits of muscle metabolism whatever the cause. It's likely that training and hard exercise increase the number of mitochondria in muscle cells and this could be the real benefit since it allows for more aerobic metabolism.
Here's just a small sample of articles debunking the lactic acid myth. Let me know if you find more.
Lactate: Not Guilty as Charged
Lactic Acid Is Not Muscles' Foe, It's Fuel
[Encouragement Credit: Ms. Sandwalk made me post this. She got tired of hearing me yell at the television and she was afraid I'd throw my textbook at the screen.]
[Image Credits: Moran, L.A., Horton, H.R., Scrimgeour, K.G., and Perry, M.D. (2012) Principles of Biochemistry 5th ed., Pearson Education Inc. (various pages) [Pearson: Principles of Biochemistry 5/E] ©2012 Pearson Education Inc.]