Today's question comes from Andy. I have decided to answer this particular one because it has to do with exercise and he recently returned from a trip to England where he, his brother and his dad biked across England. Apparantly they don't have cars in England. Biking is a big thing for the Smith clan as his brother recently biked from Beijing to London. I don't understand it either, but good for Team Smith for getting off their collective butts and enjoying this fine world of our's. I raised a pint for you each night you were in transit, feeling that you were biking enough for all of us.
On to the questions. Andy asks:
Why does lactic acid build up in muscles? And why does it cause soreness?An excellent set of questions, and they do much to support my belief that exercise hurts and should be avoided at all costs. Better to get fat and have a massive heart attack which probably hurts more, but on the plus side, doesn't hurt for nearly as long before all your hurts go away.
The simple answer to your questions is "Because it would be bad if it didn't" and "It doesn't" but that would make for a short post, so I shall endeavor to clarify things.
As you exercise, a substance called pyruvate is produced in your muscles. When enough oxygen is present, this pyruvate is converted into acetyl-coenzyme A which is an input into the Krebs Cycle. The Krebs Cycle then creates ATP which is used to transport energy between cells. This is a good thing. When there isn't enough oxygen to convert the pyruvate, as with intense exercise, the pyruvate would build up and reduce the amount of ATP that is ultimately produced, resulting in less energy for your cells. Luckily there's a substance called lactate dehydrogenase which can take the pyruvate and break it down into lactate (lactic acid) and nicotinamide adenine dinucleotide or NAD+. NAD+ is important because it can be used in the glycolysis process to create ATP, which we've already covered.
That's all well and good, but what are your muscles supposed to do with all of the lactic acid now sitting in there? For years and years, common belief was that lactic acidosis caused your muscles to burn and feel sore, namely because some dude decided to carve up frogs, shock the frog muscles into activity and then noticed that the muscles were covered in lactic acid once they stopped responding to his Frankenstonian shockings. Thus, the relationship between lactic acid and muscle fatigue was born. Why no one questioned why this guy was cutting frogs in half is beyond me, but a biologist I am not. I can also understand the notion that having muscles filled with acid would be a wee bit uncomfortable as, after all, Two-Face didn't get to look like that from harsh language.
As it turns out, lactic acid is not a bad thing. It is a very, very good thing that allows your muscles to keep on keeping on. First, your muscle cells can use the oxygen stored within them to turn the lactic acid into pyruvate, which, from what we discussed before, can then be used as an input into the Krebs cycle to create ATP. Alternatively, the lactic acid can be taken by your liver as an input into the Cori Cycle which then produces glucose which can in turn be used by your body for energy. Note the lack of soreness.
What causes the soreness in the muscles is a result of a whole lot of ATP being produced. As we talked about before, during intense exercise, aerobic metabolism can't create enough ATP so your body switches over to the glycolysis process to create ATP. This process can create a whole lot of ATP in a short amount of time. As the ATP reacts with water, it gives off hydrogen ions and overloads the support systems of the cells. These hydrogen ions then cause the pH to fall, increasing the acidity and owie, you're one hurting unit. So basically, lots of exercise causes too little ATP which causes your body to switch gears and make a whole lot of ATP which reacts with water, gives off hydrogen ions and cause acidosis in your muscles. Owie. You = hurting unit.
As we've seen that lactic acid can be used by muscles to create pyruvate and ultimately ATP, any types of exercise that increase the number of muscle cells is a good thing. More muscle cells means more lactic acid can be converted into pyruvate which means more ATP. It also means the ATP that is produced is produced at a rate that your body can handle, meaning less acidosis. Frankly, I'm quite happy with my muscle mass and the amount of ATP produced, so I'm going to stay right where I am. Then again, I've flown internationally and the pain of that experience was far worse than the sore muscles I would have received had I just biked home from Russia.
Wikipedia - Lactic Acid
PubMed - Biochemistry of exercise-induced metabolic acidosis. - Robergs RA, Ghiasvand F, Parker D, Exercise Science Program, Department of Physical Performance and Development, Johnson Center, The University of New Mexico
The New York Times - "Lactic Acid Is Not Muscles' Foe, It's Fuel" - Gina Kolata, May 16, 2006
Wikipedia - Cellular Respiration
Wikipedia - Krebs Cycle
Wikipedia - Cori Cycle