Question: Can you explain what lactic acid is and how it affects the body during exercise? I've heard it causes cramps.

Mark Burnley answered on 13 Jun 2011:

This is a great question because anything I say about lactic acid will be controversial! There are various forms of lactic acid but in the body it is a form of sugar that has only been partially broken down to release energy. You start of with glucose, and then you break that down using a process known as glycolysis (glycogen is a form of stored glucose hence glyco- and lysis means to split). The end product of this is pyruvic acid and this is either broken down futher with the use of oxygen (aerobic metabolism), or is converted to lactic acid. There are various reasons why the body choses to produce lactic acid rather than go through aerobic metabolism, but it would literally take a PhD thesis to explain it properly!

There is one other thing you need to know: lactic acid usually does not exist in the human body, because, being an acid, it tends to “dissociate” (lose a hydrogen ion) in body fluids at a pH of ~7-7.4. So we actually call the dissociated lactic acid “lactate”. So next time you hear Steve Cram or Brendan Foster talk about athletes struggling because of all the lactic acid in their muscles, you’ll know they are completely wrong!

Lactate is often seen as a cause of fatigue, but interestingly there is very little evidence to support this! Lactate is in fact a very useful substance that can be converted back to pyruvate and used to produce energy aerobically, so it is NOT a fatiguing waste product. So to answer very simply one part of the question, lactate has very little effect on the body, apart from giving you more energy!

And so to muscle cramps: we are pretty certain that lactate is not involved in muscle cramps for a few reasons. Firstly, you can do very hard exercise, which produces lots of lactate, and suffer no cramps at all. Secondly, cramps sometimes appear after very prolonged exercise, or even at rest, when the amount of lactate in the body is very low. Third, only some muscles experience cramps; when have you ever had a cramp in you gluteaus maximus (you bum muscle)? Thought not, because it is often muscles that cross two joints that suffer cramps. This suggests that something other than lactate is the cause, but we are not completely sure what that is. One thing that we are sure of it that stretching the affected muscle relieves the cramp, which is a clue to what the cause is!