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I commonly hear that muscle cramps are caused by lactic acid. For instance, this article indicates:

Another cause of muscle cramps can be the build up of lactic acid in the muscles.

However, this NYT report claims otherwise

The understanding now is that muscle cells convert glucose or glycogen to lactic acid. The lactic acid is taken up and used as a fuel by mitochondria, the energy factories in muscle cells.

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Addressing the muscle soreness point, no, lactic acid does not cause delayed soreness. Wikipedia has a nice terse summary of why, backed up by this review (of multiple related studies).

The lactic acidosis of exercise has been a classic explanation of the biochemistry of acidosis for more than 80 years. [...]

This review presents clear evidence that there is no biochemical support for lactate production causing acidosis. Lactate production retards, not causes, acidosis. Similarly, there is a wealth of research evidence to show that acidosis is caused by reactions other than lactate production. [...]

It is only when the exercise intensity increases beyond steady state that there is a need for greater reliance on ATP regeneration from glycolysis and the phosphagen system. The ATP that is supplied from these non-mitochondrial sources and is eventually used to fuel muscle contraction increases proton release and causes the acidosis of intense exercise. [...] If muscle did not produce lactate, acidosis and muscle fatigue would occur more quickly and exercise performance would be severely impaired.

— Abstract from Robergs, R A, F Ghiasvand, and D Parker, Biochemistry of exercise-induced metabolic acidosis

The muscle soreness later, post-exercise, often felt the following day, cannot be caused by lactic acid as it is "flushed from the muscle cells within 40-60 minutes", though it is unclear if the blood stream carries it away or it is simply metabolized within the cell.

Cramping seems to be a more neurological symptom, when the peripheral nerves, rather than the CNS, generate a signal directing skeletal muscles to contract. I can't find any papers that discuss lactic acid (which should be at a higher concentration with the muscle cells rather than extracellular) interfering with neurons or their receptors, so I don't know on that point.

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    Yes, I read the wikipedia article, in addition to these two: en.wikipedia.org/wiki/Cramp, en.wikipedia.org/wiki/Delayed_onset_muscle_soreness. As you can see the three do not perfectly agree on what lactic acid does or does not do.
    – Russell Steen
    Commented Mar 30, 2011 at 1:28
  • @Russell, the "lactic acid", and "delayed onset muscle soreness" article both say that lactic acid is not involved with a citation, while "cramp" lacks a citation. In any case, how does that refute the review I cited?
    – Nick T
    Commented Mar 30, 2011 at 2:03
  • @Nick -- It's nothing personal. I know you're right with regards to the conclusion. The downvote is for block voting wikipedia as a primary source. I was attempting to illustrate in a less direct fashion why wikipedia is not a quality source.
    – Russell Steen
    Commented Mar 30, 2011 at 3:09
  • fear the skeptic monkeys! :P
    – cregox
    Commented Mar 30, 2011 at 3:09
  • @Nick nothing personal here either.You were right with the conclusion from the beginning. But before the edit, you were being a poor advocate for your own position. Plus, I don't think you can base a response solely on wikipedia links. However because you now address the link between fatigue during exercise and perceived soreness later, this is much clearer statement. And with better links.
    – Monkey Tuesday
    Commented Mar 30, 2011 at 3:49

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