This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
(Rob Loszewski firstname.lastname@example.org)
"Lactic acid buildup (technically called acidosis) can cause burning pain,
especially in untrained muscles. Lactic acid accumulation can lead to
muscle exhaustion withing seconds if the blood cannot clear it away. A
strategy for dealing with lactic acid buildup is to relax the muscles at
every opportunity, so that the circulating blood can carry the lactic acid
away and bring oxygen to support aerobic metabolism. ...much of the lactic
acid is routed to the liver, where it is converted to glucose. A little
lactic acid remains in muscle tissue, where it is completely oxidized when
the oxygen supply is once again sufficient." Understanding Nutrition, 5th
ed., Whitney, Hamilton, Rolfes., West Pub. Comp. 1990, pg402- 403.