The "burn" comes from the build-up of lactic acid, which
forms as a by-product of energy production by muscle fibers. As the fibers
convert glycogen to ATP, some of the reactions take place without oxygen.
In this process, a compound called pyruvate is produced.
Some of the pyruvate is absorbed into the muscle cell's mitochondria
and converted into useful energy. But during strenuous exercise, the mitochondria
can't handle all the pyruvate that's produced. The excess pyruvate becomes
lactic acid, a dead end as far as energy production is concerned. As the
concentration of lactic acid in the muscle fiber increases, the acidity
of the cell changes, causing muscle fatigue and the all-too-familiar "burn."
The best way to relieve lactic-acid-induced soreness is to continue to
move around, but at a slower pace and without strain or with massage. Both
stimulate blood circulation, which cleans out the built-up lactic acid from