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 the muscles.