[rakgirl]

look what i found on wikipedia.

Delayed Onset Muscle Soreness (DOMS) is the pain or discomfort often felt 24 to 72 hours after exercising and subsides generally within 2 to 3 days. Once thought to be caused by lactic acid buildup, a more recent theory is that it is caused by tiny tears in the muscle fibers caused by eccentric contraction, or unaccustomed training levels. Since lactic acid disperses fairly rapidly, it could not explain pain experienced days after exercise.[1]

Although the precise cause is still unknown, the type of muscle contraction seems to be a key factor in the development of DOMS. Exercises that involve many eccentric contractions, such as downhill running, will result in the most severe DOMS. This has been shown to be the result of more muscle cell damage than is seen with typical concentric contractions, in which a muscle successfully shortens during contraction against a load.[2]

Some research claims that DOMS is not caused by the pain from damaged muscle cells, but from the reinforcement process.[3] The muscle responds to training by reinforcing itself up to and above its previous strength by increasing the size of muscle fibers (muscle hypertrophy). This reinforcement process causes the cells to swell and put pressure on nerves and arteries, causing DOMS.