Delayed onset muscle soreness (DOMS), also sometimes called muscle fever, is the pain or discomfort often felt 24 to 72 hours after exercising and subsides generally within 2 to 3 days. It is frequently said to be caused by lactic acid buildup; however, since lactic acid disperses fairly rapidly, it could not explain pain experienced days after exercise, and some concentric-only exercises produce lactic acid, but rarely produce DOMS.

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.

Some research claims that DOMS is not caused by the pain from damaged muscle cells, but from the reinforcement process. 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 in their compartment and put pressure on nerves and arteries producing pain.

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