Delayed onset muscle soreness (DOMS) describes a phenomenon of muscle pain and soreness that is felt 12-48 hours following exercise, particularly at the beginning of a new an exercise program, after a change in sports activities, or after a dramatic increase in the duration or intensity of exercise. Sometimes, people may refer to DOMS as ’stiff muscles’.

This soreness is a normal response to unusual exertion and is part of an adaptation process that leads to greater stamina and strength as the muscles recover and build. The soreness is generally at its worst within the first 2 days following the activity and subsides over the next few 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, 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 it’s compartment and put pressure on nerves and arteries producing pain.

Delayed onset muscle soreness is quite common and can be quite frustrating, particularly for those beginning an exercise program or adding new activities.

See more definitions in the Definitive Guide to Running Terms.