Causes of Delayed Muscle Soreness

There have been several theories as to the origin of delayed muscle soreness. Some of the major ones are reviewed below.

1. The lactic acid theory.
The theory that the accumulation of lactic acid somehow accounts for delayed muscle soreness is probably the most popular one in the lay exercise community. However, lactic acid concentration in the muscle drops to near normal levels within 30 to 90 minutes of exercise and delayed muscle soreness does not appear until eight or more hours afterwards. Furthermore, repeated eccentric contractions which result in the greatest degree of delayed soreness produce less lactic acid than the same volume and intensity of concentric contractions. From this evidence it is clear that lactic acid can not account for delayed muscle soreness.

2. The muscle spasm theory.
Some researchers have proposed that a muscle spasm, caused by exercise induced microtrauma, is the cause of delayed muscle soreness. Theoretically, a spasm could cut off the blood supply to the muscle and cause discomfort as the tissues became starved of oxygen. In partial support of this theory some researchers have detected elevated electrical activity (supposedly indicative of a muscle spasm) in sore muscles. However, others have detected elevated electrical activity and found it to be unrelated to the extent of soreness. Furthermore, some researchers have found that relaxation techniques and static stretching successfully reduces the muscle spasm but not delayed muscle soreness. Based on this evidence it seems very unlikely that muscle spasms are related to the degree of delayed soreness experienced.

3. Muscle fiber membrane damage.
Whilst a full explanation of DMS is yet to be established some researchers in the field have proposed a plausible mechanism which may account for the delayed nature of muscle soreness.

It is proposed that the high intramuscular tension associated with exercise (particularly eccentric contractions) damages the muscle fiber membrane allowing the leakage of calcium into the fiber. High levels of intracellular calcium then cause progressive degeneration of various intracellular components including the mitochondria. It is also apparent that various intracellular enzymes (normally found only inside muscle fibers) leak out into the spaces between muscle fibers and initiate an inflammatory response because the body recognizes the intracellular components as 'foreign'. Microscopic evidence suggests that the degree of muscle fiber membrane damage, whilst initially quite small, increases in the 30 hour period after exercise so as to parallel the extent and time course of soreness.