Muscle Soreness

Disease / Phenomenon

Delayed onset muscle soreness is the stiffness, tenderness, and reduced force production that appears roughly twelve to twenty-four hours after unfamiliar or intense exercise, peaks around forty-eight hours, and resolves within several days. It is most strongly provoked by eccentric contractions, where muscle lengthens under load — descending stairs, lowering weights, downhill running — because these produce the greatest mechanical strain per active fiber. The experience is distinct from the acute burning of metabolic fatigue during exercise, and despite persistent popular belief it is not caused by lactic acid, which clears within an hour of finishing. Soreness is a normal part of adaptation, though excessive damage can impair subsequent training quality.

The initiating event is microscopic disruption of sarcomeres and the cytoskeletal proteins desmin and titin, together with damage to the sarcolemma and t-tubules. Calcium homeostasis is disturbed, activating calpain proteases. Neutrophils and then macrophages infiltrate the tissue, generating reactive oxygen species and releasing prostaglandin E2, bradykinin, and cytokines including IL-6 and TNF-alpha, which sensitize group III and IV nociceptive afferents and produce the tenderness. Satellite cells are subsequently recruited for repair. This page separates these processes into their molecular pathways and associated biomarkers.