<P> DOMS was first described in 1902 by Theodore Hough, who concluded that this kind of soreness is "fundamentally the result of ruptures within the muscle". According to this "muscle damage" theory of DOMS, these ruptures are microscopic lesions at the Z - line of the muscle sarcomere . The soreness has been attributed to the increased tension force and muscle lengthening from eccentric exercise . This may cause the actin and myosin cross-bridges to separate prior to relaxation, ultimately causing greater tension on the remaining active motor units . This increases the risk of broadening, smearing, and damage to the sarcomere . When microtrauma occurs to these structures, nociceptors (pain receptors) within muscle connective tissues are stimulated and cause the sensation of pain . </P> <P> Another explanation for the pain associated with DOMS is the "enzyme efflux" theory . Following microtrauma, calcium that is normally stored in the sarcoplasmic reticulum accumulates in the damaged muscles . Cellular respiration is inhibited and ATP needed to actively transport calcium back into the sarcoplasmic reticulum is also slowed . This accumulation of calcium may activate proteases and phospholipases which in turn break down and degenerate muscle protein . This causes inflammation, and in turn pain due to the accumulation of histamines, prostaglandins, and potassium . </P> <P> An earlier theory posited that DOMS is connected to the build - up of lactic acid in the blood, which was thought to continue being produced following exercise . This build - up of lactic acid was thought to be a toxic metabolic waste product that caused the perception of pain at a delayed stage . This theory has been largely rejected, as concentric contractions which also produce lactic acid have been unable to cause DOMS . Additionally, lactic acid is known from multiple studies to return to normal levels within one hour of exercise, and therefore cannot cause the pain that occurs much later . </P> <P> Although delayed onset muscle soreness is a symptom associated with muscle damage, its magnitude does not necessarily reflect the magnitude of muscle damage . </P>

Why are my legs more sore the second day