Delayed onset muscle soreness (DOMS) is the pain and stiffness felt in muscles some time after unaccustomed or strenuous exercise. The soreness usually increases in intensity in the first 24 hours after exercise, and peaks from 24 to 72 hours,

It was first (medically) described in 1902, and despite intensive research, the causes are still disputed.

DOMS is characterized by intensity of symptoms beginning 24-48 hours after exercise and lasting for several days, and has been described as muscle stiffness and swelling, loss of force-generating capacity, reduced joint range of motion, and loss of proprioceptive function. The underlying physiological mechanism causing DOMS is unknown and highly disputed. At least six hypothesized theories for the mechanism of DOMS have been proposed."
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Theories include the ideas that the pain may be caused by multiple small muscle fibre ruptures, or by build up of excess calcium and/or lactic acid.

For details, see : Mechanisms and Treatment of Delayed Onset Muscle Soreness in Athletes - A Review Sports Science, Vol: 3 issue 7 - July 2022

More info at Wikipedia

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