“Pain exists to get an animal to change its behaviour.

This functional account of pain may explain some ongoing mysteries, such as the cause of the muscle soreness that follows a day of intense exercise, which has eluded physiological explanation.

The popular idea that it is due to the build-up of lactic acid has been discounted, as have other proposed theories.

Body builders have found that the optimal way to build muscle is to take a rest day after a strenuous workout.

Perhaps nature has converged on the same idea, and muscle soreness is simply a signal to rest, to enable optimal muscle building.”

Reference

Finlay, B. (2015) The Unique Pain of Being Human. New Scientist. 9 May 2015, pp.28-29.