What causes muscle soreness?

When a load that is more than what is normally experienced (called overload) has been placed upon the muscles and surround tissues, damage occurs in the form of tiny micro-tears. This can happen from strength training but also activities like running, sprinting, even walking up or down a hill. It might sound like a bad thing to allow your body to experience this damage, but it is necessary for muscle tissues to build back stronger. That is why it important to wait at least 48 hours between workouts that overload the same muscles (review recommendations in the “Muscular Strength and Muscular Endurance” section under “Components of Fitness”). It is also important that the amount of overload be appropriate and not too much. There is no additional benefit to extreme levels of overload and it could lead to a serious heath condition called rhabdomyolysis.

The ACSM provides additional information on DOMS.