Why does muscle soreness happen?
Short answer
Unfamiliar exercise or a heavy load can place small stresses on muscles and nearby tissues. The repair and inflammatory responses that follow can appear as pain hours later or on the next day.
How it works
Movements in which a muscle produces force while lengthening can place a particularly large load on it. After unfamiliar stress, small disruptions can occur in muscle fibers and connective tissue.
The body responds by repairing the area, with processes related to inflammation. These responses can stimulate pain-sensing nerves and produce delayed soreness. The common idea that leftover lactic acid causes this soreness is not accurate.
Key point
Delayed-onset muscle soreness often begins about 12 to 24 hours after exercise and can be strongest around 24 to 72 hours. More soreness does not automatically mean a workout was more effective, and no soreness does not mean the exercise had no effect.