Why do muscles hurt a day or two after exercise?

That ache the day after a new workout has a name: delayed onset muscle soreness, or DOMS. It's thought to come from tiny amounts of damage, called microtrauma, in your muscle fibres, and it's caused mainly by eccentric movements, where a muscle works while it is being stretched longer. Think of walking downhill, where your thigh muscles act as brakes, or slowly lowering a heavy box. Movements where the muscle shortens, like lifting, cause no DOMS at all.

The soreness usually builds over the first day, peaks between 24 and 72 hours, and fades within about a week. It tends to hurt only when you stretch, use or press the muscle, not when you're resting.

You may have heard it blamed on lactic acid. That idea has been largely rejected: lactic acid returns to normal within an hour of exercise, far too soon to explain pain that peaks days later, and shortening movements produce lactic acid without causing DOMS.

The good news is that muscles learn fast. After one bout of unfamiliar eccentric exercise, the same muscle adapts so that repeating it causes less damage and less soreness, a protection called the repeated-bout effect that can last several weeks. Curiously, how sore you feel doesn't necessarily reflect how much damage there is.