Why a muscle hurts the next day

Pain that shows up one or two days after a workout, and not during the workout itself, is called delayed onset muscle soreness; in everyday language people just say they are sore. It is a dull, background ache that you notice most when you press on the muscle with your fingers, when you contract it, or when you stretch it.

One particular kind of effort brings it on: eccentric exercise, the kind in which the muscle brakes while it lengthens, like walking down stairs or lowering a weight slowly. And above all when you are not used to doing it. Why that effort hurts a day later instead of at the time is still not fully understood.

Its timing, on the other hand, is well known. It is described as pain that climbs, reaches a maximum, and drops off, tracing an arc; the high point lands one to two days after exercise.

It is best not to treat it as a gauge. Hurting a lot does not mean the muscle was damaged a lot: how intense it feels correlates poorly with the other signs of muscle damage. It is an alarm that goes off late and does not seem to tell you how hard the hit was.