Why does it hurt a day or 2 later?
There are two types of exercise-related muscle soreness. Immediate muscle soreness quickly dissipates and is the pain you feel during, or immediately after, exercise. Delayed muscle soreness manifests 24 to 48 hours after the exercise session and spontaneously decreases after 72 hours.
The most current research attributes it to microscopic tears in the muscle and surrounding connective tissue following eccentric exercise. (A muscle contracts eccentrically when it lengthens under tension during exercise).
Those who experience delayed muscle soreness include conditioned individuals who increase the intensity, frequency or duration of their workouts, or participate in an activity that they are unfamiliar with. Beginning exercisers, or those who have undergone a significant lapse in training,frequently experience soreness when starting a new exercise program.
Once you induce delayed onset muscle soreness at a specific exercise intensity, you shouldn't experience that sensation again until intensity is increased. This is because delayed muscle soreness has been shown to produce a rapid adaptation response, which means that the muscles adapt to an exercise intensity.
Click to read the whole story from the American Council on Exercise.