We have all experienced this phenomenon after a long and hard workout. The muscles in our bodies start aching only on the next day, not straight away.
The scientific reasons behind this are explained in the popular answers below by four individuals.
Yes, any intense activity that you are not used to can cause this type of pain, but why not right away? Current thinking is that your nerves are actually responsible. The muscle performing this type of activity gets microscopic damage, not intense enough to cause pain right away. The repair process of this damage involves some inflamation and some immune cells so to call them the muscle produces a bunch of signal chemicals and molecules that initiate repair. This also makes the neurons that innervate the muscle to become more sensitive to movement (it changes how they make the molecules that are sensitive to movement in the muscle), but this does not happen that fast, (it takes time to make new proteins), thus you are not sore right away because it’s not the neurons sensing damage but them becoming more sensitive to movement. This makes you move your muscle a bit less so that it can heal. (Notice how in delayed onset muscle soreness it does not hurt right after a workout and it’s not really sore all the time, it just hurts when you move the affected muscle and usually a day later)
This is called delayed onset muscle soreness there are basically 3 theories 1. Micro-trauma and muscle fiber breakdown from the exercise itself that hurts and then causes the muscles to rebuild stronger. 2. Lactic acid buildup that damages muscles in a delayed fashion. 3. Enzyme damage which damages muscles in a delayed fashion.
However, 1. Things that are injured usually hurt right away. 2. This doesn’t happen. 3. This isn’t proven.
If you do an exercise and you’re not used to it, you can get some pretty extreme delayed onset muscle soreness (DOMS). But, do that same exercise 2-3 times a week for even just a couple weeks, and the DOMS goes away entirely. You can even add significant weight/resistance to the exercise. Your body just ‘gets used to it,’ even though the same ‘damage’ is taking place. Take a couple weeks off from an exercise you’re used to and come back, and the DOMS comes back for a workout or two.
This suggests that DOMS isn’t the result of damage. My anecdotal experience has been that the body can be fully recovered (only takes a day or two assuming good food and rest) but still have pretty extreme DOMS. Seems counterintuitive to work out while sore, but within a few reps the soreness dissipates significantly. (So don’t use DOMS as an excuse to not exercise!)
Short answer: We don’t know.
Long answer: Delayed Onset Muscle Soreness is poorly understood. We do not know what causes the soreness in the first place, thus we don’t know why it’s delayed. The current working hypothesis is that strong workouts cause microscopic tears, which lead to the soreness when healing.
The hypothesis that lactic acid buildup in the muscles causes the soreness has been rejected, as there is no clear correlation between acid levels and whether the muscles feel sore.