Why does it hurt the next day after doing significant exercise?

Question

I think this is a fairly common observation that if one does some significant amount of exercise, he/she may feel alright for the rest of the day, but it generally hurts bad *the next day*. Why is this the case?I would expect that if the muscles have undergone significant strain (say I started pushups/plank today), then it should cause pain while doing the strenuous activity, or during rest of the day, but it happens often that we don't feel the pain while doing the activity or even on that day, but surely and sorely feel it the next day.Another example - say after a long time, you played a long game of basketball/baseball/cricket. You generally don't feel any pain during the game/that day, but there is a good chance it will hurt bad the next day.I am trying to understand both - why does the pain not happen on that day, and why it does, the next day (or the day after that).

Answer 1

The current theory is as follows: Calcium is a key muscle cell electrolyte that is important for muscle contraction. It is stored in high concentrations within a part of the cell called the sarcoplasmic reticulum. After you have been working out for a while, your muscles have used up much of their energy stores (ATP). However, ATP is very important for getting calcium back into its storage site (the sarcoplasmic reticulum). Calcium is often also used in activating enzymes, such as proteases that breakdown cell materials. If there is a high concentration of calcium within the muscle cell that is not stored away, it is free to interact with these enzymes and turn them on. This in turn starts breaking parts of the muscle cell down, producing distress signals called cytokines. These distress signals cause inflammation in the muscle, attracting cells that dump histamine, prostaglandins, and other pro-inflammatory signals. These cells, however, have their own timelines for arriving at the inflamed muscle. Some cells will show up more immediately, but others work in 6-12 hour or 12-24 hour time frames. This delayed arrival and sequence of events is thought to be the reason why delayed onset muscle soreness occurs.

On the other hand, muscle discomfort/burning that occurs during exercise is thought to be the buildup of lactic acid when your muscles are not able to produce energy (ATP) aerobically.