Why muscles ache after exercise?
Whatever you do, you are using your muscles. Not all of them move at once - you have 639 muscles in all - but you would be surprised to know how many of your muscles you use doing something as simple as walking.
When your muscles are working, they produce an acid called lactic acid, which makes the muscles feel tired. When you rest, your body gets rid of lactic acid, and you don't feel so tired.
When you do a lot of hard exercise, like a running race, you may get a cramp, a sharp pain in the muscles you are using the most. Cramp is caused by a big build-up of lactic acid in these muscles, and a lot of rest is needed to get rid of the cramp. So, although exercise, as we know, is good for your body, rest is just as good, and just as important.
