Muscles are made up of long, thin cells called muscle fibres. But muscles differ in what they do and how they do it. When a muscle contracts, it produce an acid known as lactic acid. This acid is like a poison, with the effect of making you feel tired, by making the muscles feel tired. If the lactic acid removed from a tired muscle, it stops feeling tired and you can go right to work again.
But, of course, lactic acid is not removed normally when your exercise and various toxins are produced when muscles are active. They are carried by the blood through the body and cause tiredness through the entire body, especially in the brain. So feeling tired after exercise is really the result of a king of internal poisoning.
However, the body needs this feeling of tiredness so that it will want to rest. During rest, waste products are removed, the cells recuperate, nerve cells of the brain recharge the batteries and the joints replace their supplies of lubricant they have used up. So while exercise is good for the body and muscles, rest is just as important.