Most muscles are fastened firmly to the bones of the skeleton. The skeleton forms the framework, and the muscles move the parts of the body. Without them a person could not live. Not only would it be impossible to eat, breathe, and talk, but the heart would stop because its beating is a muscular action.
All muscle is made up of long, thin cells called "muscle fibres". But muscles differ in what they do and how they do it. They also differ in shape, appearance, size, and in other ways.
When a muscle is contracted, it produces an acid known as lactic acid. This acid is like a "poison". The effect of this lactic acid is to make you tired, by making muscles feel tired. If the lactic acid is removed from a tired muscle, it stops feeling tired and can go right to work again!
But, of course, lactic acid is not removed normally when you exercise or work. In addition, various toxins are produced when muscles are active. They arc carried by the blood through the body and they cause tiredness—not only in the muscle, but in the entire body, especially the brain.
So feeling tired after muscular exercise is really the result of a kind of internal "poisoning" that goes on in the body. But the body needs the feeling of tiredness so that it will want to rest. Because, during rest, waste products are removed the cells recuperate, nerve cells of the brain recharge their batteries, the joints of the body replace the supplies of lubricant they have used up, mid so on. So while exercise is good for the body and the muscles. rest is just as important!