When you exercise your muscles contract and produce an acid known as lactic acid. This acid acts like a ‘poison’. The effect of this lactic acid is to make you tired, by making your muscles feel tired.
If the acid is removed from a tired muscle, it stops feeling tired and can go right to work again.
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 this feeling of tiredness so that it will want to rest.
During rest the joints of the body replace the supplies of lubricants they have used up during the exercise.
It is important to stretch your muscles before and after exercise to distribute the lactic acid.
Is exercise therefore good for us?
Regular exercise is important because it keeps bones, joints, and muscles healthy. During any physical exertion, the rate at which the heart beats increases, as it pumps more oxygenated blood around the body.
How quickly the heart rate returns to normal after exercise is one way to assess how fit someone is and how exercise is actually improving their fitness.
Once almost everyone did manual work of some kind; it was essential for survival. But presently, there are hardly manual works anymore. Human bodies were not designed for the inactive lives many of us now lead. This is why exercise is important for good health.