When you exercise your muscles contract and produce an acid known as lactic acid acts like a ‘poison ‘ .the effect of this lactic acid is to make you tired , by making 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 .
*Swimming is a very good exercise as it uses lots of muscles without causing strain
*It’s important to stretch your muscles before and after exercise to distribute tha lactic acid
the source : from the book tell me why??