How Do Muscles Respond To Exercise

      So basically, muscles are made up of long, thin cells called muscle fibres and as the muscle contracts, it produces an acid known as the lactic acid. This acid is like a poison that has the effect of making you tired by making your muscles feel tired. If this acid is removed from the tired muscle, it stops feeling tired and you could get right back to work again. But, this acid is not removed normally when you exercise and various toxins are produced when your muscles are active. These toxins are carried  by the blood through the body, which cause tiredness throughout the entire body, especially the brain. Hence, the tiredness of exercise is the result of internal poisoning. Your body really needs this feeling of tiredness in order to make you want to rest. When you are resting waste products are removed, the cells recover from exertion or recuperate , nerve cells of the brain recharge their batteries. 
So while exercise is good for the body and muscles-rest is just as important