What Type Of Muscle Tires Quickly During Exercise?

If a muscle has not been exercised in some time, it will tire easily, this can be any muscle.  Including the heart.  That is why it is important to work out, run, walk, do sit ups, anything to exercise all muscles, and help pump blood to your heart, which is an involountary muscles, means it works out, whether you want if to or not, much like lungs and brain but the more you work out, the more oxygen gets to all organs.  You can also exercise the brain with puzzles or brainteasers, or coming to this site and try to answer questions