Heart Does Not Tire during Exercise
A healthy heart is so strong that it is almost never a cause
of tiredness during exercise. Tiredness during exercise comes
from your muscles. They run out of fuel or out of oxygen. Skeletal
muscles use both fat and sugar for energy. When your muscles
run out of their stored sugar supply, called glycogen, they cannot
contract and function adequately. You feel tired, your muscles hurt
and you have difficulty coordinating them. On the other hand, your
heart muscle gets energy directly from fat and sugar in your blood
and even from a breakdown product of metabolism called lactic
acid. It is virtually impossible for the heart muscle to run out of fuel
unless you are starving to death.
A healthy heart doesn't run out of oxygen either. Oxygen
comes to the heart directly through arteries on its outside surface.
If these arteries are not plugged up with plaques, they are large
enough to supply all the oxygen that the heart can possibly need.
However, fatty plaques in arteries can block the flow of blood.
When the heart does not get enough blood, it will hurt and can
start to beat irregularly. Exercise won't make a healthy heart hurt.
If you develop chest pain during exercise, something is wrong and
you need to check with a doctor immediately.
Checked 9/29/08