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Dear Dr. Mirkin: Does your heart get tired 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.