A healthy heart is so strong that it is never a cause of tiredness during exercise. Tiredness during exercise comes from our muscles, when they are running out of fuel or out of oxygen. Skeletal muscles use both fat and sugar for energy. When our muscles run out of their stored sugar supply, called glycogen, they cannot contract and function adequately. We will then feel tired, our muscles hurt and we will have difficulty coordinating them.
On the other hand, our heart muscle gets energy directly from fat and sugar in our 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 we are starving to death. It is also impossible for a healthy heart to run out of oxygen. Oxygen comes to the heart 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 can't hurt a healthy heart. If we develop heart pain during exercise, something is wrong and we need to check with a medical doctor immediately.
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