M.D. If you're afraid that exercise will hurt your heart, check with your doctor. You become tired during exercise because your muscles feel tired.
Skeletal muscles use both fat and sugar for energy.
When skeletal muscles run out of their stored sugar called glycogen, they can't contract and function adequately. They hurt and you will find it difficult to coordinate them. A healthy heart will not tire you during exercise and can withstand all the exercise you can give it. Unlike skeletal muscles that can run out of fuel, your heart muscle can never run out of fuel because it uses fat and sugar carried in your bloodstream and a breakdown product of metabolism called lactic acid. Your heart cannot run out of oxygen unless the arteries that carry blood to your heart are blocked by plaques.