Muscles use something called ATP (adenosine triphosphate) as an energy source. ATP is created in essentially two ways. One is by aerobic metabolism (using oxygen) carried in the bloodstream. This is a sure and steady way to create virtually unlimited sources of energy; however, it has a limit on how fast it can work. The cardiovascular system is limited in its ability to quickly deliver blood and oxygen to the working muscles. Because of this, during high intensity exercise, ATP is also provided by anaerobic metabolism that don't require an ongoing supply of oxygen. Anaerobic pathways can provide ATP immediately, but they also have limited stores that need to be refilled after they are used up. This may cause muscle fatigue.

This question is often asked in companies where new reps will be selling cardiovascular products; however, occasionally seen it in other companies where new reps are selling in different therapeutic areas.  The major difference between a stroke and heart attack is the part of the body that is affected: stroke affects the brain, a heart attack affects the heart.

A stroke or "brain attack" occurs when a blood clot or some other small particle blocks the blood vessel supplying that part of the brain, or when a vessel weakens, causing it to leak and bleed into the surrounding tissue.

A heart attack also occurs when there is a blockage in a blood vessel, only in this case it is in the arteries that supply the heart muscle with oxygen and blood. The blockage in the heart vessels may be due to blood clots or small particles of plaque.