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.

Genetics have a large influence over strength, muscle size and lung capacity, flexibility, and, to some extent, endurance.

One major limitation for endurance athletes is cardiac capacity, or the heart's ability to deliver enough oxygen (via the bloodstream) to the working skeletal muscles. This, too, is largely determined by genetics.

The other limitation for endurance athletes is in the muscles  ability to effectively use the oxygen and create ATP (adenosine triphosphate), the fuel that allows muscular contraction and movement. The efficiency of this process is measured by something called maximum volume of oxygen.