High density lipoproteins, or HDL's, are good for you. They are produced by the liver to carry cholesterol and other lipids (fats) from tissues and organs back to the liver for recycling or degradation. Having high levels of HDL lowers the incidence of atherosclerosis (clooged arteries) and heart disease.

The ways in which you can increase the HDL levels are:

1. Moderate exercise (about 30 minutes five times a week) not only reduces LDL cholesterol, it also can raise HDL cholesterol, too.
2. Smoking can lower HDL levels, in addition to disturbing other aspects of your lipid profile. This appears to affect women more than it does men.
3. Eat healthy by including lots of fruits, vegetables, whole grains and lean sources of protein in your diet. Also be sure to include monounsaturated fats in your diet.
4. Moderate consumption of alcohol may also positively impact HDL levels.

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.