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.

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.