Four elements of fatigue, or what I think about at mile 15 of a 19 miler.

--Lactic Acid: It builds-up in muscles when they are exerted and gives the feeling of soreness.

--Glycogen: Stored mostly in the liver, it is a source of energy. It may not completely run out but it certainly does become scarce. The body has to switch over to fat metabolism. This is an essentially bottomless well of energy, the problem is that fat cannot be converted into a directly useful form quite fast enough.

--Dehydration: I do not bring water with me on runs of any distance and can easily loose four pounds of water weight on a long run. What happens is that as a person becomes dehydrated, it causes the blood pressure to drop. (One of the main classes of hypertension medications are diuretics) When the blood pressure drops, the heart must pump more rapidly to accomplish the same circulation. Respiration seems to go hand-in-hand with this. What I find is that toward the end of a long run, I cannot maintain my starting pace without becoming winded. The drop in blood pressure is not due entirely to dehydration, so even in marathons that have hydration stations, this is still an issue.

--Will power: As exertion continues, the will to maintain it becomes depleted too. It is too bad since all of the above things make the need for will-power more acute toward the end.

In a race, much but not all of the above issues are at least of smaller concern--at least (with a marathon approaching) this is what I tell myself.