When runners say that they hit the wall during long runs or races usually they mean that they have to drop their pace significantly or even stop running. There are essentially three walls:

1. The inability of the muscle to run longer than certain distance. This happens to newbies who have not trained enough to handle the distance. Consistent long runs will eliminate this wall completely.
2. Dehydration. This is the most common wall experienced by well trained runners. It is caused by either poor hydration or the weather too hot for the hydration to keep up with the body fluid loss. Some runners are more prone to dehydration in hot weather than the others. The symptoms are numbness of hands, short breath, muscle cramp, thirst, etc. When one feels thirsty at a race, it is already too late to make up for it by any hydration process. Hydration process for a marathon should start 48 to 72 hours before the race in such a way that the urine is almost always kept clear.
3. Hypoglycemia. This happens often among less trained eager runners whose fat burning mechanism cannot keep up with their ambitious pace. The faster the pace, the more contribution of energy is from carbohydrate and less from fat. Less trained runners' carbohydrate/fat burning ratio is much higher than well trained runners. The symptoms are dizziness, light-headedness and intense craving for sweet. Consistent training will significantly elevate fat metabolic rate during running and increase the glycogen store capacity so the glycogen supply can last longer. This, in addition to proper carbo-load, can completely eliminate the wall of hypoglycemia. Since the fat has much higher energy density than the other two types of fuel - carbohydrate and protein, any healthy person has enough fat to burn for almost any distance. There is no need to do fat-load although there is a controversial study showing that fat-loading did help a group of people to enhance their racing performance.

(Originally written on 10/27/02)