Hitting the Wall: Simply put, hitting the wall (aka bonking) means you've run out of energy, usually starting at about the 20 mile mark. The medical term is hypoglycemia. Runner's World provides the best definition I've come across; hitting the wall means the "...collapse of the entire system; body and form, brain and soul." That's about it, no biggy, just the collapse of the entire friggin system!
If you've been serious about carbohydrate loading the week leading up to the marathon, you've probably managed to tuck away about 2000 calories of glycogen in the liver and muscles. Depending on pace, heat, humidity, wind, and other obstacles this is enough to get you about 20 miles into the race. If the glycogen is not replenished it's ... hello wall, think I'll just lay down a spell... At this point it's game over, you simply cannot recover from the wall, you're done like dinner.
I won't bore you with grade 10 biology (Jacque will fall asleep again) but suffice to say glycogen comes from carbohydrates. Glycogen is stored in the liver and converted to glucose as it enters the blood stream. It's the glucose that feeds the muscles, about 10 grams an hour at rest, about 60 grams an hour during a marathon! When you hit the wall you have low (hypo) blood glucose (glycemia). When your glycogen tank is empty your body attempts to consume fatty acids which eats oxygen and makes the heart work harder to carry oxygen-laden blood to the muscles. At this point your blood is a whole lot thicker (from dehydration) so your heart has to work that much harder (think pumping water vs pumping molasses). As if that wasn't enough... the brain also begins to shut down without a constant source of glycogen! One of the first signs that the wall is approaching is an overwhelming desire to stop running. In extreme cases runners will hallucinate, stumble, and imagine all sorts of nonsense. The trick is, obviously, replenish the glycogen as you run. Gels, sports drink, whatever it takes. Stay tuned for another scintillating ;>) blog entry.... only 5 more get-ups.
Rain rain go away!