The wall (Read 940 times)

It seems like a large % of marathoners hit THE WALL at 20 miles give or take a couple of miles. I have a question to bikers and ultra runners. Do you hit walls also? Is there a magic number of miles where you bonk? Ewa

Your body contains about 2000 kcal of energy stored as glycogen and another 4000 kcal for EVERY pound of fat you have (e.g., a 150 lb person with just a 5% body fat will still have almost 8 lbs of fat, worth about 32 000 kcal!). Energy expenditure while running is a function of your weight, and to a lesser extent the grade of the road, and to a far far lesser extent to your pace. So an 8 m/m runner is burning energy at about the same rate PER MILE as a 12 m/m runner with the same weight while they are both running. A 150 lb runner will burn approximately 120 kcal per mile run. Your body uses two fuel sources to run. One is glycogen. Glycogen is the primary energy source used for fight or flight type activites, which means it is the primary energy source used when running. When you run above 80-90% of your maximum effort (or VO2max, or Maximum HR), your body is burning almost entirely glycogen to fuel the effort. Below that, your body starts using the other energy source, which is fat. An innacurate but useful rule of thumb is that your body fuels its effort using glycogen as a percentage of total calories used that is equivalent to your perent effort. So if you are running at a 70% effort, about 70% of the kcal you are using to fuel the effort are coming from glycogen, and the rest come from fat. The reason you bonk is that you run out of glycogen. If you weigh 150 lbs and are running 80% effort, you will use about 2000 kcal worth of glycogen in about 21 miles. If you are running at a 70% effort, it will take you 24 miles to use 2000 kcal worth of glycogen. So why do you bonk at mile 18? Well, even if you carb load absolutely perfectly (and most of us do not), when you finish loading, you then go to bed and sleep. When you wake up marathon morning, your body has used up as much as 25-30% of your glycogen just keeping you alive overnight. And the next morning, the little bit you are able to force down into your stomach, well it does not ever get a chance to be stored as glycogen. Calories on the course? Sure, there are two general options. There are sports drinks, which deliver 4-10 kcal per cup, depending on how dilute the mix and how full the cup. And there are gu or jelly bean packets, which deliver about 100 kcal per packet, provided you can get every last bit. It takes about 1 1/2 packets of gu or about 15 cups of sports drink to fuel each additional mile (i.e., spare your body's need to burn glycogen) When you bonk, you slow down. When you slow down, your body preferentially burns fat. That is how you can finish, even after bonking. So, putting it all together, assuming that you weigh 150 lbs (thereby burning 120 kcal per mile), that you are running your marathon at 75% effort, and that you are able to store 2000 kcal, but that you also slept during the night and burned 25% of those calories, but that you take enough gu and sports drink to get 2 extra miles: ((2 000 kcal glycogen * (1 - 0.25 burned last night)) / (120 kcal per mile * .75 effort)) + 2 miles from carbs on the course = 18.6 miles You will bonk at mile 18.6. Or so. It is never quite this predictable. You can also attenuate this by long-term training (which increases your total body glycogen storage abilities and improves your fat burning at high exertion over time).

Thanks. Something that is related: The human body can contain a maximum of about 2000 calories of stored glycogen. Glycogen is the energy source we use when we run. Your body stores glycogen in the muscles and in the liver. Most of us burn about 110-140 calories for every mile we run, which means we will run out of glycogen after about 16-18 miles. After you run out of glycogen, your body will force you to stop as you transition from burning glycogen to burning protein (i.e., your leg muscles). Once your body has transitioned, you can run again. In part, it is because of this protein burning that your legs hurt for days afterwards and you require recovery before you can run again (that and the lactic acid buildup, etc). It is for this reason that many runners can go 16 miles casually, but require planning and extra effort to go beyond 20. Taking in calories during the run can extend this by preserving some of the (liver) glycogen. A packet of gu-stuff has about 100 calories (worth just under a mile of running) and a 4 ounce cup of gatorade has about 8 calories (about 50-100 yards). Even if you take in 8-10 gu packets during your run (yech, blech, eeeww, uch) you will not necessarily absorb all those calories efficiently, so you still are likely to run out. In general, the more you run in your life, the more efficient you get at burning glycogen. People who have run distance for years, and who put in 40-60+ miles per week use and replace glycogen more efficiently than folks who have only been running for a short time and who can only put in 20-30 miles per week or less. The more efficiently you burn glycogen, the less quickly you use it up, and the further you can go. Your body simply cannot store more than about 2000. Once you got to 2000 (or whatever YOUR body can hold) you will not store more. You burn glycogen all the time, whether running or not. If you were to sit at your desk all day doing nothing but surfing this board, you would still burn about 1500-1800 calories / 24 hours (although you probably burn a little more than that just by having muscles trained to run a marathon), and so all you are doing is repleting that when you carb load up every next day. When you wake up the morning of a long run or a marathon, you have already lost several hundred calories from your glycogen stores just by breathing and sleeping the night before. More if you were nervous. That is why your breakfast really counts as your carb loading unless done right before the run. If you finished dinner and went to sleep with 2000 calories in the tank, by time you woke up the next morning, you were probably down to 1400-1600. If you ate the oatmeal at 5am, you were probably close to 2000 by time the gun fired, but then you only took in about 600 during the run. If you ate after 6am, then you probably had a store of about 1500 calories when the gun fired, and the oatmeal would count in the 950. See Tim Noakes, the Lore of Running. Noakes has a long discussion in his book about energy stores and how we use them. I forget the exact chapter. Among other things, he writes: "The marathon is less a physical event than a spiritual encounter. In infinite wisdom, God built into us a 32 km racing limit, a limit imposed by inadequate sources of the marathoner's prime racing fuel -- carbohydrates. But we, in our infinite wisdom, decreed that the standard marathon be raced over 42 km...So it is in that physical no-man's-land, which begins after the 32 km mark, that is the irresistible appeal of the marathon lies. It is at this stage, as the limits to human running endurance are approached, that the marathon ceases to be a physical event...It is there that you learn something about yourself and your view of life." (Noakes, Lore of Running, p596)

My "wall" story: http://www.runningahead.com/forums/topic/96845d24283e478496b872ed73cce512