Wracked with complete body-fatigue, unsteadiness and perhaps light-headedness, a marathon-runner hits an invisible wall, an apparently insurmountable physiological barrier which stops her in her tracks. What's happened?
The Marathon is a 26 mile 385 yard long (42.195m) road-running race. At December 2002, the World Record is held by Khalid Khannouchi who completed the London Marathon on April 14, 2002 in a time of 2 hours, 5 minutes and 38 seconds. Good club distance-runners should be expect to complete a marathon in less than three hours. Novices and novelty-runners will aim for 3-5 hours and beyond. But this is digressional trivia. The point is that a marathon-run is not a cake-walk; any experienced runner knows that the 20-mile marker is the metaphorical 'half-way point', because that's the point that they will, to some degree or another, hit and try to breach the wall.
Glycogen is the substance derived from carbohydrates in our diet, which is stored in the muscles and liver, and which is the muscle's primary fuel, as it is easily converted to blood-sugar. Typically, an athlete can store up to 7500 kJ of glycogen in liver and muscles. If the runner has an average energy consumption of about 400 kJ/mile, the glycogen supply should suffice for only around 20 miles, six miles short of the finish-tape.
The wall then, in its purest form, is the appropriately-named term used to describe an event which happens to many marathon runners when they have crossed a point in the race where they have no more glycogen reserves, and hypoglycemia ensues. At this point the body, having run out of fuel, swaps over to begin using fat reserves as a fuel source, during which period performance is detrimentally affected.
Whilst a good well-habituated long-distance runner will just feel temporarily out-of-steam, inexperienced runners may also suffer and attribute additional associated physiological problems to wall-hitting including all-over muscle-cramps (as a result of lactic acid build-up) and dehydration.
Over the Wall
To runners, overcoming the wall is a fascination second-only to their PBs1 and the colour of their urine.
In the first instance, a runner preparing to run in a marathon should train one's body to accept the change from glycogen to fat by running a number of long training runs into that zone. The body will become habituated to making the switch from main to auxiliary fuel supply and so hitting the wall will become less of a shock and breaching it will be come less of a burden.
The second is to be prepared. Carbo-loading, i.e., noshing out on a pasta-supper the night before the race is a good way to encourage the storage of glycogen in the liver and muscles, and indeed, some argue that carbo-starving in the days prior to carbo-loading will accrue additional benefits.
Further, some runners argue that carbohydrate gels or drinks should be consumed during the race to keep up glycogen stores. However, others argue that the stomach cannot digest and absorb carbohydrates while under stress. Like so many aspects of sporting ability, it is an issue that depends on personal aptitudes.
Finally, it is essential during the race itself to 'take fluids on board2' to remain hydrated.
The Chequered Flag
Once a runner is 'over the wall', providing other ill-effects like blisters or cramps are avoided, it should be just a coast to the line.