After running about 20 miles, racked with total body fatigue, unsteadiness and possible light-headedness, a marathon runner hits an invisible wall, an apparently insurmountable physiological barrier which stops them in their tracks. What's happened?

The Marathon

The marathon is a 26-mile, 385-yard-long (42.195km), road-running race. As of December 2002, the world record is held by Khalid Khannouchi who completed the London Marathon on 14 April, 2002, in a time of 2hrs, 5mins, 38 seconds. Good club distance runners should be expected to complete a marathon in less than three hours. Novices and novelty runners will aim for 3-5 hours and beyond. 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 where they will, to some degree or another, hit and try to breach the 'wall'.

The Wall

Glycogen is a substance derived from carbohydrates in our diet, stored in the muscles and liver. It is the muscle's primary fuel, as it is easily converted into blood sugar. Typically an athlete can store up to 7500kJ of glycogen in the liver and muscles. If the runner's average energy consumption is about 400 kJ/mile, the glycogen supply will suffice for only around 20 miles - six miles short of the finishing 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 when hypoglycaemia ensues. At this point the body, having run out of fuel, starts using fat reserves as a fuel source, much to the detriment of a runner's performance.

While a good well-habituated long distance runner will just feel temporarily out of steam, inexperienced runners may also suffer from additional physiological problems of wall-hitting including all-over body muscle cramps (as a result of lactic acid buildup) and dehydration.

Over the Wall

To runners, overcoming the wall has a fascination second only to their PBs¹ and the colour of their urine. Firstly, a runner preparing to run in a marathon should train their body to accept the sudden change in fuel from glycogen to fat by doing a number of long training runs into this fuel change zone. The body will then become habituated to making the switch from main to auxiliary fuel supply and so hitting the wall will be less of a shock and breaching it will become less of a burden.

Secondly, a runner needs to be prepared. Carbo-loading, ie, 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. 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 aptitude.

Finally, it is essential during the race itself to 'take fluids on board²', that is, to remain hydrated.

Chequered Flag

Once a runner is 'over the wall' the worst is over, and providing other ill-effects like blisters or cramps are avoided, it should be just a coast to the finishing line.