PART 1: What is "Hitting the Wall" (aka bonking")?
According to Wikipedia: "..... In endurance sports such as cycling and running, hitting the wall or the bonk describes a condition caused by the depletion of glycogen stores in the liver and muscles, which manifests itself by sudden fatigue and loss of energy....."
The sight of a runner "hitting the wall" might seems "funny" to an outsider, but the situation can be medically dangerous to the athlete and may even lead to injury or death. Below is a YouTube video of two Ironman competitors who were suffering from a near shut-down of their bodies whether they wanted it or not.
Bonking will happen to men and women, young and old alike, if the conditions are ripe. The YouTube video below shows a young man "hitting the wall" during a 5 kilometres cross-country running event. So, it doesn't really matter whether you are doing an Ironman-distance triathlon or a 5 km event. When your glycogen runs out, your body will shut down. As shown in the video, the poor chap could no longer stand up and it was only by sheer determination that he crawled the last yards and collapsed in a heap at the finish line.
Even elite Ironman-class tri-athletes, such as Paula Newby Frazier (YouTube video below), could not escape hitting the wall when she had depleted all her glycogen. Her body forced her to come to a stand-still and there was little she could do. At one point she was seen lying on the ground, apparently dashing all hope of finishing the race. But you know what? True to her iron-will and competitive nature, she refused to accept a DNF (did not finish), got up and continued, knowing full well she would most likely lose her chance to win first place.
In Paula Newby Frazier's case, her experience as an elite triathlete probably enabled her do a quick self-assessment and make a decision that you and I would have regretted. Despite her support team's advice to quit, she got up and continued with the race. It is also possible she was experiencing a relatively "mild" bonk that was recoverable (for Paula Newby). Indeed, Wikipedia suggests that: ".... Milder instances can be remedied by brief rest and the ingestion of food or drinks containing carbohydrates...."
Stay tuned for ..... Part 2: How the body fuel itself (Glycogen vs Fat burning) during a marathon.
* Wikipedia / Hitting the Wall