Hitting the wall is a fear shared by all distance runners on race day. But glycogen depletion isn't the only cause of fatigue during long-distance events. If you're not in shape—physically and mentally—to endure the marathon distance, you should expect to hit some rough patches during the race. Here's how to train to beat marathon fatigue (warning: It's going to take added effort).
Why Runners Fatigue
Fatigue is not something specific to slow or average runners. Even world record holders fatigue; they just do it at a much faster pace than the rest of us. From a physiologist's perspective, fatigue is the inability to maintain or repeat a given level of muscle force production, resulting in an acute impairment of performance.
Indeed, fatigue is necessary to protect our bodies from damage. However, the only way to get faster is to cause some damage so fatigue occurs at a faster pace. To do that, you must repeatedly threaten the body's survival with training stimuli so that your body adapts and physiologically overcompensates. When the same stress is encountered again, it doesn't cause the same degree of physiological disruption—in other words, your body adapts to handle the threat.
Below are the main factors involved in marathon fatigue.
Limitations in Aerobic Metabolism
To provide energy for muscle contraction, a high-energy chemical compound called adenosine triphosphate (ATP) is broken down into its constituents: adenosine diphosphate (ADP) and inorganic phosphate (Pi). Since our muscles don't store much ATP, we must constantly regenerate it. The formation and regeneration of ATP is thus a circular process—ATP is broken down into ADP and Pi, and then ADP and Pi combine to regenerate ATP. Since the marathon is almost purely aerobic, limitations in the aerobic regeneration of ATP—due to inadequate blood flow to and oxygen use by the muscles—contribute to fatigue.
Endurance performance is strongly influenced by the amount of carbohydrates stored in skeletal muscles, also known as glycogen. Fatigue coincides with glycogen depletion. You have enough stored glycogen to last slightly more than two hours of sustained running at a moderate intensity. So, unless you plan on running the marathon as fast as Ryan Hall, you're going to run out of fuel. Glycogen depletion and the accompanying low blood sugar (hypoglycemia) coincide with hitting the infamous wall.