What Actually Happens When Runners 'Hit the Wall'?
Marathon runners hit 'The Wall' when their bodies exhaust glycogen, the primary fuel source for sustained high-intensity exercise.
Your body stores sugar as glycogen in your muscles and liver to fuel intense activity. Most runners have enough glycogen reserves to last about 32 kilometers (20 miles). Once depleted, your body must switch to burning fat for energy—a much slower, less efficient process. This metabolic shift causes sudden, overwhelming fatigue and a sensation of heavy, unresponsive limbs.
Nerd Mode
'The Wall' typically strikes between kilometers 29 and 35 (miles 18 to 22) of a marathon. At this point, the body's glycogen stores—averaging about 2,000 calories in a healthy adult—become severely depleted. Glycogen is a complex carbohydrate made of glucose molecules that animals and fungi use to store energy.The liver holds roughly 100 grams of glycogen, while muscles store approximately 400 grams. When these reserves drop critically low, the brain signals extreme fatigue as a protective mechanism to prevent tissue damage. This metabolic transition forces the body to rely on beta-oxidation—the breakdown of fatty acids for energy. Fat metabolism demands significantly more oxygen and is far less efficient than carbohydrate metabolism.Research published in PLOS Computational Biology by Benjamin Rapoport demonstrates that avoiding the wall is mathematically linked to aerobic capacity and leg muscle mass. To combat this, endurance athletes employ carbohydrate-loading strategies during the 48 hours before a race. During the event itself, they consume 30 to 60 grams of carbohydrates per hour via gels or sports drinks to maintain stable blood glucose levels.
Verified Fact
FP-0002854 · Feb 17, 2026