How glycogen drains during long efforts
Glycogen is the stored form of carbohydrate, held in muscle and liver. During prolonged endurance exercise, muscle glycogen powers contraction locally while liver glycogen helps maintain blood glucose. Both stores deplete over time, and when they run low the body struggles to sustain higher intensities, producing the sharp drop in capacity endurance athletes call hitting the wall or bonking.
The rate of depletion depends heavily on intensity, since harder efforts rely more on carbohydrate, and on how full the stores were at the start.
Why fueling and loading change the curve
Because the wall coincides with depleted glycogen, strategies like pre-event carbohydrate loading raise starting stores, and consuming carbohydrate during long events spares glycogen and supports blood glucose, both pushing the depletion point later. Trained athletes also store more glycogen and use fat more readily at submaximal intensities.
Individual needs vary with event duration, intensity, and body size. This is general educational information about endurance metabolism, not personalized fueling or medical advice.