Originally Posted by terrymorse

The term "bonk" gets thrown around very liberally and means different things to different people. I've heard people talk about "muscle bonk" and "brain bonk" separately, referring to two different types of glycogen depletion.

For the endurance athlete, there are two different types of glycogen depletion:

1. Muscle depletion. The glycogen stores in your muscles run out, so they must rely on fuel carried into the muscle from the bloodstream. The athlete still feels fine, but it is very difficult to work beyond about 75% of max. Legs feel like they "just won't go". I get this all the time on long training rides. In marathons, this depletion is called "hitting the wall".

2. Liver depletion. This is the more serious one, where blood sugar levels starts to drop rapidly. I had this only once on the bike, and it came upon me quickly. I felt disoriented, had trouble forming sentences, and simply couldn't continue on the bike. I had forgotten to eat. I sat on a rock, took in some carbs in liquid form, and felt fine after 15 minutes.