The expression "hitting the wall" is used to describe sensations of fatigue and discomfort in the active muscles associated with severe _

This is the lack of glycogen being produced in the muscles and the liver. A person exercising has an amount of glycogen stored in these locations, and it is depleted while being used as a source of energy. When the person runs low (or out) of this sugar, the body has ways of making more through fat breakdown, but the person will begin to feel severe discomfort and lack of energy in the meantime until this energy is created.

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