zombie asked
This happens when your body runs out of carbohydrate fuel (sometimes known as muscle glycogen) and starts burning fat for energy instead. This is a far less efficient way of producing energy and you'll start to chug along in the slow lane instead of motoring at top speed. Sometimes runners get so tired they feel they can't even put one foot in front of another any more. This urge to stop running is known as 'hitting the wall'.
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