Your muscles hoard sugar they refuse to share with the rest of your body
An adult's muscles hold roughly 400 grams of glycogen, several times what the liver stores. Yet muscle cells lack the enzyme needed to release glucose into the blood, so that fuel stays locked inside for their own use. When it runs out mid-race, runners call the collapse hitting the wall.
Glycogen is a heavily branched chain of glucose units that animals, fungi and bacteria use to store energy. It fills the middle slot among the body's reserves: creatine phosphate for seconds of effort, glycogen for the short term, and fat for the long haul. Structurally it resembles amylopectin in plant starch but is more branched and compact. Each particle is a ball of glucose chains, about a dozen layers deep, grown around a protein called glycogenin, with chains typically 11 to 15 units long. A 1999 proposal that the structure is fractal was later overturned; X-ray scattering showed it is a randomly hyperbranched particle, densest near the centre.
Storage has a clever logic. Loose glucose would pull water into cells by osmosis and could damage them, whereas glycogen is osmotically inert. It is still bulky, stored with three or four parts water per part glycogen. A 1.5 kilogram adult liver holds about 100 to 120 grams, and that reserve feeds the whole body, especially the brain, which takes around 60 percent of blood glucose in a resting, fasting person. After a meal, insulin switches on glycogen synthase; as glucose falls, glucagon triggers breakdown, and for the next 8 to 12 hours liver glycogen supplies most blood sugar.
Muscle glycogen serves the muscle alone. It powers the first minutes of activity, all anaerobic efforts such as weightlifting, and hard aerobic exercise. At maximum intensity it can deliver about 40 millimoles of glucose per kilogram of muscle each minute, versus 4 to 5 from blood glucose, which is why depleting it brings sudden fatigue.
The French physiologist Claude Bernard discovered glycogen, describing by 1857 a sugar-forming substance he isolated from liver. M. A. Sanson soon found it in muscle too, and August Kekule established its empirical formula in 1858.
Source: Glycogen
