The carbohydrate stored in the muscles and liver that provides quick energy during exercise is glycogen. It is a highly branched storage form of glucose that can be broken down rapidly when the body needs fuel.
Muscle glycogen is mainly used by the muscle that stores it. During intense activity, enzymes split glycogen into glucose units, which enter energy-producing pathways and help generate ATP for contraction. This makes glycogen especially important during sprinting, weightlifting and other high-intensity work.
Liver glycogen serves a different role. The liver can release glucose into the bloodstream, helping maintain blood-glucose levels between meals and during exercise. Muscle cells generally cannot export their stored glycogen in the same way, so their reserves are primarily local fuel.
Glycogen is not the same as glucose: glucose is the immediately usable sugar, while glycogen is its compact storage form. It is also different from insulin, a hormone that helps regulate glucose, and from keratin, a structural protein. A prolonged or very intense workout can reduce glycogen stores, contributing to fatigue or the familiar “hitting the wall” feeling.