Glycogen is mainly found in the liver and within the skeletal muscles of the human body. The National Center for Biotechnology Information report that the liver has a 10 percent glycogen concentration by weight, compared to a 2 percent concentration in skeletal muscles. However, skeletal muscles contain the bulk of the glycogen in the body due to their overall mass.
Glycogen is a storage form of glucose that the body can easily metabolize to provide required glucose for energy. Substantial quantities of glucose are stored in the form of glycogen, and not fat, for easy conversion to glucose for supply of energy between meals. Glycogen is thus a glucose reserve as well as a means for the human body to maintain balance in blood glucose levels.
The body can readily metabolize glycogen, even in the absence of oxygen, which makes glycogen an important source of energy during strenuous or sudden activities. Glycogen stored in muscles is broken down by muscle cells for use within the muscles, while glycogen stored in the liver is broken down for use in the rest of the body, including the nervous system. In contrast, energy stored in the form of fat is not readily available. The quantities of glycogen stored in muscles, the liver and the red blood cells are dependent on the body's training, its basal metabolic rate and an individual’s nutritional habits.