This is the term for glucose in storage form.  Glucose is a high powered fuel source for the body.  Free fatty acids are a body's standard fuel, while glycogen is its high powered fuel.  When the intensity of an exercise or race gets so great that fat can no longer meet the needs of the body, it taps into its glycogen reserves for the extra boost.  The glycogen that is stored in the liver helps to keep blood glucose levels normal.   The glycogen reserves stored in the muscles are utilized directly by those very muscles only when those muscles are doing intense activity.

It is important to know how to maximize your storage of glucose and how to train your body to effectively use it and replenish it.