When glucose is not available, your body will use liver glycogen to manage glucose levels in your blood. That's because only liver glycogen can be delivered to other parts of your body.
On the other hand, muscle glycogen can only be used to fuel the activity of the specific muscle it is stored in.
Also, once the intensity of your exercise reaches a certain threshold, approximately 70% of your VO2 max, glycogen will become its preferred source of energy.