Increase in cAMP releases glycogen from the muscle due to:

In muscle, the role of glycogen is to provide a source of glucose-6-phosphate for glycolysis in response to the need for ATP for muscle contraction. The enzyme is activated by phosphorylation catalyzed by phosphorylase kinase (to yield phosphorylase a) and inactivated by dephosphorylation catalyzed by phosphoprotein phosphatase (to yield phosphorylase b), in response to hormonal and other signals. Glucagon (in liver) and epinephrine (In liver and muscle) Phosphorylate glycogen phosphorylase and Glycogen Synthase. Glycogen Phosphorylase active in the phosphorylated state. Glycogen Synthase inactive in the phosphorylated state. So, Glycogen is degraded. Harper 30th edition pg -179-182