The link between decreased cAMP, increased glycogen and weaker muscle contraction?

Basically going through my clinical pharmacy and therapeutics module for the exams in january (about two weeks away, kill me now!).

Im trying to work out why a GPCR G(alpha-inhib) subunit causes a decrease in muscle contraction. From my lecture slides so far I have:

Gi causes decreased cAMP, which decreases the amount of adrenaline binding to Protein Kinase A (PKA) (but how? does it decrease the release of adrenaline?). This decreases the amount of glycogen being converted to glucose (I understand that) and so decreases muscle contraction.

It's this last bit Im confused by - how does increased glycogen (and so decreased glucose) cause decreased muscle contraction? Apologies if Im missing something really obvious...