Muscles of the back are organized into superficial, intermediate, and deep groups.
Muscles in the superficial and intermediate groups are extrinsic muscles because they originate embryologically from locations other than the back. They are innervated by anterior rami of spinal nerves:
The superficial group consists of muscles related to and involved in movements of the upper limb.
The intermediate group consists of muscles attached to the ribs and may serve a respiratory function.