Muscles of the posterior forearm. All the labelled muscles (that is, all the visible muscles except the ones on the dorsal hand and one at top left) are innervated by the radial nerve, and represent all muscles innervated by the radial nerve except for the supinator.ith the lateral and medial heads of the triceps innervated, the radial nerve emerges from the radial groove on the lateral aspect of the humerus. At this point, it pierces the lateral intermuscular septum and enters the anterior compartment of the arm.