The radial nerve comes off the posterior cord and passes through the posterior compartment of the arm.

One of the common myths is that it is contact with the humerus in the posterior compartment . In fact it is not. It comes closest to the humeral shaft as it comes through the lateral intermuscular septum to come into the anterior compartment.

The function of the muscles innervated by the radial nerve are to extend the wrist, the fingers at the metacarpophalangeal joint and the thumb. It is important to emphasize that it is metacarpophalangeal joint extension.

Occasionally I will go in and check a patient post-operatively and ask them to extend their fingers and they don't move their metacarpophalangeal joint. Instead, they extend their interphalangeal joints. That is of course a function of the (ulnar nerve innervated) intrinsic muscles - so don't be fooled.