 The brachioradialis, flexor of the elbow, is unusual in that it is located in the posterior compartment, but it is actually a muscle of flexor / anterior compartment of the forearm.The anconeus, assisting in extension of the elbow joint, is by some considered part of the posterior compartment of the arm.The majority of muscles found in the posterior compartment are extrinsic, meaning its origin has some distance from the part that it moves.