 The brachioradialis and the anconeus are considered intrinsic muscles because they both arise within the forearm and they both move the forearm.Extensor tendons pass through the extensor retinaculum at wrist joint in 6 synovial sheaths, also referred to compartments.The supinator and the anconeus are the two muscles in the posterior compartment of the forearm that do not pass through wrist extensor compartments.