A separate supernumerary muscle in the lateral cubital fossa originating from the humerus or brachioradialis and inserting into the radius, pronator teres or supinator muscle has been considered as a variation of the brachioradialis muscle (Dawson, 1822; Meckel, 1823; Lauth, 1830; Halbertsma, 1864; Gruber, 1868b; Testut, 1884; LeDouble, 1897; Spinner & Spinner, 1996). However, a similar description was used to report additional heads of the brachialis or biceps brachii muscles (Gruber, 1848; Wood, 1864, 1868; Macalister, 1864–66, 1966–69, 1875; Gruber, 1868a; Wolff-Heidegger, 1937).

The innervation of these variant muscles would be a good tool to assign each variation to its associated muscle. Consequently, innervation by the radial nerve would indicate that it is a derivative of the humero–radialis group of muscles, while innervation by the musculocutaneous nerve would support it as a derivative of the anterior musculature of the arm (Rolleston, 1887; Lewis, 1989). However, no references to the innervation were found in the available literature.

Therefore this study set out to establish the phylogenetic origin of the brachioradialis accessorius muscle and, with the help of its innervation, to determine its incidence and unreported detailed morphology.