<P> The proximal aspect of the superficial head is known as the arcade of Frohse or the supinator arch . </P> <P> It is innervated by the deep branch of the radial nerve . The deep branch then becomes the posterior interosseous nerve upon exiting the supinator muscle </P> <P> The radial nerve divides into deep and sensory superficial branches just proximal to the supinator muscle--an arrangement that can lead to entrapment and compression of the deep part, potentially resulting in selective paralysis of the muscles served by this nerve (the extensor muscles and the abductor pollicis longus .) Many possible causes are known for this nerve syndrome, known as supinator entrapment syndrome, including compression by various soft - tissued masses surrounding the nerve, and stress caused by repetitive supination and pronation . </P> <P> The deep radial nerve passes through the belly of supinator in 70% of cases and via the arcade of Frohse in remaining cases . </P>

Which of the following is the primary muscle of supination