<P> The deltoid is supplied by the posterior circumflex humeral artery and the deltoid branch of the thoracoacromial artery which branches from the axillary artery . </P> <P> The deltoid is innervated by the axillary nerve . The axillary nerve originates from the anterior rami of the cervical nerves C5 and C6, via the superior trunk, posterior division of the superior trunk, and the posterior cord of the brachial plexus . </P> <P> The axillary nerve is sometimes damaged during surgical procedures of the axilla, such as for breast cancer . It may also be injured by anterior dislocation of the head of the humerus . </P> <P> When all its fibers contract simultaneously, the deltoid is the prime mover of arm abduction along the frontal plane . The arm must be medially rotated for the deltoid to have maximum effect . This makes the deltoid an antagonist muscle of the pectoralis major and latissimus dorsi during arm adduction . </P>

Which of the following is an action of the middle fibers of the deltoid