DELTOID : flexes and medially rotates arm; prime mover of arm abduction; extends and laterally rotates arm. LATISSIMUS DORI : prime mover of arm extension; adducts and medially rotates arm (big muscle). PECTORALIS MAJOR : prime mover of arm flexion, adducts and medially rotates arm ( big muscle). TERES MAJOR : Extends, adducts and medially rotates arm. Solution DELTOID : flexes and medially rotates arm; prime mover of arm abduction; extends and laterally rotates arm. LATISSIMUS DORI : prime mover of arm extension; adducts and medially rotates arm (big muscle). PECTORALIS MAJOR : prime mover of arm flexion, adducts and medially rotates arm ( big muscle). TERES MAJOR : Extends, adducts and medially rotates arm..