The teres major is called this muscle's "little helper" as it's synergist.

Both the teres major and latissimus dorsi adduct, medially rotate and extend the arm. The deltoids actions are abduction, flexion and medial rotation of the arm. The pectoralis major is responsible for flexion, extension, adduction and medial rotation of the arm. The rectus abdominis is responsible for flexion of the vertebral column and abdominal compression.