The latissimus dorsi (also known as “the lats”) is one of the widest muscles in the human body and it belongs to the superficial back muscles. It is a large, flat triangular muscle that runs between the trunk and the humerus and thus provides movements at the shoulder joint. Together with the teres major, this muscle participates in forming the posterior axillary fold in the armpit where it is easily palpable. Besides moving the arm, the latissimus dorsi also can move the trunk and functions as an accessory muscle of respiration.

• Origin:
• Spinous processes of seventh thoracic to fifth lumbar vertebrae (T7 - L5)
• Iliac crest of the sacrum
• Thoracolumbar fascia
• Inferior angle of the scapula
• Lower three or four ribs

• Insertion: intertubercular sulcus of the humerus, between the pectoralis major and teres major muscles. 
• Action: by acting on the shoulder joint, the latissimus dorsi provides internal rotation, adduction, and extension of the arm. When the humerus is fixed, this muscle may participate in trunk movements: it may elevate the trunk (important during climbing and pull-ups) or move the trunk anteriorly (useful, for example, in cross-country skiing). It also supports other muscles of the back during lateral flexion of the lumbar spine.It is also one of the accessory respiratory muscles as contractions of the latissimus dorsi on both sides of the body compresses the rib cage, therefore, facilitating expiration. 
• Innervation: thoracodorsal nerve (C6-C8), which is a branch of the brachial plexus.