The latissimus dorsi muscle medially rotates, adducts, extends, and also depresses the shoulder girdle (not the humeral head).  The range of shoulder flexion/elevation is limited when the latissimus dorsi is short.  When acting bilaterally, the latissimus dorsi extends the spine and anteriorly rotates the hips.  If the abdominals are weak, shoulder flexion is compensated for with lumbar spine extension. 

Actions

• Extends the shoulder (glenohumeral joint)
• Adducts the shoulder (g/h joint)
• Medially rotates the shoulder (g/h joint)

Origin

• Spinous processes of the last 6 thoracic vertebrae
• Last 3 or 4 ribs
• Thoracolumbar aponeurosis
• Posterior iliac crest

Insertion 

• Groove of the humerus

Nerve Innervation

• Brachial plexus