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Latissimus Dorsi

Key Points:
• Latissimus dorsi abducts, extends and internally rotates the shoulder.
• Important muscle to examine because its function is often maintained in even very severe brachial plexus injuries where it can act as a donor nerve for nerve transfer procedures.
Muscle Function:
• Origin Fixed:
• Shoulder abduction, extension, and internal rotation.
• Scapular depression.
• May assist in lateral flexion of the trunk
 Insertion Fixed:
• Assist in tilting the pelvis anteriorly and laterally.
• Acting bilaterally, may assist in hyperextending the spine and anteriorly tilting the pelvis.
• May act as an accessory muscle of respiration.\
• At the posterior axillary fold where the latissimus dorsi inserts and the entire length of the muscle may be palpated.  Ask the patient to cough and you can often feel it in this area.
• To assess and make sure that substitutions (see below) aren’t been made, both sides should be examined simultaneously as the patient coughs.  The muscle can be felt between the examiner’s thumb and fingers bilaterally (examiner stands behind patient) as the patient coughs.  The examiner will feel the normal latissimus dorsi contract and can compare the test side with the contralateral normal side more exactly with this double simultaneous exam technique.
Strength Testing: 
Gravity-lessened Test: Position –prone. The hand of the arm being tested is placed on the buttock on the same side, with the shoulder adducted, medially rotated, and extended with elbow extension. Stabilize – on the table. Resist – on the forearm against adduction and extension with the shoulder in medial rotation without scapular motion. Primary emphasis for resistance is against shoulder adduction. Subject can be asked to reach for his opposite foot.
Anti-gravity Test: Same as the gravity-lessened test except that the primary emphasis for resistance is against shoulder flexion.
Alternate Anti-gravity Test: This muscle may also be tested in sitting with a gross test being a chair pushup from a sitting position. The patient is asked to place their arms on the chair and to push up and out of the chair with their arms supported on the chair. In this test position, the latissimus dorsi is assisted by the triceps brachii and pectoralis muscles. This is considered more of a functional test rather than a specific manual muscle test.  
Possible Substitutions: Lateral abdominals, posterior deltoid, teres major, quadratus labarum, and the elbow flexion using biceps if resistance is given distally.   See palpation details on exam for techniques to assess.
Relevant Anatomy:
• Roots: C6, C7, C8.
• Nerve: Thoracodorsal nerve.
• Innervation Route: C6, C7, C8 → thoracodorsal nerve → latissimus dorsi.
Origin: Spinous processes of inferior six thoracic (T6-T12) vertebrae, last three or four ribs, through the thoracolumbar fascia from lumbar and sacral vertebrae and posterior ⅓ of external lip of iliac crest, and a slip from inferior angle of scapula.
Insertion: Intertubercular groove of humerus.