The anatomy of latissimus dorsi seems simple but ends up attaching to more bones in the back than almost any other muscle. It connects the mid-back, low-back, and pelvis to the humerus through the thoracic vertebrae, clavicle, and scapula.
The latissimus dorsi is a broad, flat muscle covering the mid-back. It is named for its size and location. It has 3 compartments and twists nearly 180 degrees around the teres major. The distal end of latissimus dorsi is often in the same compartment as teres major. It is usually innervated in six separate sections by the thoracodorsal nerve. It originates along the axial skeleton and attaches to the humerus, making it an extrinsic back muscle.
It has statistically significant variations in its attachments. It may cross over the bicipital groove and blend with the attachment of pectoralis major. It may also blend with the fibers of triceps brachii. The lateral fibers may extend all the way down to the ilium. A little more than 40% of the time, it attaches to the scapula’s inferior angle. A little less than 40% of the time, it attaches via soft connective tissue, and about 20% of the time, there is no attachment to the scapula at all.
Latissimus dorsi usually attaches to the inferior angle of the scapula, either directly or through fascial slips.
Origin – through the lumbar aponeurosis, it attaches to T7-T12, sacrum, and ilium. It also attaches to the last 3-4 ribs along the lateral border of this muscle.
Insertion – the lateral lip of the bicipital groove, usually fused with the tendon of teres major.
Function – it is consistently credited with extension, adduction, and assisting internal rotation of the scapula. Also, It depresses the scapula and extends the thoracic spine. This influences neck and pelvic posture.
This transparent view shows the complexity of its attachments. You can see how the scapula and lower 4 ribs get bound between the humerus and spine.
Latissimus dorsi attaches to most of the bones of the axial skeleton below T6, except for a few ribs. It attaches to the spine and pelvis through the thoracolumbar fascia, which originates along the iliac crest, sacrum, lumbar vertebrae, and the last 7 Thoracic vertebrae. The muscle fibers of latissimus dorsi usually stop short of the iliac crest and lumbar vertebrae.
This muscle’s superior border attaches to the scapula’s inferior border directly or by a slip of connective tissue about 75% of the time. It twists around the teres major and often shares the same compartment as it passes along the axilla. It inserts along the medial lip and floor of the bicipital groove in a tendon that usually fuses with the teres major’s fibers.
The latissimus dorsi depresses the humerus and shoulder girdle. Because of the mobility of the glenohumeral joint, this can make naming its functions complex. It is largely dependant on the humerus’ starting position.
When in front of the body, as when chopping wood, it extends the humerus. When the humerus is behind the body, as when dipping on bars, latissimus dorsi flexes the humerus. When the arm is abducted laterally, as when dribbling a basketball out to the side, it is adducting the arm. It even has some ability to assist in internal rotation.
The lateral compartment is more involved and developed by movements that pull the down from overhead. The transverse compartment is more involved in movements that pull the humerus and shoulder girdle posteriorly, as when rowing.
Its broad attachment to the mid and low make it suited to support the body weight when hanging by the upper extremity. The biceps slips between the lats and pecs offering a perfect structure for swinging from a tree branch.
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