Latissimus dorsi muscle
The latissimus dorsi muscle ('the lats') is the widest muscle in the human body. It is relatively thin and covers almost all back muscles at the posterior trunk (except the trapezius). Together with the teres major it forms the posterior axillary fold in the armpit where it is the thickest and therefore easy to palpate.
Origins & Insertions
Depending on the origin, one can distinguish four parts:
- Vertebral part: originating from the spinous processes of the 7th to 12th thoracic vertebrae and the thoracolumbar fascia
- Iliacal part: starting from the iliac crest
- Costal part: having origins from the 9th to 12th ribs
- Scapular part: starting from the inferior angle of the scapula (inconstant)
All fibers together attach to the floor of the intertubercular sulcus of the humerus between the pectoralis major and teres major. During its course, the fibers turn spirally around the teres major (torsion) so that the lower part of the latissimus dorsi inserts proximally at the humerus and the upper part more distally.
The innervation is carried by the thoracodorsal nerve (C6-C8), a branch of the brachial plexus.
Upper Limb Movements
The latissimus dorsi supports various different movements of the shoulder. Due to the crossing over of the muscle fibers at the insertion point, the muscle's contraction leads to an inward rotation of the humerus. Moreover it pulls the humerus towards the trunk (adduction) and behind (retroversion). This motion is most powerful in a previously elevated arm whereby the torsion is relieved and the muscle maximally stretched (e.g. wood chopping). Owing to the three movements at the shoulder joint, the latissimus dorsi is referred to as one of the swimming muscles of the human body.
When the humerus is fixed, the contraction of the latissimus dorsi leads to two different movements of the trunk. On one hand, it is elevated which is particularly effective during climbing and pull-ups. On the other hand, the trunk is moved anteriorly, e.g. useful during cross-country skiing.
Accessory Muscle of Respiration
A third important function of the muscle is support during breathing. The activation of the latissimus dorsi muscle on both sides causes a compression of the ribcage, facilitating expiration (accessory muscle of expiration). For this reason, the muscle is particularly strained during coughing attacks (“coughing muscle”).
Damages of the brachial plexus may paralyze the thoracodorsal nerve and thus affect the latissimus dorsi. In comparison, an isolated lesion of the nerve (e.g. by trauma) occurs very rarely. Clinical signs are the missing of a muscular relief at the back and the posterior axillary fold. In everyday life, the paralysis of the muscle may cause a light weakness in the shoulder, however usually the deficit is well compensated by the remaining shoulder musculature.
The latissimus dorsi is often used for surgical reconstruction of the anterior trunk, e.g. after mastectomy or larger injuries of the breast. Hereby it is removed from its' origin and implanted at the anterior trunk wall together with skin, subcutaneous fat tissue and an artificial implant (if necessary). In the process, the muscle gets rotated around the neurovascular bundle in the posterior axilla, and due to that the muscle is still well nutritioned despite its' transfer. This so-called pedicled latissimus dorsi musculocutaneous flap is one of the most common methods in breast reconstruction worldwide.