The latissimus dorsi (plural: latissimi dorsi), meaning 'broadest muscle of the back' (Latin latus meaning 'broad', latissimus meaning 'broadest' and dorsum meaning the back), is the larger, flat, dorso-lateral muscle on the trunk, posterior to the arm, and partly covered by the trapezius on its median dorsal region.
The latissimus dorsi is responsible for extension, adduction, transverse extension also known as horizontal abduction, flexion from an extended position, and (medial) internal rotation of the shoulder joint. It also has a synergistic role in extension and lateral flexion of the lumbar spine.
Due to bypassing the scapulothoracic joint and attaching directly to the spine, the actions the lat has on moving the arm can also influence the movement of the scapula, such as their downward rotation during a pull up.
[edit] Variations
The number of dorsal vertebra to which it is attached vary from four to eight; the number of costal attachments varies; muscle fibers may or may not reach the crest of the ilium.
A muscular slip, the axillary arch, varying from 7 to 10 cm in length, and from 5 to 15 mm in breadth, occasionally springs from the upper edge of the latissimus dorsi about the middle of the posterior fold of the axilla, and crosses the axilla in front of the axillary vessels and nerves, to join the under surface of the tendon of the pectoralis major, the coracobrachialis, or the fascia over the biceps brachii. This axillary arch crosses the axillary artery, just above the spot usually selected for the application of a ligature, and may mislead a surgeon. It is present in about 7% os and may be easily recognized by the transverse direction of its fibers. Guy et. al. extensively described this muscular variant using MRI data and positively correlated its presence with symptoms of neurological impingement.[1]
A fibrous slip usually passes from the upper border of the tendon of the Latissimus dorsi, near its insertion, to the long head of the triceps brachii. This is occasionally muscular, and is the representative of the dorsoepitrochlearis brachii of apes.
[edit] Triangles
- Another triangle is situated behind the scapula. It is bounded above by the trapezius, below by the latissimus dorsi, and laterally by the vertebral border of the scapula; the floor is partly formed by the rhomboideus major. If the scapula is drawn forward by folding the arms across the chest, and the trunk bent forward, parts of the sixth and seventh ribs and the interspace between them become subcutaneous and available for auscultation. The space is therefore known as the triangle of auscultation.
- The Latissimus Dorsi can be remembered best for insertion as "The Lady Between Two Majors". As the Latissimus Dorsi inserts into the floor of the intertubercular groove of the humerus it is surrounded by two major muscles. The Teres Major inserts medially on the medial lip of the intertubercular groove and laterally the Pectoralis Major inserts into the lateral lip.
[edit] Innervation
The Latissimus dorsi is supplied by the sixth, seventh, and eighth cervical nerves through the Thoracodorsal (long scapular) (long subscapular) nerve. Electromyography suggests that it consists of six groups of muscle fibres that can be independently coordinated by the central nervous system.[2]
[edit] Training
The power/size/strength of this muscle can be trained with a variety of different exercises. Some of these include:
Lifting under control can help reduce chances of injury.
[edit] Synergists
Most latissimus dorsi exercises concurrently recruit the teres major, posterior fibers of the deltoid, long head of the triceps brachii, among numerous other stabilizing muscles. Compound exercises for the 'lats' typically involve elbow flexion and tend to recruit the biceps brachii, brachialis, and brachioradialis for this function. Depending on the line of pull, the trapezius muscles can be recruited as well; horizontal pulling motions such as rows recruit both latissimus dorsi and trapezius heavily.
Tight latissimus dorsi has been shown to be one cause of chronic shoulder pain and chronic back pain.[3] Because the latissimus dorsi connects the spine to the humerus, tightness in this muscle can manifest as either sub-optimal glenohumeral joint (shoulder) function which leads to chronic pain or tendinitis in the tendinous fasciae connecting the latissimus dorsi to the thoracic and lumbar spine.[4]
[edit] Cardiac support
For heart patients with low cardiac output and who are not candidates for cardiac transplantation, a procedure called cardiomyoplasty may support the failing heart. This procedure involves wrapping the latissimus dorsi muscles around the heart and electrostimulating them in synchrony with ventricular systole.
[edit] References
- ^ Guy MS, Sandhu SK, Gowdy JM, Cartier CC, Adams JH. MRI of the axillary arch muscle: prevalence, anatomic relations, and potential consequences. AJR Am J Roentgenol. 2011 Jan;196(1):W52-7.
- ^ Brown JM, Wickham JB, McAndrew DJ, Huang XF. (2007). Muscles within muscles: Coordination of 19 muscle segments within three shoulder muscles during isometric motor tasks. J Electromyogr Kinesiol. 17(1):57-73. PMID 16458022 doi:10.1016/j.jelekin.2005.10.007
- ^ Arnheim, D.D., Prentice, W.E., Principles of athletic training. 9th ed. McGraw Hill, pp 570-574, 1997.
- ^ Francis, P., Applied anatomy and kinesiology, supplemental materials. KB Books., p 19-25, 1999.
[edit] Additional images
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Left humerus. Anterior view.
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Right hip bone. External surface.
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Deep muscles of the chest and front of the arm, with the boundaries of the axilla.
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Muscles on the dorsum of the scapula, and the Triceps brachii.
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The axillary artery and its branches.
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The veins of the right axilla, viewed from in front.
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The right brachial plexus (infraclavicular portion) in the axillary fossa; viewed from below and in front.
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Surface anatomy of the back.
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The left side of the thorax.
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[edit] External links
