The latissimus dorsi (plural: latissimi dorsi) 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.
Actions
The latissimus dorsi is responsible for extension, adduction, transverse extension also known as horizontal abduction, flexion from an extended position, and internal rotation of the shoulder joint. It also has a synergistic role in extension and lateral flexion of the lumbar spine.
Variations
The number of dorsal vertebræ 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.
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.
Triangles
- The lateral margin of the latissimus dorsi is separated below from the obliquus externus abdominis by a small triangular interval, the lumbar triangle of Petit , the base of which is formed by the iliac crest, and its floor by the obliquus internus abdominis.
- 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 .
Nerves
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.
Training
To increase the power of this muscle (commonly known as the V-taper to bodybuilders), the muscle can be trained with the following exercises:
- pull-down (compound)
- bent-over row (compound)
- chin-up and pull-up (compound)
- pull-over (single-joint)
- dumbbell-rows (compound)
- T-bar row (compound)
Weight training must be carefully controlled due to the importance of the muscle. Instead, swimming is a safe method of working the lats.
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. Because the latissimus dorsi connects the spine to the humerus, tightness in this muscle can manifest as either sub-optimal glenohumeral joint function (which leads to chronic shoulder pain) or tendinitis in the tendinous fasciae connecting the latissimus dorsi to the thoracic and lumbar spine.
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.
References
- ^ 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.]
Additional images
External links
- LUC lat
- latissimus dorsi at GPnotebook
- SUNY Figs 01:03-08 - "Superficial layer of the extrinsic muscles of the back."
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