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01.10.2012.

Muscles extending the arm at the shoulder joint



Latissimus dorsi
Teres major
Triceps(long head)

Latissimus dorsi

Latissimus dorsi is a large flat triangular sheet of muscle running between the trunk, via an extensive attachment, and the humerus by a narrow tendon. Consequently, it acts on the shoulder joint. The superior surface of the muscle forms the lower border of the triangle of auscultation, while its lateral border forms the medial border of the lumbar triangle.



Latissimus dorsi arises from the posterior layer of the thoracolumbar fascia, which attaches to the spinous processes of the lower six thoracic and all of the lumbar and sacral vertebrae, as well as to the intervening supraspinous and interspinous ligaments.
That part arising from the lower six thoracic vertebrae is covered by trapezius. In addition to this vertebral attachment, latissimus dorsi arises from the posterior part of the outer lip of the iliac crest, most laterally by direct muscular slips. As the muscle fibres sweep upwards and laterally across the lower part of the thorax, they attach to the outer surfaces of the lower three or four ribs and via fascia to the inferior angle of the scapula. From this widespread origin the fibres converge as they pass to the humerus and form a thin flattened tendon. The tendon winds around and adheres to the lower border of teres major, and inserts into the floor of the intertubercular groove anterior to the tendon of teres major, being separated from it by a bursa. The effect of twisting the muscle through 180° means that the anterior surface of the tendon is continuous with the posterior surface of the rest of the muscle. Consequently, the fibres with the lowest origin on the trunk gain the highest attachment on the humerus.

Nerve supply

Latissimus dorsi is supplied by the thoracodorsal nerve, root value C6, 7, 8, which enters the muscle on its deep surface. The skin covering the muscle is supplied by roots T4 to T12 inclusive, by both ventral and dorsal rami, and L1 to L3 by the dorsal rami.

Action

Latissimus dorsi is a strong extensor of the flexed arm; however, if the humerus is fixed relative to the scapula it retracts the pectoral girdle. It is also a strong adductor and medial rotator of the humerus at the shoulder joint.

Functional activity

Functionally, latissimus dorsi is a climbing muscle, and with the arms fixed above the head it can raise the trunk upwards, in conjuction with pectoralis major. Latissimus dorsi has an important function in rowing and during the downstroke in swimming. Attachment of the muscle to the ribs means that it is active in violent expiration, and can be felt pressing forcibly inwards during a cough or sneeze, as it acts to compress the thorax and abdomen.
The attachment to the inferior angle of the scapula allows latissimus dorsi to assist in holding it against the thorax during movements of the upper limb.
If the humerus becomes fixed point when standing, as for example when using crutches, latissimus dorsi is able to pull the trunk forwards relative to the arms; associated with this is a lifting of the pelvis. In patients with paralysis of the lower half of the body, the fact that latissimus dorsi attaches to the pelvis and is still innervated allows it to be used to produce movement of the pelvis and trunk. Consequently, patients wearing calipers and using crutches can produce a modified gait by fixing the arms and hitching the hips by the alternate contraction of each latissimus dorsi.

Palpation

In a lean subject, latissimus dorsi can be made to stand out relative to the thorax by asking the subject to raise his or her arm to 90° flexion and to hold it steady against an upwardly directed pressure. The muscle can be felt contracting if the posterior axillary fold is held between the finger and thumb while the subject coughs. Adduction of the abducted arm against resistance also enables latissimus dorsi to be seen and felt.

Teres major

In the posterior part of the axilla, teres major forms the lower boundary of both the upper triangular and quadrangular spaces. It is a thick, chunky muscle, forming, with latissimus dorsi, the posterior fold of the axilla. It arises from an oval area on the dorsal surface of the scapula near the inferior angle, and from the fascia between it and adjacent muscles. The muscle fibers, which adhere to those of latissimus dorsi, run upwards and laterally to form a broad, flat tendon which attaches along the medial lip of the intertubercular groove. The tendon is separated from that of latissimus dorsi by a bursa, with the latter muscle virtually covering the whole of teres major.



Nerve supply

Teres major is supplied by the lower subscapular nerve, root value C6 and 7.

Action

Teres major adducts and medially rotates the humerus at the shoulder joint. In addition it can help to extend the flexed arm.

Functional activity

Teres major, like latissimus dorsi, is a climbing muscle and works with the latter and pectoralis major to pull the trunk upwards when the arms are fixed. In conjunction with latissimus dorsi and pectoralis major, teres major is important in stabilizing the shoulder joint.

Palpation

Teres major is covered by latissimus dorsi, and as these two muscles have similar actions, considerable care must be exercised when it is tested. The inferior angle of the scapula must first be found, the fingers are then moved upwards and laterally into the posterior wall of the axilla. The subject should abduct the arm to 90° and then adduct against an upwardly directed resistance. The rounded contour of teres major should now be palpable. During this same manoeuvre the flattened tendon of latissimus dorsi, as it twists around teres major, may also be felt.

Triceps brachii

Triceps brachii is situated on the back of the arm and, as suggested by its name, arises by three heads. Two of the heads arise from the humerus, being separated by the spiral groove, and the third comes from the scapula. The three heads are reffered to as: the long head, lateral head and medial head. The muscle attaches via a tendon of the ulna.



Long head. The tendinous long head comes from the infraglenoid tubercle of the scapula and the adjacent glenoid labrum, where it blends with the lower part of the shoulder joint capsule. Named after its length, the long head is also the most medial of the three, the fibres running downwards superficial to the medial head before joining the tendon of insertion. As the long head descends from the infraglenoid tubercle, it passes between teres minor, to which it is anterior, and teres major, to which it is posterior. In its course it forms the medial border of the quadrilateral and lower triangular spaces, and the lateral border of the upper triangular space.

Lateral head. The fleshy lateral head arises above and lateral to the spinal groove on the posterior surface of the humerus between the attachments of teres minor and deltoid. As the fibres pass to join with those of the medial head, they cover the spiral groove.

Medial head. The large, fleshy medial head lies deep of the other two, and arises from the posterior surface of the humerus, below and medial to the spiral groove as far distally as the olecranon fossa. It has an additional attachment to the posterior aspect of the medial and lateral intermuscular septa.

The three heads of triceps come together to form a broad, laminated tendon; the superficial part of which covers the posterior aspect of the lower third of the muscle, while the deeper part arises from within the substance of the muscle. Such an arrangement creates a larger surface area for the attachment of the muscle fibres. Both laminae blend to form a single tendon which attaches to the posterior part of the proximal surface of the olecranon of the ulna, and to the deep fascia of the forearm on either side. Some muscle fibers from the medial head attach to the posterior part of the capsule of the elbow joint and serve to pull it clear of the moving bones and prevent it becoming trapped during extension of the joint.

Nerve supply

All three parts of the muscle are supplied separately by branches from the radial nerve. The branch to the lateral head is derived from C6, 7 and 8, while those to the long and medial heads come from C7 and 8. Of these, the medial head receives two branches, one of which accompanies the ulnar nerve for a considerable distance before entering the distal part of the muscle. The other branch enters more proximally, continuing through the substance of the muscle to end in, and supply, anconeus. The skin over the muscle is supplied by roots C5, 7, T1 and T2.

Action

Triceps brachii is the extensor of the elbow joint. The long head can also adduct the arm and extend it from a flexed position.

Functional activity

Once the elbow has been flexed, gravity often provides the necessary force for extension, with the elbow flexors working eccentrically to control the movement. Triceps only becomes active in this form of extension when the speed of the movement becomes important as in executing a karate chop. Triceps works strongly in pushing and punching activities, and when performing “press-ups”. In the latter it is working concentrically in the upward movement, and eccentrically in the downward movement. It works in a similar manner when using the arms to get out of, or to lower oneself into, a chair with arms, or when using crutches or parallel bars to relieve body-weight from the legs during walking. When using a wheelchair, triceps brachii works strongly to push the wheel round and so propel the chair forwards.
Triceps brachii is also an important extensile ligament on the under surface of the shoulder joint capsule during abduction of the arm.

Palpation

The bulk of triceps is easy to see and feel on the posterior aspect of the arm. All three heads can be felt contracting if the subject flexes the elbow to 90° with the hand resting on a table, and then alternately presses downwards and relaxes. The long head can be felt high up on the back of the arm almost at the axilla; careful palpation enables it to be traced almost to its insertion on the scapula. The lateral head can be felt on the upper lateral part of the arm, extending as far round as the biceps brachii, while the medial head, covered by the other two heads, can be felt contracting just above the olecranon.
The thick tendon of triceps can be easily gripped between the thumb and index finger of the examiner’s hand, just above the olecranon of the ulna. The triceps reflex is elicited by tapping the tendon just above its insertion, with the elbow slightly flexed.


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