The action of a muscle crossing the shoulder joint can be predicted not only from its origin and in- sertion, but also from its relation to the shoulder joint. These details, along with the nerve supply to the muscle, are shown in Table 6.1. In Table 6.2,
Table 6.1 Muscles acting on the shoulder joint
Muscle Origin Insertion Relation to joint Action Nerve supply
Pectoralis major Clavicle, medial two-thirds
Humerus, lateral lip of intertubercular sulcus
Anterior Flexion and medial rotation
Medial and lateral pectoral nerves
Sternum
Costal cartilages 1–6
Humerus, lateral lip of intertubercular sulcus
Anterior Adduction and medial rotation
Medial and lateral pectoral nerves
Latissimus dorsi Lower ribs Thoracolumbar fascia
Iliac crest
Humerus, intertubercular sulcus
Inferior Adduction, medial rotation, extension if flexed
Thoracodorsal nerve
Deltoid Clavicle, lateral one-third
Humerus, deltoid tuberosity
Anterior Flexion and medial rotation
Axillary nerve
Scapula, acromion, spine
Humerus, deltoid tuberosity
Superior Posterior
Abduction, extension, lateral rotation
Axillary nerve
Biceps brachii short head
Scapula, coracoid process
Radius Anterior Flexion
Stabilization
Musculocutaneous nerve
Biceps brachii, long head
Scapula, supraglenoid tubercle
Radius Anterior Flexion
Stabilization
Musculocutaneous nerve
Coracobrachialis Scapula, coracoid process
Humerus, middle of body medially
Anterior Flexion Musculocutaneous
nerve Teres major Scapula, lateral
margin inferior one-third
Humerus, medial lip of intertubercular sulcus
Inferior Adduction, medial rotation
Lower subscapular nerve
Supraspinatus Scapula, supraspinous fossa
Humerus, greater tubercle superior surface
Superior Abduction, stabilization
Suprascapular nerve
Infraspinatus Scapula, infraspinous fossa
Humerus, greater tubercle posterosuperior surface
Posterior Lateral rotation, stabilization
Suprascapular nerve
Teres minor Scapula, lateral margin superior two-thirds
Humerus, greater tubercle posterior surface
Posterior Lateral rotation, stabilization*
Axillary nerve
Subscapularis Scapula, subscapular fossa
Humerus, lesser tubercle
Anterior Medial rotation, stabilization*
Upper and lower subscapular nerves Triceps, long head Scapula, infraglenoid
tubercle
Ulna, olecranon process
Inferior Stabilization Radial nerve
* These muscles stabilize the shoulder joint in abduction and prevent the head of the humerus from rising in the glenoid and hitting on the acromion when the deltoid contracts.
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The shoulder
See Clinical Applications 6.1 and 6.2 for the practical implications of the anatomy in this chapter.
shoulder abduction, extension, and lateral rota- tion. In such an injury, the arm is held in a position of adduction, medial rotation, and flexion.
Table 6.2 Movements at the shoulder joint
Movement Muscles Nerve supply
Flexion Pectoralis major, clavicular part Pectoral nerves Deltoid, clavicular part Axillary
Biceps, short head Musculocutaneous
Coracobrachialis Musculocutaneous
Extension Deltoid, posterior part Axillary
Latissimus dorsi (if shoulder flexed) Thoracodorsal Teres major (if shoulder flexed) Subscapular
Abduction Deltoid, acromial part Axillary
Supraspinatus Suprascapular
Adduction Pectoralis major, sternocostal part Pectoral
Latissimus dorsi Thoracodorsal
Teres major Subscapular
Lateral rotation of humerus Deltoid, posterior part Axillary
Infraspinatus Suprascapular
Teres minor Axillary
Medial rotation of humerus Pectoralis major Pectoral
Latissimus dorsi Thoracodorsal
Deltoid, clavicular part Axillary
Teres major Subscapular
Subscapularis Subscapular
Stabilization* Subscapularis Subscapular
Supraspinatus Suprascapular
Infraspinatus Suprascapular
Teres minor Axillary
Triceps, long head Radial
Biceps, long head Musculocutaneous
The actions of muscles shown in Table 6.2 presuppose a fixed scapula.
* All the muscles of stabilization are attached close to the shoulder joint, have a poor mechanical advantage over it, and are more effective in holding the joint surfaces together than in moving it.
CLINICAL APPLICATION 6.1 Anastomosis around the scapula Arteries supplying the muscles on the dorsal surface of
the scapula come from two distinct sources. The dorsal scapular artery and the suprascapular artery are branches of the thyrocervical trunk of the subclavian artery. The posterior humeral circumflex artery and the circumflex scapular arteries are branches of the axillary artery. The anastomosis around the scapula [Fig. 6.9] ensures that the mobile scapula is not dependent on only one source.
The anastomosis forms an alternate route through which blood from the first part of the subclavian artery can reach the third part of the axillary artery when the main stem is blocked between these two points. Note that slow occlusion will allow time for collateral circulation to be established. Sudden occlusion is more dangerous, as the collateral circulation may not be ready to take over the increased supply.
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Muscles, movements and nerves
CLINICAL APPLICATION 6.2 Rotator cuff injury A young swimmer came to the doctor with a history of
sudden pain in the right shoulder, and inability to sleep on the injured side. After examining the patient, a diag- nosis of rotator cuff injury was made.
Study question 1: what are the muscles which form the
‘rotator cuff’? (Answer: supraspinatus, infraspinatus, teres major, teres minor.)
Study question 2: apart from the movement these muscles bring about at the shoulder, what is the other
important function they perform? (Answer: they act as expansile ligaments of the shoulder joint and steady it.) The doctor suspects injury to the supraspinatus.
Study question 3: where is the supraspinatus inserted?
What is its action? Under which bony projection does the tendon pass? (Answer: it is inserted into the upper facet on the greater tubercle and produces abduction of shoulder. The tendon passes under the acromion process of the scapula.)
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CHAPTER 7
The arm
The deep fascia enclosing the arm sends septa be- tween the groups of muscles to allow them to slide on each other and to give an increased area for origin. Two of these septa—the lateral and medial intermuscular septa—pass to the correspond- ing supracondylar lines and epicondyles of the humerus, thus dividing the distal part of the arm into anterior and posterior compartments [Fig. 7.1].