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25. 9. 2012.

Muscles laterally rotating the pectoral(shoulder) girdle

As it can be seen in the pictures below, both muscles are well positioned to cause the inferior angle of the scapula to move laterally around the thoracic wall. The clavicle acting as a strut restricts movement at the acromion process, so that the overall effect of the action of these two muscles is to elevate the acromion process and move the inferior angle laterally, thereby enabling the glenoid fossa to face more directly upwards. This movement of the pectoral girdle is extremely important for increasing the range of movement possible, particularly in terms of abduction and flexion of the upper limb at the shoulder.

Trapezius contributes to the rotation by contraction of its upper fibres which lift the lateral end of the clavicle and acromion process upwards while at the same time its lower fibres pull downwards on the medial end of the spine of the scapula.
Serratusanterior, the more important of the two muscles in this movement, pulls strongly on the inferior angle of the scapula, where the majority of its muscle fibres insert, to pull it laterally around the chest wall. The notional axis about which this rotation takes place is just below the spine of the scapula towards the base. Resultant movements are shown in the picture below. 

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