• The serratus anterior muscle abducts and upwardly (laterally) rotates the scapula and holds the scapula flat against the rib cage.  The upward rotation is produced by the force couple action of the serratus anterior muscle and the trapezius muscle.
• The serratus anterior is the primary, and only effective abductor of the scapula.
• Full range of flexion/elevation motion is not possible if the serratus anterior is paralyzed or weak.
• Impaired control of the scapula by the serratus anterior is common, and results from the serratus being weak, long, short, or altered in the timing of its movement of the scapula.  If the scapula is not correctly positioned during shoulder flexion or abduction the scapulohumeral muscles (rotator cuff) will not be able to maintain their optimal length and tension relationships.
• Deficient control of the serratus anterior, causing impairment in timing and range of scapular motion, can cause stress at the glenohumeral joint.  This stress results from the incorrect positioning of the glenoid for glenohumeral joint motion when there is insufficient abduction and upward rotation of the scapula.
• Observation of the degree of abduction or adduction of the scapula is necessary to distinguish insufficient performance of the trapezius or serratus anterior.
• Although both muscles are upward rotators, because the trapezius is an adductor and the serratus an abductor, the medial/lateral position of the scapula can be a guide as to which muscle should be emphasized for lengthening or strengthening.
• The primary indicators of impaired performance are inadequate abduction and inadequate upward rotation of the scapula during shoulder flexion and abduction.

Actions

With the origin fixed

• Abducts the scapula
• Depresses the scapula
• Holds the medial border of the scapula against the rib cage

If the scapula is stabilized

• May act in forced inhalation

Origin

• Surfaces of the upper 8 or 9 ribs

Insertion

• Anterior surface of the medial border of the scapula

Nerve Innervation

• Long thoracic from the brachial plexus