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Serratus Anterior

Key Points:

• Responsible for shoulder flexion especially beyond 90° of flexion.
• Patients cannot forward flex the shoulder past 90° and exhibit scapular winging when the serratus anterior is not functioning.
• Patients with serratus anterior weakness, patient is able to fully flex the shoulder but exhibit a degree of scapular winging.

Muscle Function: 
Origin Fixed:
• Abduction (protraction) of the scapula.
• Upward rotation of the scapula and rotates the glenoid cavity cranially.
• Holds the medial border of the scapula firmly against the rib cage.
• Upper fibers may slightly elevate the scapula.
• Lower fibers may depress the scapula.
Insertion Fixed: May assist in forced inspiration when the scapula is stabilized in adduction by the rhomboids.

Palpation: Difficult to palpate on most people, one may be able to palpate along the lateral ribs anterior to the latissimus muscle.
Strength Testing: Position – in sitting with humerus flexed to approximately 130º with slight horizontal abduction. If the patient has weak shoulder flexors and is unable to reach 130º, position the patient’s arm on the tester’s shoulder. Tester is positioned in front and to the side of the patient to monitor the position of the scapula during the test. Stabilize – the shoulder and arm. Resist – against abduction of the scapula while maintain upward rotation by having the patient reach forward and upward with the upper extremity. The inferior angle of the scapula should come towards the mid-axillary line.
Possible Substitutions: None.
Relevant Anatomy:
• Roots: C5, C6, C7, C8.
• Nerve: Long thoracic nerve.
• Innervation Route: C5, C6, C7, C8 → long thoracic nerve → serratus anterior branches.
Origins: External lateral surfaces and superior borders of the first through eighth ribs.
Insertion: Costal surface of the medial border of the scapula.