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Key Points:
• Coracobrachialis is useful for shoulder flexion when the elbow is flexed (which shortens the biceps muscle diminishing its contribution to shoulder flexion.
• To discern the coracobrachialis from the short head of the biceps brachii, test coracobrachialis when the elbow is passively flexed and the forearm supinated.
• Reinnervation of the coracobrachialis may be helpful to augment anterior deltoid function in patients with upper brachial plexus injuries.
Muscle Function:
Flexes and adducts the shoulder joint. 
Palpation: The coracobrachialis is deep to the short head of the biceps brachii. To discern the coracobrachialis from the short head of the biceps, it is important to have the forearm passively flexed at the elbow during palpation and resistance to disengage the biceps brachii and contract the coracobrachialis. If there is a doubt, allowing the subject to perform elbow flexion will help differentiate the two muscles by contracting the biceps brachii, but not the coracobrachialis.
Strength Testing: Position – subject seated or supine with arm flexed and laterally rotated at the shoulder joint, the elbow completely flexed, and the forearm supinated. Elbow flexion and forearm supination shortens the biceps brachii and decreases the assistances from the biceps brachii during shoulder flexion. Stabilization – if the trunk is stable, no fixation by the examiner is necessary. Resist – against shoulder flexion in lateral rotation on the anteromedial surface of the lower ⅓ of the humerus, in the direction of shoulder extension and sight abduction.
Possible Substitutions: Biceps brachii, specifically the short head of the biceps, when the elbow is not passively flexed. Anterior deltoid is responsible for shoulder flexion while the serratus anterior allows this movement by stabilizing and upwardly rotating the scapula.
Relevant Anatomy:
• Roots: C6, C7.
• Nerve: Musculocutaneous nerve.
• Innervation Route: C6, C7 → musculocutaneous nerve → coracobrachialis.
Origin: Apex of the coracoids process of the scapula.
Insertion: Medial surface of the middle of the shaft of the humerus, (opposite of the deltoid tuberosity).