The middle scalene is the largest of the three scalenes, running from the C2 through C7 posterior tubercles to the first rib. The brachial plexus exits between the anterior and middle scalenes in the interscalene triangle, and the middle scalene forms the posterior boundary of this space. The dorsal scapular nerve, which supplies the rhomboids and levator scapulae, typically pierces through the middle scalene belly, making middle scalene hypertrophy a potential site of dorsal scapular nerve entrapment.
| Origin | Posterior tubercles of the transverse processes of C2 through C7 |
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| Insertion | Superior surface of the first rib posterior to the subclavian groove |
| Nerve Supply | Ventral rami of the cervical spinal nerves C3 through C8 |
| Blood Supply | Ascending cervical artery |
| Actions | Elevation of the first rib during forced inspiration; Ipsilateral lateral flexion of the neck; Bilateral contraction assists in neck flexion |
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As the primary rib elevator of the three scalenes, the middle scalene is the most important accessory inspiratory muscle during exercise, lifting the first rib to expand the upper thoracic aperture when the diaphragm alone cannot meet respiratory demand.
Interscalene brachial plexus block is performed by inserting a needle between the anterior and middle scalenes at the C6 level, targeting the brachial plexus trunks in this groove. The middle scalene is specifically implicated in dorsal scapular nerve entrapment, producing rhomboid and levator scapulae weakness with periscapular pain in a pattern that mimics cervical radiculopathy.
The middle scalene is palpable in the lateral neck posterior to the anterior scalene and anterior to the posterior scalene, most accessible in the posterior triangle between the trapezius and sternocleidomastoid during lateral neck flexion against resistance.
Compression of the dorsal scapular nerve as it pierces the middle scalene belly producing rhomboid and levator scapulae weakness with interscapular pain, managed with scalene release and dry needling.
