During bilateral contraction, which joint is primarily extended by the rectus capitis posterior major?

During bilateral contraction, which joint is primarily extended by the rectus capitis posterior major?

• A. Atlantooccipital joint
• B. Zygapophysial joints of the cervical spine
• C. Vertebral joints of the thoracic spine
• D. Glenohumeral joint

Answer: (A)

The key idea is that this muscle is a suboccipital muscle that extends the head at the atlas–occipital junction. Rectus capitis posterior major runs from the posterior surface of the axis (C2) to the inferior nuchal line of the occipital bone. When both sides contract, it pulls the occipital bone backward toward the atlas, producing extension at the atlantooccipital joint (the joint between the skull and the first cervical vertebra). The other joints listed aren’t the primary site for this action: the cervical facet joints (zygapophysial joints) mainly guide intervertebral movements, the thoracic vertebral joints are far down the spine, and the glenohumeral joint is the shoulder joint—unrelated to head extension.

The key idea is that this muscle is a suboccipital muscle that extends the head at the atlas–occipital junction. Rectus capitis posterior major runs from the posterior surface of the axis (C2) to the inferior nuchal line of the occipital bone. When both sides contract, it pulls the occipital bone backward toward the atlas, producing extension at the atlantooccipital joint (the joint between the skull and the first cervical vertebra). The other joints listed aren’t the primary site for this action: the cervical facet joints (zygapophysial joints) mainly guide intervertebral movements, the thoracic vertebral joints are far down the spine, and the glenohumeral joint is the shoulder joint—unrelated to head extension.