M01.09.003 Intermediate Muscles

The back muscles are categorized into three major groups based on their location and function:

Classification of Back Muscles

1. Superficial Group
   • Associated with: Shoulder movements.
   • Development: Extrinsic, as they do not develop embryologically in the back.
2. Intermediate Group
   • Associated with: Movements of the thoracic cage.
   • Development: Extrinsic, similar to the superficial group.
3. Deep Group
   • Associated with: Movements of the vertebral column.
   • Development: Intrinsic, as they develop embryologically in the back.

This guide focuses on the intermediate back muscles, including their attachments, innervation, and functions.

Intermediate Back Muscles

The intermediate group comprises two muscles:

• Serratus Posterior Superior
• Serratus Posterior Inferior

These muscles extend from the vertebral column to the rib cage and contribute slightly to respiratory functions by elevating or depressing the ribs.

Serratus Posterior Superior

• Shape: Thin and rectangular.
• Location: Lies deep into the rhomboid muscles in the upper back.

Serratus Posterior Inferior

• Shape: Broad and strong.
• Location: Lies underneath the latissimus dorsi.

Key Points to Remember

1. Intermediate muscles are extrinsic, meaning they develop outside the back and migrate during embryogenesis.
2. Function: These muscles primarily assist with thoracic cage movement and may play a minor role in respiration.
3. Innervation: Both muscles are innervated by the intercostal nerves.
4. Serratus Posterior Superior: Elevates ribs 2–5 for inspiration.
5. Serratus Posterior Inferior: Depresses ribs 9–12 for expiration.

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