The first two groups (superficial and intermediate) are referred to as the extrinsic back muscles. The deep group is the intrinsic muscle group. This article will focus on the intermediate group.
The intermediate back muscles are covered superficially by the superficial back muscles. The intermediate group includes:
Serratus posterior superior
Serratus posterior inferior
Serratus posterior superior (SPS)
The serratusposteriorsuperior (SPS) attaches the vertebral column to the ribs, providing stabilisation to the ribs and promoting forceful expiration. When looking at the SPS on both sides of the spine, it has the appearance of a triangle.
Origin and insertion
The SPS originates from the inferior portion of the nuchal ligament, and the spinous processes of C7-T3. It inserts onto the posterior aspect of ribs 2-5.
The SPS consists of ‘fingers’ of muscle fibres that each resemble a short strap.
The function of the SPS is to stabilise the ribs during passive respiration and elevate the ribs (2-5) during active or forceful inspiration.
The intercostal nerves of ribs 2-5 supply sensorimotorinnervation to the SPS.
The intercostal arteries, as part of the intercostal neurovascular bundle, supply the SPS.
Serratus posterior inferior (SPI)
The serratus posterior inferior (SPI) has a similar shape to the SPS but appears lower on the vertebral column and has an inverted appearance. It looks like an inverted triangle.
Origin and insertion
The SPI originates from the spinous processes of T11-L3 and inserts onto the posterior surface of ribs 9-12.
Like the SPS, the fibres of SPI are finger-like projections of a strapmuscle.
The function of SPI is to stabilise the thorax during passive respiration and depress ribs 9-12 during active inspiration.
Sensorimotorinnervation is supplied to the SPI through the intercostalnerves, and occasionally the subcostalnerve (T12).
The intercostalarteries, as part of the intercostal neurovascular bundle, supply the SPI.
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