The serratus posterior inferior muscle is one of the back's two intermediate muscles (the other is called the serratus posterior superior muscle), which means it’s neither a surface nor a deep muscle.

The serratus posterior inferior muscle is located in the mid to lower back and forms a V-shape. It originates from the spinal processes (the points of the vertebrae closest to the skin) of vertebrae T11 to T12 and L1 to L2. It travels upward from its origin and inserts into the lower borders of ribs 9-12.

The serratus posterior inferior muscle is thin but becomes more muscular at its lowest points. It is supplied with blood by the lowest posterior intercostal artery, the subcostal artery, and the first two lumbar arteries. The nerves in this muscle come from the ventral primary rami. 

The serratus posterior inferior muscle is a respiratory muscle. The function of the serratus posterior inferior muscle is to pull down the lower ribs, assisting with forced exhaling.