The serratus posterior inferior muscle is a broad, flat muscle located in the posterior (back) region of the chest. It originates at the spinous processes of the lower six or seven thoracic vertebrae, and inserts into the inferior (lower) borders of the ninth to twelfth ribs. The serratus posterior inferior muscle is one of the muscles that make up the chest wall, and is located just below the serratus posterior superior muscle.

The main function of the serratus posterior inferior muscle is to depress (lower) the ribs, and assist in expiration (breathing out). This movement is essential for activities such as breathing, and helps to provide stability and support for the rib cage. The serratus posterior inferior muscle also helps to stabilize the ribs, and assist in rotation of the rib cage.

The serratus posterior inferior muscle is used in many activities that involve depression of the ribs, such as breathing and coughing. It is also used in activities that require stability and support for the rib cage, such as lifting a heavy object or bending over to pick something up.

To strengthen the serratus posterior inferior muscle, exercises that involve depression of the ribs, such as rib cage compressions and deep breathing, can be effective. Other exercises that target the muscles of the chest wall, such as push-ups