Which accessory muscle is primarily engaged in costal breathing?

The correct answer involves the sternocleidomastoid muscle, which is primarily engaged during costal breathing. Costal breathing, also referred to as thoracic or rib breathing, involves the expansion and contraction of the rib cage to facilitate breathing. The sternocleidomastoid muscle, located in the neck, assists in elevating the sternum and the upper ribs, thus increasing the capacity of the thoracic cavity during inhalation.

While the other muscles listed do play roles in respiration, they are not primarily responsible for costal breathing. The rectus abdominis, for example, is more involved in forced expiration rather than in the act of inhaling related to rib cage expansion. The intercostal muscles are essential for breathing as they are located between the ribs and help in the expansion and contraction of the ribcage, but when focusing specifically on accessory muscles involved in elevating the thoracic structure during costal breathing, the sternocleidomastoid takes precedence. The scalene muscles also assist in breathing but are primarily engaged when there is increased demand for airflow, often during deep or labored breathing rather than in typical costal breathing scenarios.