Which of the following muscles is NOT considered an accessory respiratory muscle?

The rectus abdominis is primarily a muscle involved in trunk flexion and stabilization rather than respiration. Accessory respiratory muscles are those that assist the primary muscles of respiration, especially during periods of heightened respiratory demand, such as during exercise or in respiratory distress.

Muscles like the pectoralis minor and sternocleidomastoid are considered accessory muscles because they aid in increasing the volume of the thoracic cavity when inhaling, especially during deep or labored breathing. The pectoralis minor elevates the ribs, and the sternocleidomastoid can raise the sternum.

The erector spinae, while primarily associated with the extension and lateral movement of the vertebral column, can also assist in breathing by stabilizing the spine and allowing for proper posture during respiratory movements.

In contrast, the rectus abdominis does not significantly contribute to respiratory mechanics, rather it functions mainly to flex the vertebral column and compress the abdominal contents, which aids in expiration but is not considered an accessory muscle for inspiration.