Which muscle is NOT considered an accessory muscle of inspiration?

The rectus abdominis muscle is not considered an accessory muscle of inspiration because it primarily functions in expiration rather than inspiration. During normal breathing, the diaphragm is the main muscle used for inhalation, while accessory muscles such as the sternocleidomastoid, pectoralis muscles, and scalenes can aid in deeper or more forceful inhalation when needed, particularly during physical exertion or distress.

In situations where more air needs to be drawn into the lungs rapidly, these accessory muscles assist by elevating the rib cage and expanding the thoracic cavity. On the other hand, the rectus abdominis plays a critical role in forced expiration, working to push the diaphragm upwards and expel air from the lungs by compressing the abdominal contents, which supports its function during activities like coughing or intense exercise. Thus, identifying the rectus abdominis as an accessory muscle of inspiration misrepresents its role within the respiratory mechanics.