Which muscle is NOT involved in the process of inhalation?

Inhalation is primarily facilitated by the contraction of specific muscles that expand the thoracic cavity, allowing air to flow into the lungs. The diaphragm, a dome-shaped muscle at the base of the thoracic cavity, plays a crucial role by contracting and moving downwards, increasing the volume of the thoracic cavity. The external intercostals assist by elevating the rib cage during inhalation, further increasing lung capacity and facilitating airflow. The sternocleidomastoid, while not a primary muscle for normal breathing, can assist during deep inhalation or in situations of increased respiratory demand by elevating the sternum.

In contrast, the rectus abdominis is primarily a muscle involved in exhalation. Its contraction helps to compress the abdominal contents and push the diaphragm upwards, thereby aiding in the expulsion of air from the lungs.

Thus, the correct choice is the rectus abdominis, as it does not play a role in the inhalation process but rather contributes to forced exhalation. Understanding the function of these muscles is essential for nurses and paramedics to assess respiratory mechanics accurately and provide appropriate interventions.