All of the following are muscles of inspiration, EXCEPT:

In the context of muscles that are involved in the act of inspiration, the quadriceps muscle does not play a role. Muscles of inspiration are primarily responsible for expanding the thoracic cavity and facilitating the intake of air into the lungs.

The diaphragm is the main muscle of inspiration, contracting and flattening when inhaling, which increases the volume of the thoracic cavity. The external intercostals are also involved in this process as they lift the rib cage and expand the chest. The sternocleidomastoid, although primarily associated with neck movements, can assist in deep inhalation by elevating the sternum and hence increasing the thoracic volume.

In contrast, the quadriceps is primarily a muscle of the anterior thigh, responsible for extending the knee, and does not contribute to the inhalation process, which involves the muscles of the thorax and abdomen. Therefore, identifying the quadriceps as not being a muscle of inspiration aligns with the functional anatomy of the respiratory system.