Final answer:
The sternocleidomastoid muscle is a major neck muscle that plays a key role in forced inspiration, helping to lift the thoracic wall and increase lung volume during forceful inhalation.
Step-by-step explanation:
The Sternocleidomastoid Muscle and Forced Breathing
The sternocleidomastoid muscle is a significant muscle involved in the movement of the head and neck. Originating at the sternum and clavicle and inserting onto the mastoid process of the temporal bone, this muscle plays a crucial role in head motion. It is also active during forced breathing, which involves additional muscle contractions beyond the normal action of the diaphragm and intercostal muscles. Forced inspiration involves the contraction of accessory muscles such as the sternocleidomastoid and scalenes, which help lift the thoracic wall to increase lung volume.
During forced expiration, other muscles including the abdomen's obliques come into play, pushing abdominal organs upward against the diaphragm to expel more air. Considering its involvement in lifting the thoracic wall, the sternocleidomastoid muscle is a muscle of forced inspiration, not expiration. This muscle's contraction helps increase the thoracic cavity's volume and thereby contributes to drawing air into the lungs during forceful breathing efforts.