Final answer:
The sternocleidomastoid muscle assists in forced inspiration by lifting the thoracic wall to increase lung volume.2. inspiration
Step-by-step explanation:
The sternocleidomastoid muscle is a prominent muscle in the neck that plays a role in various movements of the head.
During forced breathing, or hyperpnea, which can occur during exercise or actions that require the active manipulation of breathing such as singing, multiple muscle groups are involved. Specifically, the sternocleidomastoid muscle assists in forced inspiration. When the sternocleidomastoid muscles contract, they work with other neck muscles like the scalenes to lift the thoracic wall, thereby increasing lung volume. This clearly indicates that among the given choices, the sternocleidomastoid muscle is a muscle of forced inspiration rather than expiration.