Final answer:
The sternocleidomastoid is a primary muscle for rotating and flexing the head, while the infrahyoid muscles like sternohyoid and sternothyroid, involved with laryngeal positioning, do not contribute to these movements.
Step-by-step explanation:
The muscles involved in flexing, extending, and rotating the head include a variety of axial muscles, notably the sternocleidomastoid, which is key for laterally flexing and rotating the head. Other muscles contribute to these movements, including the splenius cervicis, which inserts into the cervical region, providing posture and rotation functions to the head and neck. The infrahyoid muscles like the sternohyoid and sternothyroid are primarily involved with the positioning of the larynx and hyoid bone, and thus, they would not be typically responsible for flexing, extending, or rotating the head.
The muscles that flex, extend, and rotate the head include all of the following except the sternocleidomastoid muscle. The sternocleidomastoid muscle is a major muscle that laterally flexes and rotates the head. It divides the neck into anterior and posterior triangles. When it contracts bilaterally, it flexes or extends the head. However, it is not involved in rotating the head.