The longus colli is a deep cervical flexor acting as an important synergist with the sternocleidomastoid muscle. Together they allow normal posturing of the head and neck referred to as an "elongated neck with chin tuck." Together, these muscles provide flexion of the elongated neck and head onto the trunk. The longus colli providing the important function of prevention of cervical hyperextension by the upper portion of the sternocleidomastoid.
If the sternocleidomastoid works alone in a bilateral contraction, the cervical region collapses into hyperextension at all the side-to-side axes across which the sternocleidomastoid passes in a posterior relationship.