This statement is true. The sternocleidomastoid muscles are indeed the primary flexors of the head. They are located on each side of the neck and are responsible for flexing the neck and rotating the head.
In addition to the sternocleidomastoid, the muscles associated with the hyoid bone, including the infrahyoid and suprahyoid muscles, assist in head movement and stabilization. These muscles play a supporting role in head flexion and help adjust the position of the head relative to the body.
Thus, the statement accurately reflects the contributions of both the sternocleidomastoid and the hyoid muscles in flexing the head.