When both muscles of this pair are contracted simultaneously, they flex the neck forward. When both muscles of this pair are contracted simultaneously, they flex the neck forward. a. platysma b. semispinalis c. sternocleidomastoid d. trapezius

Sternocleidomastoid muscles.

Explanation:

The neck movements like flexion, rotation, extension, lateral rotation and flexion are carried out by neck muscles which include the sternocleidomastoid muscle,  

The sternocleidomastoid muscle is a pair of muscle located superficially in the anterior neck.

The sternocleidomastoid muscles connects skull with the clavicle and hence is involved the head and neck motions.

The `bilateral contraction movements of sternocleidomastoid muscles aids in flexion of the neck and brings the chin down towards the breastbone.  

Platysma is a facial muscle and involved in facial expressions and mandibular movements

Semispinalis muscle is in the posterior neck and maintains posture and head movements.

Trapezius muscles lies at the back of the head.