Which muscles act as agonists in left lateral flexion of the neck?

• A. Left SCM, Left Scalenes, Left Trapezius, Left Levator Scapulae
• B. Right SCM, Right Scalenes, Right Trapezius, Right Levator Scapulae
• C. Pectoralis Major
• D. Latissimus Dorsi

Answer: (A)

Left lateral flexion of the neck is produced by contracting muscles on the left side that shorten the left side of the neck. The sternocleidomastoid on the left, when it contracts unilaterally, pulls the head toward the left shoulder and tilts the head to the same side (with rotation to the opposite side). The scalene muscles on the left do the same, contributing to lateral flexion and helping with rib elevation during breathing. The levator scapulae crosses the neck as well, and when the scapula is stabilized, its left-sided contraction contributes to bending the neck to the left. The trapezius, especially its upper fibers, can assist with lateral flexion when the shoulder girdle is stabilized, adding to the leftward bend of the neck.

The other options rely on muscles that either act on the opposite side or on areas outside neck lateral flexion (such as the chest or broad back), so they don’t serve as the primary muscles driving left-side bending of the neck.