According to Inner Body, the trapezius muscle is responsible for shoulder shrugging and head extension. The trapezius is a wide, superficial muscle that covers most of the upper back and the posterior side of the neck. There is a trapezius muscle on the left and right side of the body.
The trapezius is responsible for several dynamic movements. Inner Body states that the trapezius muscles support the shoulders when carrying weight, help the head extend backwards and help the head rotate from left to right. The trapezius muscles bring the shoulder blades together as well. The superior fibers, middle fibers and inferior fibers control these motions.