This is an excerpt from Jim Stoppani's Encyclopedia of Muscle & Strength-3rd Edition by Jim Stoppani.
The trapezius is the large diamond-shaped muscle on the upper back, often referred to as traps. This muscle has upper, middle, and lower portions that all perform different movements (see figure 5.4). The upper trapezius primarily lifts and rotates the shoulder blades upward as when shrugging the shoulders (such as during dumbbell shrugs). The middle trapezius primarily pulls the shoulder blades together (such as during prone incline bench dumbbell or cable shrugs). The lower trapezius rotates the shoulder blades downward (such as during straight-arm dips). Trapezius training can be paired with shoulders or back. Most bodybuilders train the traps after shoulders because their primary interest is in developing the upper portion of the traps. The upper traps are involved in most deltoid exercises. Therefore, they are sufficiently warmed up after training shoulders. However, because it is technically a back muscle and assists during many back exercises, upper traps are often trained with back. Most lifters typically pick one or two exercises for trap workouts and perform three to eight sets. If both a barbell and a dumbbell trap exercise are done in the same workout, the barbell exercise is typically done first. For detailed descriptions of trapezius exercises, see chapter 18. See table 5.14 for basic guidelines for designing a trapezius workout based on the current training split used.