I found this on one site I frequent. This very question was asked by someone else. This woman does only one set of lat pulldowns and no shrugs or upright rows, yet still has large traps. This guy seems to think the levator scapula, a muscle below the traps which is stimulated by pulldown movements, could be the source of the overdeveloped trap look. Read below........
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Q: My trapezius muscle easily gets overdeveloped giving me a thick-neck/round-shouldered look, yet I don't do shrugs or upright rows. Could my one set of lat-pulldowns be causing this?
When most people mention the trapezius they are commonly referring to the upper trapezius. I assume this is your problem area. Incidentally, the middle and lower trapezius rotate the scapula upward when the arm is raised as in the shoulder presses, upright rows, lateral and front raises. The Upper trapezius will act a stabilizer during these motions and other exercises that require holding a heavy weight. This may include exercises from any type of deadlift, Olympic type lift to even heavy arm curls. Pulldowns involve opposite muscles (levator scapulae, rhomboids, pectoralis minor) since the scapula downward rotates as the shoulder adducts or extends. It is possible the levator scapula beneath the trapezius can be developed from pulldowns.
If your shoulder width is narrow, you may attempt to further the develop the lateral deltoid within minimum stimulation of the upper trapezius. This may require you to perform side delt exercises like lying lateral raise and lever lateral raise. Front deltoid movements that involve minimal amount of upper trapezeus stabilization include cable front raises, triceps dips, or bench or chest press movements with the bar to the lower chest (Like powerlifter style bench presses with a slightly narrower grip and your elbows closer to the sides). Usually most get a sufficient front delt stimulation from basic chest work already so those movements would be optional or incidental.
If you have protracted shoulders, which is often associated with the rounded shoulder look, you may need to strengthen the rhomboids, middle and lower trapezius and stretch the pectoralis minor and major. You should be performing approximately equal volume of exercise for the chest as back, particular balancing your general chest (ie: chest and bench press work) with general back work (ie rows).