This altered position requires the levator scapulae and upper fibers of trapezius to increase activation in an attempt to maintain glenohumeral stability. (exclusive.multibriefs.com)
Recent studies have shown that as surprising as it may sound, decline is the most effective angle for training the entire chest area, including its upper fibers. (fitnessandpower.com)
That being said, the trapezius muscles are comprised of upper, middle and lower fibers, even though the upper fibers usually get all the love because they are most visible from the front. (fitnessandpower.com)