The lower trapezius is one of three divisions of the trapezius muscle in the upper back. A large muscle stretching from the back of the neck outward to the shoulders and midway down the back, the trapezius performs a number of functions in the upper body, depending on which section of the muscle is being activated. The lower or inferior section is responsible for the medial rotation and depression, or turning inward and pulling downward, of the shoulder blades.
Although it is one continuous muscle, the trapezius has three distinct sections: the upper, middle, and lower trapezius. It has a quadrilateral or diamond shape, with the triangle of the upper trapezius attaching to the occipital bone at the base of the skull and spreading outward to the top of either shoulder blade. The middle trapezius extends horizontally between the upper thoracic vertebra and the top outer edges of the shoulder blades. Beneath that is the inverted triangle of the lower trapezius, which runs diagonally on either side of the spine from the spinous processes of the middle-to-lower thoracic vertebra to the lower inside edges of the shoulder blades.
As its fibers run obliquely, beginning in the middle of the back and extending upward and outward to attach to either shoulder blade, the lower trapezius is positioned to pull downward and inward on the shoulder blades. As it does so, it rotates the inferior angle of the scapula, which is the bottommost point of the triangle-shaped shoulder blade, inward toward the spine. It also works in opposition to the upper trapezius, which is the division of the muscle whose job is to elevate the shoulder blades, or shrug the shoulders. The upper trapezius is often overdeveloped, especially in individuals who work in front of a computer, so strengthening the lower fibers of this muscle is a recommended strategy for getting the tight upper fibers to relax.
To strengthen the lower trapezius and pull the shoulders down and back, several exercises are recommended. One such exercise is the wall slide, which works both the lower and middle fibers. To perform wall slides, one should stand with his head, shoulder blades, and tailbone pressed against a wall and feet ten or 12 inches out from the wall. Squeezing the shoulder blades back and together, he should firmly press his elbows against the wall to either side of the body with hands up and knuckles touching the wall. Maintaining this retracted and depressed shoulder blade position, he should then slowly slide his elbows up the wall to shoulder height and all the way back down towards his sides, never dropping the hands forward and never releasing tension in the shoulder blades.
Another recommended exercise for the lower trapezius is the chin-up, which uses the lower fibers to pull the shoulder blades down and together as the body is pulled upward. For anyone unable to perform an unassisted chin-up, the assisted chin-up machine, which is found in most health clubs, is an acceptable substitute. As with the wall slide, two to four sets should be performed and up to 12 repetitions for increased strength.