The lower trapezius (G. trapeza, four-sided or four-legged table.) is not an individual muscle, itself, but rather it is the lowermost section of fibers in the trapezius muscle. It acts on the scapula, with its main role as the prime mover in scapular depression.
Also worth noting is its important role as a scapular stabilizer: it is a synergist in scapular upward rotation, which is required for raising the arm overhead through a full range of motion.
It is a superficial posterior axioappendicular (extrinsic shoulder) muscle, located on the thoracic spinal region of the back. It’s in the most superficial layer of muscle, sitting atop portions of the rhomboids, lats and infraspinatus.
The lower trapezius originates from the upper 6 thoracic vertebrae. It has parallel-oriented fibers that run superolaterally and converge as they approach their insertion on the medial scapular spine.
The lower traps, much like the middle traps, is an oft-ignored muscle. Most people don’t even know it exists, thinking the upper traps make up the entire trapezius muscle…
…People’s lack of awareness of the lower traps is one reason why it is weak in the vast majority of people, weight trainees, athletes and general population alike.
Inhibited/Lengthened Lower Trapezius: The lower trapezius is inhibited and long in individuals with upper crossed syndrome (UCS). The excessive thoracic kyphosis and scapular protraction that comes with UCS, causes the lower trapezius to get stretched out and weak. One effect of lower trapezius inhibition is the reduced ability to posteriorly tilt the scapula during scapular upward rotation, which is required for proper overhead arm movement. When this can’t happen properly, there’s increased chance of shoulder impingement and overhead range of motion is restricted. The reduced overhead mobility is often compensated for with excessive lumbar extension and jutting the head forward. Also, lower trapezius inhibition causes poor scapular depression, which leads to poor technique on upper body pull exercises because your scapulae tend to stay in an elevated position. This restricts scapular mobility and forces you to rely on arm strength and lumbar extension (for momentum) to lift the weight on pull exercises.
If you have inhibited/lengthened lower trapezius, do the following:
Strengthen lower trapezius with direct exercises. I recommend starting with scapular wall slides and unilateral prone floor Y raises. Once you get the hang of those exercises, you can add another exercise or two, and use the bilateral Y raise variations.
Release and stretch the pectoralis major, pectoralis minor and the latissimus dorsi. These muscles, which are very tight and short, are the main culprits responsible for pulling you into the hunched-over posture that’s stretching out your lower traps.
Reduce the use of heavy overhead presses until you restore your lower trapezius strength and improve your overhead range of motion. In the mean time, you can do the high incline shoulder press for heavy shoulder work.
Address the other issues related to upper crossed syndrome, which is (most likely) the root cause of your lower trapezius weakness. See how to fix upper crossed syndrome (article coming soon) for specific details.
Even if you don’t have any postural issues or any (noticeable) lower trapezius dysfunction, it’s still a good idea to include some lower trapezius exercises somewhere in your training. Pretty much everyone stands to benefit from stronger lower traps.