The biggest muscles that work the shoulder are the pectoralis and the latissimus dorsi muscles. Their job is to move the arm. One thing they do really well is to turn the arm inward. Because the musculoskeletal system
works in a chain reaction fashion, when the arm rotates inward, it takes the shoulder with it. As this happens, the muscles at the front of the shoulder tighten, while the muscles of the upper back, especially the rhomboids, become overstretched and weak.
The resultant picture is:
- kyphosis with forward head posture
- tight muscles in the front of the shoulder
- weak muscles in the upper back
. People who work for long hours at a computer are prone to this muscle imbalance.