One of the often overlooked muscles affected by poor rounded shoulder posture is the latissimus dorsi muscle. They are the largest muscles in the back and function primary to adduct the arm and to assist in medially rotating the arm. Chronic strain causes trigger points to build up in the muscle tissue causing pain, referred pain, stiffness and weakness. Trigger points in the lats refer pain into the back, shoulder, and down the arm into the fingers.
The supraspinatus muscle is one of your rotator cuff muscles in the shoulder. It helps to move and stabilize the shoulder joint. Poor posture can cause this muscle to become strained and irritated. Rounded shoulders can cause strain in this muscle and can even cause the tendon of this muscle to become impinged with overhead shoulder movement. Trigger points that form in this muscle will cause pain to be felt in the shoulder, elbow, and down the arm.
The pectoralis major is the main chest muscle. In mainly functions to adduct the arm, and medially rotate the arm. This muscle is always right with a rounded shoulder posture. When trigger points form in this muscle they produce pain felt in the chest and down the arm.
The scalenes muscles are located in the neck and help with neck movements and inspiration. The anterior scalene runs directly over the brachial plexus that gives nerve and blood supply to the arm. Poor neck and head posture as well as poor breathing mechanics can cause this muscle to become overloaded and develope trigger points. These points will refer pain into the shoulder, arm, and hand. As well as the shoulder blade. When tight, the anterior scalene muscle can put pressure on the brachial plexus causing numbness and tingling felt in the fingers and hand.