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 pectoralis minor muscle is a small strip like muscle located underneath the pectoralis major. It functions to protect the shoulder and aid in scapular movement and stabilization. This muscle almost always has trigger points in it due to chronic poor posture where the shoulders are rounded. Federal pain from these trigger points will cause pain to be felt in the chest and down the arm.
The Infraspinatus muscle is part of the rotator cuff. It acts to move and stabilize the shoulder. This muscle is often chronically strained in people who have a rounded shoulder posture. With this type of strain trigger points can develop. These knots in the muscle will cause pain to be felt in the front of the shoulder and down the arm.