Several muscle groups surround the shoulder and help it both with movement and with remaining stable as it does so.
Commonest are the rotator cuff muscles - supraspinatus, infraspinatus, subscapularis and teres minor. They all act close to the shoulder and help to rotate it in the socket but also are crucial in keeping it stable and secure as we move and go about our day to day life.
The larger deltoid muscle overlies the rotator cuff and is the muscle that forms the rounded shape of our shoulder. The deltoid muscle is closely related to the pectoral muscles of the chest wall.
Paradoxically though the commonest reason for sore shoulder muscles is likely to be due to fatigue in the trapezius muscles - the large diamond shape group that attach from the neck down to the lower part of the scapula. These often become sore and painful after workouts or unaccustomed use of the arms in the overhead position.