Upper Crossed Syndrome
Upper crossed syndrome (UCS) occurs when the muscles in the neck, shoulders, and chest become deformed, usually as a result of poor posture.
The muscles that are typically the most affected are the upper trapezius and the levator scapula, which are the back muscles of the shoulders and neck. First, they become extremely strained and overactive. Then, the muscles in the front of the chest, called the major and minor pectoralis, become tight and shortened.
When these muscles are overactive, the surrounding counter muscles are underused and become weak. The overactive muscles and under active muscles can then overlap, causing an X shape to develop.