Scapular dyskinesis is a condition characterized by abnormal movement or positioning of the shoulder blade (scapula). This alteration in mechanics is not a diagnosis itself, but an impairment resulting from various underlying issues that disrupt the shoulder’s proper function. Understanding the causes behind this abnormal movement is necessary for correction and prevention of further shoulder problems.
Defining Scapular Dyskinesis
The scapula is a triangular bone on the back of the ribcage that acts as the foundation for arm movements. Its normal motion is precisely coordinated with the upper arm bone (humerus) in the scapulohumeral rhythm. This synchronized movement provides a stable base for the rotator cuff muscles and maintains adequate space within the shoulder joint.
When this rhythm is disrupted, the condition is termed scapular dyskinesis, meaning “alteration of motion.” Dyskinesis often manifests as “winging,” where the shoulder blade lifts away from the chest wall, or “tipping,” involving the inferior angle moving away from the ribcage. These changes reduce shoulder efficiency, often contributing to pain and weakness.
Primary Muscular and Neurological Drivers
The most common cause of scapular dyskinesis involves alterations in the activation and strength of the periscapular muscles. Because the shoulder blade has limited bony attachments, it is highly dependent on muscular control for stability and movement. Weakness or delayed firing in specific muscles immediately compromises the scapula’s ability to maintain proper position against the ribcage.
Muscles like the serratus anterior and the upper, middle, and lower trapezius work together as a “force couple” to ensure smooth rotation during arm elevation. Reduced strength in the serratus anterior and lower trapezius is frequently seen in shoulder dysfunction. This imbalance causes the scapula to lose the posterior tilt and upward rotation necessary for full overhead arm movement. The rhomboid muscles also help stabilize the scapula and control its translation.
Less commonly, dyskinesis results from a direct injury to the nerves controlling these muscles. Damage to the long thoracic nerve, for example, can cause paralysis or severe weakness in the serratus anterior, leading to pronounced “winging.” Injury to the spinal accessory nerve can impair the trapezius muscle, resulting in a lower and more lateral posture of the scapula.
Structural and Postural Contributors
Beyond muscle activation issues, the underlying skeletal framework and soft tissue restrictions can force the scapula into an abnormal position. Poor posture, often seen as a forward head and rounded shoulders, is a common contributor. This habitual alignment pulls the shoulder girdle forward, causing the scapula to protract and internally rotate at rest.
Excessive rounding of the upper back, known as thoracic kyphosis, alters the surface contour on which the scapula glides. This abnormal curvature provides a less-than-optimal base for scapular motion, leading to maltracking. Furthermore, tightness in anterior muscles, such as the pectoralis minor, can pull the scapula into an anteriorly tilted position, impairing its ability to posteriorly tilt during arm elevation.
Acute Injury and Repetitive Strain
External factors, including sudden trauma and chronic overuse, can initiate scapular dyskinesis. Acute injuries like clavicle fractures or high-grade sprains of the acromioclavicular (AC) joint directly alter the bony attachment points of the shoulder girdle. By disrupting structural integrity and stability, these injuries inhibit muscle function and lead to abnormal scapular motion.
Chronic repetitive strain is common in overhead athletes, such as baseball pitchers, swimmers, and tennis players. The repeated, high-velocity movements cause microtrauma and fatigue in the scapular stabilizing muscles. As these muscles tire, their ability to maintain smooth, coordinated control breaks down, leading to dysfunctional movement patterns. Overuse can also lead to secondary issues like posterior shoulder capsule tightness, which further restricts normal scapular mechanics.