Understanding Shoulder Movement: The Role of Scapular Control & Joint Function

The shoulder girdle is one of the most complex collections of joints in the body, allowing it to have such a range of movements. It's complexity allows us to move our shoulders and arms in almost any direction. It is made up of four key joints that work together:

1. The glenohumeral joint is the main ball-and-socket joint between the upper arm bone (humerus) and the shoulder blade (scapula),

2. The acromioclavicular (AC) joint connects the collarbone (clavicle) to a bony projection of the scapula, the acromion.

3. The sternoclavicular (SC) joint links the clavicle to the breastbone (sternum)

4. The scapulothoracic joint isn’t a true anatomical joint, but rather a functional one. It refers to the space where the scapula glides over the ribcage during shoulder movement.

Movement and Centration

The shoulder's remarkable range of motion is due to the coordinated interaction between the scapula and the humerus. For optimal shoulder movement, the socket of the scapula must be correctly positioned to guide the humeral head – a concept known as scapular control and centration. When we lift our arm, the scapula should move in a sequence to ensure centration of the socket is correct:

• Upward Rotation: The scapula turns towards the spine, allowing the socket to point upwards.

• Posterior Tilt: The top of the scapula tilts backward, helping to prevent impingement and maintain smooth shoulder mechanics.

• External Rotation: The scapula turns to ensure the socket faces outwards to make space for the humerus to move.

A Perfectly Synchronised Pulley System

These movements are facilitated by the coordinated action of several muscles; the serratus anterior, trapezius, and rhomboids. Their actions are balanced by opposing muscles such as the subscapularis, pectoralis minor, coracobrachialis, latissimus dorsi, teres major and triceps. Check out a video of the pulley system here.

Image of the muscles of the Shoulder

If any of these opposing muscles are overused or too tight (from posture, stress or repetitive motions), they will pull the scapula down, in and forward adversely affecting the position of the socket. This loss of optimal centration limits the range of motion that can be achieved, ultimately restricting healthy shoulder movement. This reduction in motion is a result of abnormal function and is unlikely to appear normal on static scans (MRIs and X-rays), as investigate structural changes.

Problems Associated with Abnormal and Reduced Movement in the Shoulder

Limited mobility can contribute to several well-known shoulder conditions, including:

• Impingement syndrome, where poor shoulder mechanics cause the tendons of the rotator cuff to become compressed, leading to inflammation and pain.

• Increased stress on structures can increase the risk of tendonitis or bursitis.

• Prolonged inflammation from a persistent strain is one factor that can contribute towards fibrotic changes in the joint capsule, which occurs as part of Frozen Shoulder.

• Over time, these muscular imbalances can lead to poor posture and strain in the neck, upper back, and even further restrict shoulder movement.

  
  
  
  

One of the injuries we commonly see in clinic associated with abnormal movement is rotator cuff tendon irritation. We will take a closer look at the rotator cuff, injuries that can occur and how to avoid this in July's blog.

Other Factors Limiting Shoulder Mobility

In the clinic, most shoulder restrictions we see are a result of imbalances within the pulley system described above. However, other contributing factors include spinal stiffness, osteoarthritis of the shoulder, muscle/tendon tears, and structural changes from previous trauma.