Introduction to Shoulder Mechanics

The shoulder is an intricate area of the body, a common place for pain and injury. A major reason the shoulder is so intricate is that it only has only one bony attachment, where the clavicle meets the sternum. This is called the sternoclavicular joint. Due to this factor, the shoulder is a very unstable joint and for optimal motion to transpire, proper timing of every muscle it critical. Not only is proper timing of the surrounding musculature in the shoulder critical, but there is a direct relationship between other regions in the body that can have an impact on shoulder mechanics. This makes pinpointing shoulder dysfunction a difficult task. Limitations, among others, in trunk motion or ankle motion can dramatically impact shoulder motion.
Another reason the shoulder is so complex is that there is no true law of reciprocal inhibition. The law of reciprocal inhibition states that muscles on one side of a joint relax when muscles on the other side of a joint contract. The shoulder does not operate about a single axis, requiring every muscle in the shoulder to be active during all motions.  As a result, shoulder stability comes from the compression of the humeral head in the glenoid from the rotator cuff muscles and a special relationship between force couples of shoulder muscles. The muscles that create compression are the rotator cuff muscles, which include the infraspinatus, supraspinatus, subscapularis, and teres minor. Some of the major scapular stabilizers are the traps, serratus and rhomboids.
Shoulder pain? Tightness in the front of the shoulder?
This is a typical symptom that occurs when the supraspinatus muscle gets jammed underneath the AC joint resulting in pinching, inflammation, and inhibiting overhead motion. This occurs because when the shoulder moves overhead, external rotator muscles externally rotate the glenohumeral joint, so the supraspinatus tissue does not get impinged under the acromion. If shoulder muscles are inhibited, or from stress and trauma, muscle proprioception can be impacted affecting muscle activation and thus stability of the shoulder girdle.
4 major joints of the shoulder complex
  • GH- Glenohumeral Joint, which is very unstable
  • AC- acromioclavicular joint
  • SC- Sternoclavicular joint
  • ST- Scapulothoracic Joint, which is a physiological joint and does not have a joint capsule
Shoulder planes of motion
  • Flexion and extension
  • Abduction/ Adduction
  • Internal/ external rotation
  • Horizontal Adduction/Abduction
The diagram above shows some of the various joints and muscles comprising the shoulder girdle.  With all the joints and planes of motion the shoulder is capable of, ensuring that the scapular stabilizers and rotator cuff muscles are working in harmony can safeguard a stable shoulder. To determine why your shoulder is hurting or injured, a thorough evaluation needs to be conducted. As stated, shoulder motion can be impacted if other regions in the body are not functioning optimally. How would one know that a limitation is due to ankle dysfunction, trunk motion limitations, or hip stability without a thorough evaluation?
At the Athletic Training Institute, we start everyone, including athletes, with a baseline assessment to determine the underlying cause of shoulder dysfunction, then address weak musculature for proper progression.

Introduction to Shoulder Mechanics

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