Shoulder Pain – Part 1

Shoulder pain can be due to tendinitis, tears of the rotator cuff, a labral tear, an unstable joint, referred pain from the neck, an acromioclavicular joint separation, a dislocation, or buritis, to name a few musculoskeletal causes. This post is the begining of a series addressing shoulder pain, a fairly common problem.  In 2003, the Center for Disease Control (CDC) estimated shoulder pain affected 13.7 million people in the United States.

A brief description of the anatomy will help with understanding the reason for the different causes of shoulder pain.

Consider the bony construction of the joint:  There is very little bony stability in the design of the shoulder.  It is designed for mobility.  The head of the humerus (the ball) sits in the glenoid fossa (the socket) like a basket ball sits on a tea cup saucer.  Yes, thats the geometry, a really big ball on a small shallow socket.  Clearly this is built for mobility and not stability.  Contrast this anatomy with the hip joint which is build for stability.  Here the head of the femur (the ball) sits deep in the acetabulum (the socket) which almost surrounds the ball (it is deeper than the glenoid fossa) giving far greater structural support to the joint from the bony anatomy.  This contrast in design between the hip and the shoulder fits perfectly with the function of the joints:  the hip is a weight bearing joint for walking while the shoulder is a non-weight bearing joint which serves to place our hand anywhere in space so we can use the fine motor skills of our hand e.g. scratch your back, tie your shoes, put on a shirt, push open a door, pull a rope, play tennis, throw a ball, shoot hoops!

The glenoid fossa is part of the scapular (shoulder blade) which is attached to the rest of the skeleton via the clavicle.  The acromioclavicular joint (A-C joint) is the joint between the part of the scapular called the acromion and the distal (away from the midline of the body) end of the clavicle.  This is where shoulder pain may be felt from an A-C joint separation (more in later posts).  The sternoclavicular joint (S-C joint) is the joint between the proximal (midline) end of the clavicle and the sternum (“breast bone”) and is the only bony attachment of the shoulder to the rest of the skeleton!  Yes, the scapula floats freely on the back of the rib cage and is held in place by numerous muscles which orchestrate its movement when you use your arm.  A floating scapula and a huge ball in a shallow, small socket … now that is a joint made for movement!

Why is this important to physical therapy?

Look at the moving parts that have to function together for optimal use of your arm!  The scapula has to rotate to angle the glenoid fossa correctly with various arm movements e.g. reaching to a cupboard above shoulder height.  This is not possible if the A-C joint and the S-C joint are not moving correctly.  So these joints have a combined function – if one is not moving well then the arm may not move correctly and you may have shoulder pain.   The muscles controlling scapular motion also need to be contracting at the right time with the right force to ensure correct positioning of the glenoid.  Physical therapists evaluate and treat all these aspects of shoulder function in patients with shoulder pain.

Consider the soft tissue construction of the joint:  What is the rotator cuff?

The rotator cuff is a set of 4 muscles which originate on the scapula and insert strategically around the humeral head.  Working together, these muscles control the biomechanics of the glenohumeral joint keeping the ball centered in the socket.  You can visualize the rotator cuff as the seal balancing a ball (the humeral head) on its nose (the glenoid fossa or socket).  They are frequently referred to by the acronym SITS muscles:

  1. Supraspinatus
  2. Infraspinatus
  3. Teres minor
  4. Subscapularis

The supraspinatus muscle is the most commonly involved muscle in shoulder pain as it passes through the small space between the top of the humeral head and beneath the acromion of the scapula.  It can thus be compressed between these 2 bones (referred to as impingement).  If the biomechanics of the joint is abnormal (various reasons) there can be repeated tendon irritation against the acromion thus creating shoulder pain.  Alternately, the shoulder pain can be a consequence of the supraspinatus tendon being forcefully compressed between the humeral head and the acromion e.g. jamming the shoulder joint by falling forward onto your hand with your elbow straight, or pinching the tendon if you fall onto an outstretched hand forcing your arm all the way above your head.  The resultant inflammation of the supraspinatus tendon is referred to as rotator cuff tendinits.  Repetitive overhead tasks (e.g. electrician working overhead, pitching in baseball, lifting packages to shelves above shoulder height) may also inflame this tendon as the rotator cuff, as a whole, may fatigue and hence alter the biomechanics of the shoulder resulting in impingement and hence shoulder pain.

The other 3 rotator cuff muscles (infraspinatus, teres minor and subscapularis) serve as shoulder depressors.  That is, they pull the head of the humerus (the ball) downwards as the arm is elevated.  This reduces the compression of the supraspinatus tendon as described above.  So, if you are lifting a heavy object overhead to a shelf and the shoulder depressors are over powered by the muscles doing the lifting,  the humeral head will ride high in the glenoid (the socket) and result in impringement as described above.

In summary, the shoulder is a very complex joint.  It’s function is dependent on synchronous functioning of the muscles which move the scapula as well as the rotator cuff which keep the ball of the joint centered in the socket.  These muscles must have suitable strength, balanced (front and back of shoulder) flexibility as well as sufficient endurance.

A well trained physical therapist will evaluate all these aspects of shoulder function and determine what deficits are causing the shoulder pain.  He/she will then design a rehabilitation program to address the cause of the shoulder pain, correcting the deficiencies thereby abolishing the pain and regaining full function.


  1. Post a question (or topic) you would like to see addressed.
  2. Forward this to a friend, family member or co-worker.
  3. Call us with any questions you might have.
  4. Await the next post on shoulder pain: Glenohumeral instability. 

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