Shoulder Mobility and Stability

By Nate Christopher, PT, OCS
Physical Therapist

The shoulder is the most mobile joint in the body.   You can use your shoulder to reach overhead, behind your back, behind your head, and in many other directions.  Imagine moving your knee in the ways that you can move your shoulder; you would be rushed to the hospital because it would mean that the joint is not properly attached!

How can the shoulder move through so great a range of motion?  With human joints mobility and stability are balanced.  The shoulder is a joint that is extremely mobile, but to an extent, this is at the expense of stability.  In fact, the shoulder is the most frequently dislocated large joint in the body.  The shoulder is a ball and socket joint, but there is little coverage of the socket over the ball.  The shoulder joint has been compared to a golf ball on a tee.  Think of how much force it takes to knock a golf ball off of a tee – not much!  Because the ball does not cover much of the socket, the shoulder joint is free to move to the extreme ranges that we are accustomed to, but it is also susceptible to problems of instability.

So why doesn’t everyone dislocate their shoulder?  The shoulder has several built in stabilizing structures including the labrum, which is a ring of cartilage that serves to deepen the socket, and to an extent, vacuum seals the joint like a plunger over the ball shaped part of the humerus bone.  There is also a structure called the shoulder capsule.  This can be thought of as a shrink wrap that envelopes the ball and socket providing stability.  The shoulder also has ligaments—structures that attach bone to bone, which also act to restrain movement.  While these structures provide stability to the joint, the shoulder is also very dependent on what is called “dynamic stability” or in other words, stability provided by muscle contraction.

The major source of dynamic stability in the shoulder is the rotator cuff.  This is a group of four muscles that attach from the scapula (otherwise known as the shoulder blade) to the humerus, (the long bone of the upper arm).  The rotator cuff helps to center and compress the ball of the joint into the socket during movement.  In addition to helping stabilize the shoulder, it serves to fine tune the mechanics of the joint so that the larger, more powerful deltoid muscle can do the “heavy lifting.”  It has been said that the rotator cuff steers the ship by maintaining proper mechanics and centering the ball within the socket, and the deltoid is the powerful engine that provides movement and strength.

Another large contributor to proper shoulder mechanics is the influence of the shoulder blade, aka the scapula.  About 1/3 of our overhead reaching range of motion comes from movement of the scapula.   This is to say that without our shoulder blade properly moving in rhythm with the rest of our shoulder we would only be able to lift our arm about 120 degrees overhead, versus the normal 180 degrees.  This would make it very difficult to reach into cupboards, wash our hair, or do anything that involves overhead reaching.  Again, there are specific muscles that contribute to the proper mechanics of the scapula with movement.

So why is all of this important?  When people have shoulder pain and dysfunction, they usually demonstrate weakness or alterations of how they contract their rotator cuff and scapular musculature.   A physical therapist can analyze the movements of the shoulder and test strength to prescribe exercise that can restore normal mechanics to the shoulder and alleviate pain.  People can have successful outcomes with this type of treatment even when they have a wide array of shoulder diagnoses ranging from tendinitis, bursitis, instabilities, impingements, and even rotator cuff tears!  In fact, a 2013 study of 452 individuals who had a traumatic full-thickness rotator cuff tears showed that physical therapy was effective in successfully treating 75% of these patients without surgery.  This is good evidence as to why your doctor will usually recommend at least a trial of physical therapy prior to surgery.   Physical therapists can also use hands on techniques to treat movement restrictions and imbalances that may be contributing to pain and dysfunction.  If you’re suffering from a “bad shoulder,” you don’t have to live with it – have it evaluated by your doctor, and see if Physical Therapy is a good option for you!