Daily Archives: September 27, 2012

The Rotator Cuff

Arguably one of the most injured and dysfunctional muscle groups in the shoulder, the rotator cuff is essential for all shoulder motions.

Anatomy of the Rotator Cuff:

The rotator cuff is composed of four small, short muscles that originate on the scapula and pass around the shoulder where their tendons fuse together and attach on the humerus. Known as the “SITS” muscles, they are:

  1. supraspinatus – anchors the humeral head in the glenoid.  It is thought to assist with abduction (lateral raising of the shoulder) in the early phase of the motion. Of the four rotator cuff muscles, this is the most often torn.
  2. subscapularis – this muscle sits between the shoulder blade and the back of the rib cage (on the anterior surface of the scapula).  It’s main functions are internal rotation (twisting the arm in) and adduction (bringing the arm tight against your side).  It is the largest of the rotator cuff muscles.  It is also assisted by the pectoralis major muscle, and thus not as susceptible to injury.
  3. infraspinatus – this muscle sits on the back of the scapula (shoulder blade) and under the supraspinatus and scapular spine (that bony ridge you can feel if your reach over your shoulder and touch the top of your shoulder blade.  It’s main function is external rotation (twisting the arm out) and it assists with extension (bringing the arm back) and horizontal abduction (pulling the arm back at shoulder height).  It is much smaller than the subscapularis.
  4. teres minor – this muscle sits just inferior to the infraspinatus.  It is also an external rotator of the shoulder and assists with extension and horizontal abduction.  This muscle is about the same size as the subscapularis.

As a group, these muscles have two main functions:

  1. First, they stabilize the joint and help center the humeral head in the glenoid fossa (important in preventing pain and injury). During abduction (raising up sideways) of the arm, the rotator cuff anchors the humeral head, forcing it to pivot while the deltoid contracts to raise the arm.  If the deltoid were to work without the rotator cuff, the humeral head would be pulled up and  toward the top of the glenoid fossa – causing it to bump into the acromium and making the motion of abduction limited (by joint space and ultimately pain).
  2. Second, the rotator cuff muscles control rotation of the shoulder and allow for complex movements.  They are some of the prime movers with external rotation and internal rotation of the shoulder.

Injury to the Rotator Cuff:

There are several mechanisms by which the rotator cuff can be injured/damaged.  These include:

  1. Direct trauma: such as a fall or unexpected force through the joint
  2. Overuse/repetitive stress: this can include chronic impingement (slowly saws through the rotator cuff tendon), chronic tendonitis, and even degenerative joint disease (the arthritis and loss of cartilage changes the joint space and thus how the rotator cuff can contract).
  3. Pathologic weakness: Pathologic weakness (such as systemic muscular/joint disease and nerve injury) can this make it more susceptible to injury.  The rotator cuff is innervated (controlled by the nerves) C5-C6 which exit the neck just under the clavicle.  If there is any compression on these nerves, the signals they send to control the rotator cuff can be decreased, and the muscles become functionally weaker.  If that’s the case, even a small stress can  cause a traumatic tear (reaching for a purse, lifting a pot, doing a muscle up, etc)

Gymnastics and the rotator cuff:

Warning: the following contains some complex thinking and is mostly personal theory (so take what you want from it and ignore the rest).

It should come as no surprise that gymnastics puts a huge strain on the rotator cuff.  The various body positions required often bias the gymnast’s arm and shoulder into positions where the rotator cuff is disadvantaged or put in stretched position and forced to contract to stabilize the joint (and sometimes act as a prime mover).

In gymnastics, the shoulder is required to be a weight bearing joint.  (Warning: this is not what it was designed for).  There are two types of weight bearing encountered by the shoulder in gymnastics: compression/approximation & traction/distraction.  Compression is the traditionally accepted term for weight bearing.  Compression can be defined as the force of gravity and weight as they pass through and approximate the bones of the joint  (as in a handstand).  The term I’m adding as a “weight bearing” activity is traction/distraction.  In this case, the force is through the joint capsule, the ligaments, and the surrounding muscles. In the shoulder, arguably the rotator cuff plays a huge role in traction weight bearing – it is the muscular stabilizer!

Structurally and anatomically, weight bearing is not what the shoulder is designed for.  If you look at the hip, a truly weight bearing joint, the joint congruity (the amount of surface area in contact between the 2 bones of the joint)  is very high.  This spreads the forces out and gives stability.  The hip also has significant musculature (specifically the gluteals and hip rotators)  that assists in stabilizing and adding congruity to the joint.  This is an evolutionary upgrade – something we took on when we began to walk upright on two legs as our primary means of transport.  This is NOT the case in the shoulder.  It has a very low level of congruity (think tennis ball balanced on a quarter) and is designed for mobility.  It also does not have as thick of a joint capsule and has less dense and less numerous ligaments around it than the hip.  Finally, in the hip, the gluteal muscles and the hip rotators are some of the thickest muscles in the body.  Thier “analogous” structures in the shoulder – the pectorals and the rotator cuff are itty bitty in comparison.

See the dilemma? We gymnasts are fighting the evolutionary chain of events and trying to return to the activities of our now far-estranged primate ancestors.  Swinging, standing, jumping, and walking on our hands and arms comes at a price that the shoulder isn’t structurally able to pay on cash.

Drum roll for some fun structural facts:  According to a study published in the Journal of Clinical Sports Medicine, the shoulder capsule (the ligaments surrounding the shoulder) have s tensile strength of 100-180 lbs at full maturity.  This means that any force through the joint that exceeds that amount requires the rotator cuff to contract and provide enough force to protect the joint.

Now – think about a giant swing on uneven bars or a dislocate on rings.  For all you physics whizzes out there – combine your body weight with gravity (F = ma) and then add in torque (T = Fd) and factor in some momentum ( p = mv) and TADA! Your rotator cuff just tried to provide enough force to substitute for a cable on the nearest suspension bridge.  Potential for injury? I should say so….

Coming up next:

  • Common rotator cuff injuries
  • And Angie, I don’t care why – just tell me how to fix them!