Dynamic Stabilizers of the glenohumeral joint include the contractile tissues and the associated sensorimotor system involved with proprioception, kinesesia, and the sensation of resistance. For optimal shoulder stabilization the dynamic stabilizers must be working in an efficient synergistic fashion.
The rotaor cuff muscles have a smaller cross-section area and size, a closer to center of rotation on which they act, their lever arm is shorter, and they generate smaller forces. Which means they are made to provide stability to a dynamic fulcrum during GH movement. It is also important to realize that the rotator cuff tendons blend with the joint capsule. They not only help to dynamically stabilize and move the humerus, but they reinforce the joint capsule (dynamic ligament tension).
In addition to the concatity-compression effect to stabilize the humeral head in the center of the glenoid fossa, the rotator cuff helps to externally rotate and depress the humeral head to avoid contact (impingement) of the greater tubercle with the acromion. While external rotation comes mainly from the teres minor/infraspinatus, the downward depressive moment comes from the force couple of the subscapularis and teres minor/infraspinatus. This downward force component helps to prevent the dominance of the deltoid’s upward force (depressive forces at maximum between 60 and 80 degrees of elevation). This is synergistic relationship often referred to as the RTC/Deltoid force couple.
Assessing for adequate dynamic stability in every direction requires knowledge of the contractile structures around the joint and how they function with each humeral vector force. There are 3 ways I tend to look at this dynamic stability.
- Resistance to translation from opposite side pull of muscles
- Support of same side structures through muscle stiffness/capsular tensioning
- Synergistic force coupling for efficient controlled axis of rotation/motion
We’ll use anterior translation of the humerus during external rotation at 90° as an example since it is the most common clinically. On the posterior side of the GH joint, the external rotators would fire to “pull” the humeral head back. It has been proven in research that the external rotators help to prevent anterior translation of humerus (Kuhn et al 2005). On the anterior side of the joint, the internal rotator would provide anterior capsule tightening and act as a sling to prevent excess anterior translation. The force couples of the scapular musculature, teres minor/infraspinatus and subscapularis, and supraspinatus would help to stabilize the humeral head in the glenoid fossa.
- Supraspinatus - compression, abducts, and generates a small ER torque , peaks at 30°-60°, generates most force in scapular plane
- Infraspinatus & Teres Minor - compression, generates inferoposterior force, provides great ER torque, generates most force at 0° abduction
- Subscapularis - compression, provides anterior stability, generates IR torque, generates most force at 0° abduction, primary IR at 90° abduction
Long Head of Biceps Brachii
The LHB as a stabilizer of the shoulder joint has been a topic of controversy for a long time now. Some believe that the forces of the LHB are negligible and do not function to stabilize the glenohumeral joint. Others believe it is anywhere from a secondary to tertiary stabilizer of the shoulder. Although, there are many different views as to how the muscle functions as a stabilizer. It has been theorized that the LHB is a humeral head depressor, reduces anterior translation in late cocking phase of throwing and can increase the torsional rigidity of the joint resisting ER, and some have pointed out that at low levels of elevation it stabilizes the joint anteriorly when the arm is in IR and posteriorly when the arm is in ER.
Research still seems to be inconclusive as to the function of the LHB in stabilizing the shoulder joint. However, the fact that the biceps tendon often plays a role in symptoms (anterior shoulder pain) and pathologies (SLAP lesions) leads me to believe that the LBH plays a role in stabilizing the shoulder joint.
The scapula is of great importance when considering shoulder stability. Achieving proper balance of force couples, resolving any muscle-length limitations, and ensuring dynamic stabilization of the scapula throughout the entire ROM is necessary for optimal functioning of the shoulder joint. It is important to ensure a retracted and slightly depressed scapula during all shoulder exercises.
A discussion of the influence of the scapula is beyond the scope of this article. A future post will provide more detail.
Examination and assessment of the dynamic stability of the shoulder complex can be very difficult. Most special tests do not provide adequate specificity or sensitivity, subjective complaints may be misleading, and there are many structures they may be involved. I have found it helpful to assess the shoulder in various degrees of motion in all 3 cardinal planes.
For example, I will preform the kennedy-hawkins in 3 different transverse plane degrees to attempt to differentiate the influence of pain from the acromial and/or coracoid. I usually MMT IR/ER and abduction strength in 3 different frontal plane degrees in attempt to determine a more specific directional instability and to assess the RTC force coupling efficiency. I also find it helpful to consider the sagittal plane and palpate the humerus and scapula to determine if and where any uncontrolled motion is coming from.
Assessing and treating dynamic stability of the shoulder joint is a very complex task. A thorough understanding of the kinesiology and structures involved is necessary to determine the specific dynamic stability impairments. Since the shoulder joint has the most mobile joint in the body, it is important to consider all planes of motion to locate the uncontrolled motion.
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