The mechanism of injury for shoulder pain can come from a single traumatic experience like throwing a baseball without a proper warm-up or repetitive lifting of luggage into the trunk of the car during a week of vacation. It could also not be attributed to anything but an insidious onset of symptoms.

Shoulder pain usually occurs because of components of both instability and impingement in the shoulder-girdle complex. Instability of the shoulder-girdle complex can directly change the biomechanical nature of the shoulder and cause impingement pain. This leads to injury during overhead activity in sports, as well as in repetitive daily activities where pain can initially be a mild discomfort, but it can also lead down a path of debility where use of that arm is avoided.

Starting Point
To successfully evaluate a person for shoulder pain, look closely at the relationship between the scapula on thoracic spine position, the shoulder active range of motion (AROM), and the strength of the scapular-stabilization and rotator cuff muscles. By exploring the connection between the shoulder-girdle complex and the thorax in both static and dynamic motion, a proper treatment protocol can be devised to eliminate the problem.

Postural alignment is one of the first subjects that a PT learns in school, but its value is easily overlooked during a physical therapy evaluation. A proper stance posture from a lateral view includes dropping a plumb line in which the ears are in alignment with the acromio-clavicular joint, the greater trochanter of the hip, the knee, and the talocrural joint. While sitting, most patients present with a slight forward-head posture, with bilateral anterior humeral head positions, a protracted scapulae, and an increased thoracic spine curvature due to slouching. Now, what are the chances that the posture will miraculously become more ideal in dynamic shoulder movements? Almost none.

The human body is a vehicle with multiple systems within itself. If one system is not working efficiently and effectively, it will break down; and indirectly, it can be injurious to other areas. People use their hands and shoulder complex constantly throughout their days, so it’s important to identify the pain and diagnose the injury as soon as possible. To prevent the shoulder pain from worsening and affecting other systems, you need to ensure that the body is being used correctly with respect to the biomechanical nature of the system, as well as to what positions we decide to put the shoulder in to complete its motion. We spend most of our day in different postures, whether they be sitting at a desk or in a car, or standing and talking to friends. If we can focus our patients on the importance of their posture in static and dynamic motion, we can greatly affect their mechanics and their shoulder pain.

When examining the patient for shoulder pain, note that spinal alignment and scapular position will affect their shoulder-girdle function. There are numerous muscle attachments to the spine, scapulae, clavicle, and humerus. Therefore, the different positions in which these bones sit on top of one another will directly influence the muscle length and will force generation of the muscle.

During shoulder activity, various muscles are recruited to turn on at different times to create a motion. Therefore, the sequencing on a motion is important. Scapulohumeral rhythm describes the natural relationship between the scapulae and humerus throughout the full shoulder-abduction range of motion. Initially, the first 30º of motion is due to pure glenohumeral motion. From 30º to 90º, the scapula abducts and upwardly rotates 1º for every 2º of humeral elevation. From 90º to full abduction, the scapula abducts and upwardly rotates 1º for each 1º of humeral elevation. For the scapula to abduct and upwardly rotate throughout 180º of humeral abduction, clavicular motion must include elevating approximately 40º and rotating in a posteriorsuperior direction.¹ A discrepancy within the natural scapulohumeral rhythm can cause muscle recruitment and timing to be off secondary to malalignment of the bones during shoulder motion. With enough repetitions or a great force during the faulty irregular biomechanics, a shoulder injury can occur.

Understanding Functions
There is very little research literature that shows the unique relationship between the thoracic spine alignment and the shoulder girdle for shoulder movements. Kendall et al² and Kelley and Clark³ have suggested that an increased kyphotic posture will cause shoulder-complex muscle weakness and restricted shoulder AROM. Culham and Peat⁴ concluded in their study that an upper-thoracic slope caused an increased anterior tilt of the scapula. As age and upper-thoracic slope increased, both shoulder and scapular motion decreased. In a study by Kebaetse et al,⁵ they concluded that there was significantly less shoulder AROM with a slouched posture with an average difference of about 23.6º. The scapula in the slouched posture exhibited abnormal scapulohumeral rhythm throughout shoulder abduction. Compared to normal subjects, those with slouched posture had the following: 1) more scapula internal rotation, 2) more superior translation with increasing abduction from neutral to 90º of shoulder abduction, and 3) significantly less posterior tilting and upward rotation to the thoracic spine from 90º to full shoulder abduction. This study greatly reinforces the importance of the relationship between the thoracic spine, the scapulae, and the humerus, and the integration of the muscles attaching to those bones. By simply affecting the joint positions of the shoulder complex and posture, the mechanical level of the muscles are also modified.

The shoulder-girdle complex allows the greatest degree of mobility in the human body while sacrificing stability. Anatomically, the glenohumeral joint is not as protected as the hip joint because it consists of a rounded humeral head that is oversized compared to a smaller, flat glenoid fossa of the scapula. Passive structures like the ligaments, the labrum, and the capsule hold the joint intact. The muscles that maintain the scapula on the thorax are very vital to stabilizing the glenoid for the moving humerus. Movement is sloppy and inefficient if both ends are moving. That would not be the case if the base, which consists of the scapula-thorax component, is stable.

Muscles that incorporate scapulae stabilization are: the levator scapula, the upper trapezius (I, II), the rhomboids major and minor, and the superior fibers of the serratus anterior, which elevate; the inferior fibers of the serratus anterior and the lower trapezius (IV), which depress; the upper, middle, and lower trapezius, and the rhomboids major and minor, which adduct; the serratus anterior and pectoralis minor, which abduct; the trapezius (I, II, IV) and the inferior fibers of the serratus anterior muscles, which upwardly rotate; and the rhomboids major and minor, the subclavius, and the superior fibers of the serratus anterior muscles, which downwardly rotate the scapulae.

A patient’s resting scapula positions can give great insight on which muscles are lengthened and shortened. Generally, muscles that are in a lengthened position are not being used as much; and when contracting, they do not generate as much force as a muscle at optimal or a shortened length. During the scapulohumeral rhythm examination, the symmetry between the bilateral scapulae should be noted to determine if there are any sequencing problems that need to be addressed. An appropriate exercise program can then be developed to strengthen the weaker muscles and/or work on the muscle-timing issues.

Shoulder impingement occurs when the rotator-cuff tendons running through the subacromial space get pinched during shoulder elevation. That space is very narrow already, and with the slouching posture, there is even more narrowing, which ultimately results in microtears of the tendons and shoulder pain. Also in multidirection instability, the humerus rides higher into the subacromial space and the rotator cuff tendons compress excessively.

So what keeps the humeral head from constantly abutting the roof of the subacromial space? Out of the 26 muscles that control the shoulder girdle, only the four rotator cuff muscles, as well as the biceps brachii (long head), play a significant role in dynamic stability of the shoulder complex during shoulder movements. The infraspinatus, teres minor, and subscapularis muscles cause a compression and inferior shear force on the shoulder complex, which results in a downward glide of the humerus. The supraspinatus muscle is the only one that causes a compression and minimal vertical shear, causing an upward glide of the humerus. The compression force created by the rotator cuff muscles are needed to stablilize the humeral head in the glenoid fossa. Therefore, strengthening of the rotator cuff muscles should be incorporated into every shoulder program. It would be beneficial to strengthen the rotator cuff muscles throughout the full shoulder AROM (0º, 30º, 60º, 90º of shoulder elevation).

In certain shoulder-pain cases, biceps brachii tendinitis is a secondary result from severe rotator cuff weakness or rotator cuff tendinitis. At times, the biceps brachii muscle can be considered an honorary rotator cuff muscle, where, for example, biceps brachii tendinitis results from being overused to compensate for the rotator cuff weakness.

A Well-Rounded Approach
A complete shoulder-therapy program for such patients as an elderly person who needs his shoulder to be pain free when doing daily activities like reaching above into the cupboard for a plate, or a professional athlete who requires enough power and endurance to pitch nine innings of baseball, need a few simple considerations in their shoulder-rehabilitation program. First, a proper postural assessment of their scapula-thorax and shoulder-girdle complex in static and dynamic motion will determine what to focus on. By looking at their task analysis when they perform shoulder AROM, note if they are putting themselves in biomechanically poor alignments where they will not be successful with their shoulder movements. Then, it is important to determine why the patient is not achieving the desired posture or shoulder AROM. Most of the time, patients will be unable to hold themselves in good alignment because they are asking their muscles to work hard to fight against the gravity that has been pushing them down into the slouched posture, and their bodies now have restrictions.

If there are soft-tissue restrictions like tightness in the latissimus dorsi or pect major or minor, then a stretching or soft-tissue-mobilization program needs to be implemented to give the muscles the flexibility to get into their needed range of motion. If there are joint restrictions, then joint mobilizations such as posterior glides, inferior glides, or scapula mobilizations can be implemented for the following: to get the thoracic spine into further extension, to decrease posterior capsule tightness, to prevent excessive superior shearing force, or to get more posterior tilting or upward rotation. Lastly, if the rotator cuff muscles or postural scapula stabilization muscles are weak, then a strengthening and endurance program must also be included.

Rehabilitation programs should allow patients to strengthen their muscles in proper alignment, because there is no point being strong in limited shoulder range of motion. Patients with shoulder pain will succeed more in therapy if the PT can take the focus away from just the shoulder complex and start looking at how the whole upper quadrant can affect each other.

Susan Fu, DPT, ATC, is a physical therapist at Congress Physical Therapy and Rehabilitation, Pasadena, Calif.

References
1. Arnheim D, Prentice W. Principles of Athletic Training. 10th ed. Boston, Mass: McGraw-Hill Companies Inc; 2000.

2. Kendall FP, McCreary EK, Provance PG. Muscles-testing and Function. 4th ed. Baltimore, Md: Williams & Wilkins; 1993.

3. Kelly MJ, Clark WA. Orthopedic Therapy of the Shoulder. Philadelphia, Pa: JB Lippincott Co; 1995.

4. Culham E, Peat M. Functional anatomy of the shoulder complex. J Orthop Sports Phys Ther. 1993;18:342–350.

5. Kebaetse A, McClure P, Pratt N. Thoracic position effect on shoulder range of motion, strength, and three-dimensional scapular kinematics. Arch Phys Med Rehabil. 1999:80:945–950.