Evidence-Based Strength Training: Rotator Cuff

This will be the first in a series of monthly posts that I will be contributing to MedBridge Education, who is an online continuing education resource for physical and occupation therapists…

According to Sipes et al, 30% of athletes suffer a shoulder injury during their career. Of those injuries, subacromial impingement syndrome and rotator cuff tendonitis were the most common shoulder injuries for each individual sport and accounted for 27% and 24% of total shoulder injuries, respectively. More specifically, over-head throwing athletes are especially susceptible to shoulder pathology as 28% of all injuries in professional baseball have been shown to occur at the shoulder (Cante et al).

Additionally, van der Windt et al conducted a prospective evaluation of over 300 patients. This analysis found that approximately 48% of shoulder injuries in a general population were diagnosed as subacromial impingement. Stability at the glenohumeral joint is dependent upon the passive stabilization of the ligamentous tissues and the dynamic stability provided by the rotator cuff musculature, as well as indirectly by the stability of the scapula and the muscles that support it. The rotator cuff’s primary role is to center the humeral head in the glenoid fossa, which requires adequate muscular strength and endurance. Because the rotator cuff muscles all originate from the scapula, appropriate functional control of the scapula is also an important rehab target.

As weakness of the rotator cuff and scapular stabilizing musculature can predispose individuals to subsequent pathological conditions, understanding what exercises are most effective in training these muscles is of utmost importance to the practicing clinician and student alike.  Evidence supports the use of appropriate exercises and manual therapy to improve the strength and function of these muscles. For example, in a clinical trial (Bang et al) comparing an exercise program to the same program with the addition of manual therapy, strength of patients in the manual therapy program improved significantly whereas the exercise-only group did not. In addition, based on the unique ROM, capsular laxity, strength, proprioception, and osseous anomalies specific to the over-head athlete, they too require the creation of a rehabilitation program specific to the demands of their sport/position (see: “Rehabilitation of the Overhead Throwing Athlete”). This review will touch on strength training interventions targeting the rotator cuff, but please remember this is only the tip of the iceberg in terms of rehabilitative principles.

Supraspinatus:

The supraspinatus originates from the supraspinous fossa and inserts laterally to the superior facet of the greater tubercle of the humerus. Based on this muscle’s line of pull, its primary responsibility involves concentric abduction of the humerus. In addition to this primary action, the supraspinatus also drives the superior roll of the humeral head and compresses the humeral head firmly against the glenoid fossa during shoulder abduction. Secondarily, this muscle contributes a small external rotation torque. Based on these biomechanical and anatomical considerations, Reinold et al investigated the electromographic (EMG) activity of the supraspinatus through  three common therapeutic exercises, which also serve as diagnostic tests for shoulder impingement when performed isometrically (Full Can, Empty Can , and Prone Full Can). All three of these exercises had nearly identical EMG data ranging between 62% (Full Can) and 67% Maximal Voluntary Isometric Contraction (MVIC) (Prone Full Can). It should also be noted that there was significantly greater middle and posterior deltoid EMG activity during the empty can exercise, which can contribute to superior humeral head migration and a predisposition to subacromial impingement. Biomechanically speaking, when the humerus is elevated in a position of internal rotation, it does not allow the greater tuberosity to clear from under the acromion as it does in a neutral or externally rotated position. Based on these findings, why are we still using the more provocative Empty Can when the better tolerated full can exercise is just as effective?The answer for this is still unclear.

Earlier in 2004, Reinold et al conducted a similar study looking at a broader range of exercises and muscular contributions of the supraspinatus, infraspinatus, teres minor, and posterior & middle deltoid. It was determined that the top three exercises based on %MVIC were Prone Horizontal Abduction at 100° with full external rotation (82% MVIC), Prone External Rotation at 90° of abduction (68% MVIC), and Standing External Rotation at 90° of abduction (57% MVIC). As previously stated, selecting exercises that activate the supraspinatus while minimizing the activity of the deltoid musculature is of importance in the rehabilitation of most shoulder pathologies. This information could make utilizing the Prone Horizontal Abduction at 100° with Full External Rotation and Prone External Rotation at 90° of abduction detrimental to proper rehabilitation as substantial deltoid activity was recorded at 82% and 79% MVIC, respectively.

In lieu of this additional consideration, the most appropriate exercises for isolated strengthening of the supraspinatus are the Full Can, Prone Full Can, and Side-lying ER (51% MVIC). Once progressing to a more functional program targeting scapular and glenohumeral stability, it must be noted that this does not necessarily warrant the discharge of more targeted rotator cuff exercises in favor of seemingly more difficult weight-bearing exercises. Uhl et al evaluated several common weight-bearing exercises and of the exercises evaluated, the most intensely the supraspinatus was engaged was only 29% MVIC during a single-arm push-up.

Infraspinatus:

The infraspinatus originates distally to the supraspinatus at the infraspinous fossa of the scapula and attaches laterally at the middle facet of greater tubercle of the humerus. Based on this muscle’s origin and insertion, its primary action involves external rotation of the humerus. The secondary actions of the infraspinatus include horizontal abduction, glenohumeral compression at the glenoid fossa, and resistance to superior and anterior humeral head translation. This is why many diagnostic special tests for impingement or rotator cuff integrity will also assess the strength of the infraspinatus (e.g. the Resisted Isometric External Rotation Test). The previously mentioned study conducted by Reinold and colleagues also evaluated the %MVIC of the infraspinatus musculature during the same exercises. The three most demanding exercises for the infraspinatus included Sidelying External Rotation at 0° of abduction (62% MVIC), Standing External Rotation in the scapular plane (53% MVIC), and Prone External Rotation at 90° of abduction (50% MVIC). Based on the line of pull and perceived action of the infraspinatus, this data seems to make reasonable sense. Additionally, when having your patient perform side-lying external rotation, consider having them place a rolled towel between their arm and their torso. By making this simple adjustment, it is postulated that the muscles controlling adduction and those performing external rotation are more appropriately balanced. The data from these two exercises agree with this theory as %MVIC of the infraspinatus increased from 20% to 25% MVIC with the addition of a rolled towel. When progressing to more functional weight bearing exercises, Uhl et al determined that infraspinatus activity was substantially more active than the other rotator cuff musculature measured. They found the Push-up with feet elevated (52% MVIC) and One-armed Push-up (82% MVIC) to be especially challenging. Obviously, these are more challenging activities and are not appropriate for every patient, but may be beneficial adjunctive exercises for the more advanced clientele.

Teres Minor:

The smaller, but still important, teres minor attaches inferior to the infraspinatus at the lateral border of the scapula and inserts onto the inferior facet of the greater tubercle of the humerus. The teres minor’s primary role is external rotation and stabilization of the humeral head within the glenoid fossa. Secondary actions include adduction and horizontal abduction of the humerus. While, in general, teres minor performs similar actions to the infraspinatus. It provides drastically less activity during flexion, abduction, and scapular abduction than the infraspinatus. It may be isolated with the Dropping Sign Test, which has been shown to have poor sensitivity, but good specificity (Hertel et al).  With regards to exercise prescription, the three most demanding exercises (as determined by Reinold and colleagues) are Sidelying External Rotation at 0° of abduction (67% MVIC), Standing External Rotation in the scapular plane (55% MVIC), and Prone External Rotation at 90° of abduction (48% MVIC). Once again, given this muscle’s line of pull, the data with regards to these exercises make both biomechanical and anatomical sense. Horizontal abduction was also evaluated as it was shown to have substantial activation (74% MVIC) in a previous study conducted by Townsend et al. However, Reinold’s more recent study showed a relatively small contraction of both the infraspinatus (39% MVIC) and teres minor (44% MVIC). As both the infraspinatus and teres minor are the primary external rotators at the glenohumeral joint, it is important to understand which exercisesimultaneously activates both muscles. Reinold has determined the exercises that elicite the highest combined EMG activation are shoulder ER in side-lying, standing ER in the scapular plane at 45° of abduction, and prone ER in 90° of abduction.

Subscapularis:

The subscapularis is the only rotator cuff muscle that is located on the anterior surface of the scapula. With an origin at the subscapular fossa and insertion laterally onto the lesser tubercle of the humerus, its primary action is internal rotation of the humerus. Secondary actions include humeral adduction, production of an abduction torque during arm elevation, glenohumeral compression, and anterior stabilization of the glenohumeral joint. As with the supraspinatus, the subscapularis can produce its maximal force when the humerus is positioned at 0° of abduction. As the abduction angle increases, the moment arms of the inferior and middle heads stay relatively constant.However, the moment arm of the superior head progressively decreases until approximately 60° abduction, which translates into diminished torque production. There have been many studies with conflicting results in terms of optimal abduction angle for subscapularis force production. In place of definitive EMG conclusions, potential for compensation should be taken into consideration. With the arm positioned at 0° of abduction, Decker et al found increased activation of the pectoralis major, latissimus dorsi, and teres major, which indicates a greater potential for substitution and masking of subscapularis weakness. In contrast, it was determined that pectoralis major activity decreased substantially when internal rotation was performed at 90° of abduction.

While this study did not clear up any of the murkiness in regards to subscapularis activation, it did offer assistance in avoiding or detecting compensatory muscular substitution. Decker et al also determined three more complex movements that created substantial subscapularis activity. The Push-up Plus (135.5% MVIC), Diagonal (99.7% MVIC), and Dynamic Hug (94.1% MVIC) exercises all created subscapularis activity that exceeded performance during isolated internal rotation exercises. Although these values are much greater than those reported for isolated internal rotation exercises, the potential for muscular substitution is likely with these more dynamic and functional exercises. Based on these considerations, isolated internal rotation should not be dismissed, but rather used in conjunction with these more demanding activities.

Additional Considerations:

Movement at the glenohumeral joint is complex and dependent on many different muscular actions as well as the contributions of several other joints. While strengthening of the rotator cuff and scapular stabilizers is often directed for patients with shoulder pathology, other regions and joints must also be evaluated. Contributions of the cervical spine, thoracic spine , scapulothoracic joint, acromioclavicular joint , sternoclavicular joint, and passive ligamenous structures can affect the underlying pathology and subsequent rehabilitation process. This is why manual therapy is often directed at these areas. Not only can it increase  mobility, but it can also improve the quality and strength of the exercise (Bang et al).

Additionally, scapulothoracic and pectoral musculature is also necessary for optimal shoulder biomechanics and must be addressed when appropriate. A recent continuing education course taught by Lenny Macrina, MSPT, SCS, CSCS provides a solid evidence-based explanation of the various factors involved in a successful rotator cuff and/or subacromial impingement program (“Glenohumeral Joint Biomechanics and Rehabilitation Implementation”). A comprehensive impairment-based program focused on muscular strengthening, neuromuscular control, endurance, joint hypo/hypermobility, and surgical precautions must be implemented to treat pathologies related to the glenohumeral joint.