Subacromial impingement syndrome, the most common cause of shoulder pain, has been implicated as contributing to rotator cuff tears and glenohumeral joint instability. 19 Clinically, the most common symptom of subacromial impingement is pain during active arm elevation, particularly between 70° to 120°. 45 Along with shoulder pain, crepitus, weakness, and tenderness have been reported. 40,44 Impingement is most common in individuals who do repeated overhead activities for work or sport. 40,52
Several authors described the etiology of subacromial impingement syndrome as a mechanical phenomenon in which there is repetitive compression of the supraspinatus tendon between the humeral head and the acromion and coracoacromial ligamentous complex. 9,22,45,53 This process is thought to cause inflammation, fibrosis, and ultimately rupture of the rotator cuff, or more specifically, the supraspinatus tendon. 11,40,44 Irritation and tearing of the supraspinatus tendon leads to shoulder pain and impairment. Several factors have been hypothesized to contribute to this syndrome, including rotator cuff weakness, instability patterns, glenohumeral capsular tightness, muscle imbalance, shape of the acromion, and altered glenohumeral rhythm. 1–4,7,10–14,17,19,20,24,29,35,37,51
One component that is addressed in the conservative treatment of subacromial impingement syndrome is rotator cuff muscle strengthening (supraspinatus, infraspinatus, teres minor, and subscapularis). 26,30,52 This premise is based on the belief that with subacromial impingement, there is weakness or an imbalance of muscle forces acting at the shoulder, resulting in excessive humeral head elevation, and ultimately, compression of the subacromial tissues between the greater tuberosity and the acromium. 42,46
Adequate strength of the rotator cuff has been reported to offset the upward translation force of the deltoid thereby reducing the superior shear of the humeral head on the glenoid fossa. 42,46 In addition, the teres minor and infraspinatus muscles have lines of action that provide lateral rotation, a motion necessary to clear the greater tuberosity from under the acromial head during arm elevation. 8,47 The actions of the rotator cuff and deltoid form a force couple that ensures that the head of the humerus spins around a fixed axis of rotation with minimal superior translation. 37,50,55
Exercises to strengthen the rotator cuff muscles have been defined as an integral part of treatment for subacromial impingement syndrome. 5,30,35,43,55 Because the mechanical nature of subacromial impingement specifically involves the supraspinatus tendon, understanding the exercises and resulting stresses applied to this structure during the rehabilitative process warrant special consideration. The clinical rationale and the specific exercises advocated for strengthening the supraspinatus were evaluated.
The Role of the Supraspinatus in Normal Shoulder Function
The supraspinatus is a key component of the rotator cuff and a necessary element of normal shoulder function. It arises from the supraspinous fossa of the scapula and inserts on the superior facet and superior ½ of the middle facet of the greater tuberosity. 41 Given its origin and insertion, the supraspinatus is an abductor of the shoulder. 15,25 Contrary to early views, the supraspinatus is active throughout all ranges of arm elevation as opposed to being only the initiator of abduction. 25,27,28,39
As a result of its fiber orientation, the supraspinatus also is an effective stabilizer of the glenohumeral joint, especially at 90° shoulder abduction. The compressive forces of the supraspinatus and other glenohumeral muscles center the humeral head in the socket which forms a dynamic fulcrum for the deltoid (Fig 1). 8,18,42,55 However, unlike the other cuff muscles, the supraspinatus (along with the deltoid) contributes to a slight superior translational force on the humeral head rather than a downward force, particularly at the initial angles of arm elevation. Although strengthening the supraspinatus might seem to facilitate impingement, the amount of the total muscle force that contributes to superior translation is only 4%, whereas the remaining 96% of the total force production provides compression (Fig 2). 46 This compressive force centers the humeral head in the glenoid fossa providing a stable pivot point.
If the supraspinatus becomes weakened or injured, the normal balance of forces acting on the glenohumeral joint are altered. With supraspinatus weakness, there is less of a dynamic fulcrum from which the deltoid can pivot during abduction, causing the humeral head to translate superiorly (Fig 3). 16,18,55 In addition, loss of supraspinatus strength and subsequent decrease in glenohumeral compressive forces can contribute to increased joint translation. 27,49,50
Mechanical Function of the Supraspinatus
The ability of the supraspinatus to generate torque about the shoulder is dependent on its mechanical leverage. Torque is a function of a muscle’s force-producing capacity and the location of its line of action relative to an axis of rotation (joint center). 38,47 The perpendicular distance from the muscle’s line of action to the joint center is referred to as its moment arm (Fig 4). Mathematically, torque is the product of force and the moment arm length. 38,46 The larger a muscle’s moment arm, the greater its capacity to generate torque required to produce motion and counteract external loads. However, when a muscle’s lever arm is decreased, it must generate a larger amount of force to compensate for the diminished mechanical leverage.
The supraspinatus moment arm length has been quantified in vitro in two studies 47,50 and in vivo with the use of magnetic resonance imaging (MRI). 33,34 Using fresh-frozen specimens from cadavers, Poppen and Walker 50 reported a moment arm length of 2.2 cm throughout the abduction range except at maximum elevation. Measurements in their study were made from radiographs obtained with the arm in a neutral position (0° internal and external rotation). Otis et al 47 estimated the supraspinatus moment arm length on 10 fresh-frozen cadavers by measuring muscle excursion with known torques applied to the arm. The results showed a moment arm length of 2.5 cm for the anterior portion of the supraspinatus during abduction in neutral rotation. With the arm in internal rotation, the abductor moment arm of the supraspinatus decreased from 2.5 to 2.2 cm, which is consistent with the tendon having dropped slightly from the top of the humeral head. With the arm in external rotation, the abductor moment arm length increased to 2.8 cm. The results of Otis et al 47 are consistent with results of a recent in vivo MRI study that found the moment arm length of the supraspinatus with the arm in neutral rotation and 0° abduction to be 2.4 cm, and 2.6 cm at 34° abduction with slight external rotation. 33
Strengthening of the Supraspinatus: The Empty Can Exercise
One of the most common exercises used to strengthen the supraspinatus is the empty can exercise, which first was advocated by Jobe and Moynes. 31 Justification for this exercise was based on an electromyographic record from one individual who had maximum supraspinatus activity when asked to resist a downward force applied at the wrist with the shoulder abducted 90° (elbow extended), horizontally flexed 30°, and full internal rotation (Fig 5). 30 Although Jobe and Jobe 30 concluded that the empty can position maximally challenged the supraspinatus, the validity of this premise can be questioned as only one subject was evaluated.
Additional evidence in support of the empty can exercise was provided by Townsend et al 54 who used electromyography to evaluate 17 shoulder exercises to establish which exercises were most effective to strengthen the rotator cuff muscles. 54 With respect to the supraspinatus, four exercises were found to produce high levels of electromyographic output. Of those four, the highest electromyographic output was the military press, which resulted in an activity level of 80% of the electromyography obtained during a maximal manual muscle test. This was followed by abduction in the scapular plane with the arm in internal rotation (74% maximal manual muscle test), forward flexion (67% maximal manual muscle test), and finally abduction in the scapular plane with the arm in external rotation (64% maximal manual muscle test). Scapular plane elevation with internal rotation (the empty can exercise) was determined to be the leading exercise as it resulted in not only the second highest electromyographic output for the supraspinatus but also the greatest electromyographic activity for the anterior and middle deltoid and subscapularis. Based on these data, Townsend et al 54 recommended that the empty can be one of the core exercises in a shoulder rehabilitation program for the rotator cuff and glenohumeral muscles. 54
Despite widespread use of the empty can exercise for strengthening the supraspinatus in patients with subacromial impingement syndrome, several authors caution against using this exercise, especially between 70° to 120° arm elevation. 32,36,54,56 This recommendation is based on recent MRI studies that have shown the subacromial space is diminished significantly with the arm abducted and internally rotated. 6,16,17,23,32
Using radiographs and MRI, Brossmann et al 6 investigated the relationship of the distal supraspinatus tendon to the coracoacromial arch at various shoulder positions in three cadavers. Using radiopaque and gadolinium-impregnated markers sutured to the distal aspect of the supraspinatus tendon and along the coracoacromial ligament, Brossmann et al found that impingement was most evident with the shoulder placed in a position similar to that of the empty can exercise (60° flexion, 60° abduction, and internal rotation).
Graichen et al 16 measured subacromial space changes using an open MRI system and three-dimensional reconstruction techniques in 12 healthy volunteers (five positions of abduction and three positions of rotation). 16 They found that at 90° abduction and internal rotation, the minimal acromiohumeral distance passed directly through the supraspinatus at precisely the location where most rotator cuff tears occur.
Given the limitations noted above, the tradeoff for eliciting greater motor unit activity from the supraspinatus using the empty can exercise is the risk of contributing to subacromial impingement. Because mechanical impingement is thought to be a primary cause of supraspinatus degradation, it would seem that preventing additional impingement should take priority when prescribing exercises for this condition.
Alternative to the Empty Can Exercise: The Full Can
Given the recognized limitations with the empty can exercise, several authors have advocated an alternative, the full can exercise. 5,16,28,36 The full can differs from the empty can in that patients elevate their arm with the elbow extended in the scapular plane with the arm in external rotation instead of internal rotation (Fig 6). The obvious advantage of the full can relative to the empty can is that the external rotation clears the greater tuberosity from under the acromion during elevation and minimizes potential impingement. 16,21,48
Although Townsend et al 54 reported that the full can exercise did not generate as high supraspinatus electromyography as the empty can exercise, the difference in recorded electromyography can be explained by the biomechanical differences between these two exercises. The 15% reduction in supraspinatus electromyography with the full can exercise corresponds with the fact that in external rotation the supraspinatus lever arm is 22% greater as compared with internal rotation. Therefore, for a given external load, and a larger moment arm, less force would be required of the supraspinatus. With the arm in external rotation, the supraspinatus is functioning at a mechanical advantage and thereby would require less force production.
Additional evidence in support of the full can exercise was provided by Kelly et al 37 who used electromyographic data to test 29 positions used for strength testing of the rotator cuff. The criterion for identifying the optimal manual muscle test for each rotator cuff muscle in their study were maximal activation of the cuff muscle, minimal contribution from involved shoulder synergists, minimal provocation of pain, and good test and retest reliability. Given these standards, Kelly et al 37 found that the best test position for the supraspinatus was 90° scapular elevation and 45° external rotation.
Repeated mechanical compression of the supraspinatus and subsequent weakness has been identified as a causative factor of subacromial impingement. Rotator cuff weakness is thought to perpetuate impingement as the ability to counter the superior shear forces produced by the deltoid during arm elevation is diminished. Therefore, strengthening the supraspinatus has become a main component in rehabilitation for individuals with subacromial impingement.
Based on the data presented above, the empty can position seems to have two disadvantages. First, placement of the arm in internal rotation does not allow for the greater tubercle to clear from under the acromium and places the patient at risk of impingement during arm elevation. Second, an internally rotated position decreases the moment arm length of the supraspinatus, which increases the demand placed on the muscle. By exercising a muscle in a position of diminished mechanical leverage, increased forces, and tensile stresses would likely result. As one of the primary goals in the rehabilitation of an individual with subacromial impingement syndrome is to promote supraspinatus tendon healing and restore normal glenohumeral joint mechanics, the exercises prescribed should minimize stress on the injured tissues.
To clear the greater tubercle and avoid mechanical impingement, external rotation or neutral humeral position rather than internal rotation seems to be indicated. The externally rotated position has been shown to increase the subacromial space, thereby minimizing impingement of the subacromial tissues between the acromion and the greater tubercle. 32 In addition, the moment arm of the supraspinatus is increased with external rotation, providing greater leverage and an increased ability to provide compressive force and stability compared with internal rotation during arm elevation. 50,56
Clinicians should be cognizant of the advantages and disadvantages of commonly prescribed exercises used to strengthen the supraspinatus. Traditionally the empty can exercise has been prescribed to isolate and strengthen the supraspinatus in persons with subacromial impingement. However, the empty can exercise has significant disadvantages: (1) it places the arm in elevation with internal rotation which decreases the subacromial space and increases the risk for mechanical impingement, and (2) the internally rotated position places the supraspinatus at a mechanical disadvantage by decreasing its lever arm.
However, the full can places the arm in external rotation, thereby increasing clearance of the greater tuberosity during arm elevation. Additionally, the moment arm of the supraspinatus is increased with external rotation, thereby providing a better mechanical advantage for protecting and strengthening the supraspinatus. From a biomechanical perspective, the full can exercise seems to be a viable alternative for strengthening the supraspinatus in individuals with subacromial impingement.
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