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
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.
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
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.
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
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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