The rotator cuff muscles group consists of Supraspinatus, Infraspinatus, Teres minor, and the Subscapularis muscles. The primary function of the rotator cuff is to keep the head of the humerus depressed and centered into the glenoid fossa permitting a single center of rotation while allowing efficient abduction or forward elevation of the arm (Saha 1971, Basmajian et al., 1959). Supraspinatus is the primary muscles among other rotator cuff muscles to provide stability of the humerus in the glenoid fossa and also aids in abduction along with the deltoid (https://healthnphysio.com/supraspinatus/). Rotator cuff weakness or injury to supraspinatus muscle has been reported the primary cause of shoulder or subacromial impingement syndrome (https://healthnphysio.com/subacromial-impingement-syndrome/). Supraspinatus strengthening is thus the significant part of shoulder injury rehabilitation to improve or maintain the shoulder stability.
Supraspinatus strengthening has been postulated to offset the upward translation force of the deltoid thereby reducing the superior shear of the humeral head on the glenoid fossa (Wendy S., et al 2002). Rotator cuff strengthening exercises, mainly supraspinatus strengthening exercises have been advocated by many studies as an integral part of treatment for subacromial impingement. Among many exercises reported to strengthen the supraspinatus, the empty can (EC) and the full can (FC) has been researched extensively. Although electromyography and magnetic resonance imaging studies (Malanga et al., 1996; Yoshitsugu et al., 2002)on the activity of supraspinatus muscle have shown both EC and FC exercises as the most effective exercise for supraspinatus strengthening, recent studies have recommended FC over EC. FC has been shown to be more effective in supraspinatus strengthening without increasing impingement by limiting the reduction of the subacromial space (Tino et al., 2010).
The Empty Can Exercise
The empty can exercise was first advocated by Jobe and Moynes, 1932. The EC is performed by elevating the arm 90 degrees in the scapular plane (30 degrees anterior to the frontal plane) while maintaining shoulder internal rotation. Townsend et al., 1991, who used electromyography to evaluate 17 shoulder exercises to establish which exercises were most effective to strengthen the rotator cuff muscles. With respect to the supraspinatus, four exercises were found to produce high levels of electromyography output. Of those four, the highest electromyography 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 electromyography output for the supraspinatus but also the greatest electromyography activity for the anterior and middle deltoid and Subscapularis. Based on these data, Townsend et al., recommended that the empty can be one of the core exercises in a shoulder rehabilitation program for the rotator cuff and glenohumeral muscles. However, athletes and patients reported pain in their shoulder during the EC. The pain associated with the EC has been attributed to the mechanical compression and impingement from the reduction of the subacromial space.
A recent study done by Yasojima et al., 2008, has shown the increased activity of middle deltoid when performing empty can (EC) compared to full can (FC). This excess amount of deltoid activation in comparison with the supraspinatus activity leads to superior humeral head migration and a decrease in subacromial space (Reinold et al., 2009). The scapular protraction has been reported to cause internal rotation and anterior tilting, decreasing the subacromial space. The EC exercise has been shown to produce more internal rotation and anterior tipping of scapula compared to FC exercise even if EC is done at just 30 and 60 degrees elevation (Thigpen et al., 2006). Many recent MRI studies have also demonstrated the significant diminish in subacromial space with the arm abducted and internally rotated position (Jull et al., 2000).
Based on these data empty can exercise may not be the ideal exercise for supraspinatus strengthening especially during the subacromial impingement syndrome rehabilitation. Recent studies have proposed full can exercise over empty can for supraspinatus strengthening exercise.