With many studies reporting limitations with the empty can exercise, several studies have recommended the full can exercise as an effective alternative. Empty can exercise is emerging as the least choice in shoulder rehab exercises mainly in athletic population as it increases the possibility of subacromial impingement by activating the middle deltoid, and also by increasing the scapular internal rotation and anterior tipping decreasing the volume of the supraspinatus outlet (Thigpen et al., 2006).
To perform full can exercise, the patients elevate their arm in scapular plane with the arm in the external rotation instead of internal rotation, elbow extended, and thumbs pointing up towards the ceiling. 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 (Graichen et al., 1999).
Reinold et al., 2007, compared electromyographic analysis of 3 common shoulder rehab exercises; Full can, Empty can and prone Full can (prone horizontal abduction at 100 degrees with glenohumeral external rotation). ” While all 3 shoulder rehab exercises produced similar amounts of supraspinatus activity, the full-can exercise produced significantly less activity of the deltoid muscles and may be the optimal position to recruit the supraspinatus muscle for rehabilitation and testing. The empty-can exercise may be a good exercise to recruit the middle deltoid muscle, and the prone full-can exercise may be a good exercise to recruit the posterior deltoid muscle.”
Kelly et al., performed electromyographic analysis of 29 shoulder positions used for strength testing of the rotator cuff. Maximal activation of the cuff muscles, a minimal contribution from involved shoulder synergists, minimal provocation of pain, and good test and retest reliability were the criteria for identifying the optimal manual muscle test. The results showed the best position for the supraspinatus was 900 scapular elevations and 450 external rotations.
Based on the recent evidences full can exercise seems to be the most effective shoulder rehab exercises for strengthening the supraspinatus as: 1) It places the arm in elevation with external rotation which increases the subacromial space, thereby minimizing the impingement of subacromial tissue between the acromion and the greater tubercle, 2) the externally rotation position increases the moment arm of the supraspinatus providing mechanical advantage with greater leverage, an increased ability to provide compressive force, and stability compared with internal rotation during arm elevation. By exercising a muscle in a position of diminished mechanical leverage as in empty can exercise increases tensile stress for small output (Burke et al., 2002).
Thus from the biomechanical, muscle activation, stability, impingement, and pain perspectives, the full can exercise seems to be more viable than the empty can exercise.