For years, the debate has continued on the ‘best’ exercises for strengthening shoulder muscles. Using electromyography (EMG) to measure muscle activity, researchers have attempted to determine which exercises have the highest levels of muscles activity for strengthening exercises. Several studies have examined EMG of common shoulder exercises to find the best exercises to strengthen the rotator cuff; in particular, the supraspinatus muscle. The “empty can” or “full can” exercises have been the most commonly performed exercise for supraspinatus strengthening, but controversy still exists in the literature.
While high muscle activation levels are often desirable, what’s more important is the relative activation of other muscles during the movement. For example, Dr. Michael Reinold and colleagues suggested in their 2007 paper that a good rotator cuff exercise should produce the greatest supraspinatus activity while minimizing the deltoid activation. Theoretically, reducing deltoid activation decreases the upward shear of the humerus during arm elevation, which may be desirable when prescribing exercise to strengthen the suprapsinatus in impingement patients.
In a study published in Medicine and Science and Sports and Exercise, subjects performed 5 isometric exercises in random order while measuring the EMG activity of the deltoid, supraspinatus, and infraspinatus. The exercises were: full can, empty can, prone elevation, elastic external rotation, and prone external rotation. The researchers noted that all exercises produced similar high levels of supraspinatus activity, while the full and empty can exercises also had higher levels of deltoid activity.
The researchers concluded that shoulder external rotation at 0° of abduction with an elastic band and prone external rotation were preferable exercises for the supraspinatus. While the full- and empty-can exercises are traditionally favored to isolate the supraspinatus, the authors noted that elastic external rotation and prone external rotation did not activate the deltoid at high levels compared to the full- and empty-can exercises. In addition, the exercises also exhibited high levels of infraspinatus activation.
In summary, clinicians should consider using Thera-Band® exercise bands for resisted external rotation and isotonic prone shoulder external rotation exercises because they preferentially activate the supraspinatus and infraspinatus without activating the deltoid, which may be more beneficial for patients with impingement.
Reference: Boettcher CE, Ginn KA, Cathers I. Which is the optimal exercise to strengthen supraspinatus? Med Sci Sports Exerc. 2009;41(11):1979-83.