I’m no stranger to shoulder problems as I suffered through four shoulder surgeries during my time as an athlete. Throughout all my rehab I always wondered what was the purpose of placing a towel under the arm during external rotation movements. Well if you don’t know why you’re doing it either then you’re in the right place.
Cable External Rotation
The towel actually increases activation of the rotator cuff 23% according to a study in the Journal of Orthopaedic and Sports Physical Therapy. It helps to de-activate the deltoid and avoid compensation patterns. Without the towel the deltoid can assist in the movement and actually overpower the rotator cuff.
Side Lying External Rotation
When this happens the humeral head can superiorly migrate. This isn’t a good thing when doing anything as the two main functions of the rotator cuff are to 1. stabilize and 2. depress the head of the humerus in the glenoid fossa. This is why its important not to overdevelop the deltoid, or negate training the rotator cuff.
Deltoid pulls humerus superior
The deltoid and rotator cuff are in a constant battle for humeral head placement. A weakened rotator cuff or too much deltoid strength causes the upward shift, and with this upward movement comes impingement, bursitis, rotator cuff tears, etc. The sub-acromial space is already small enough and we don’t want to shrink it even more.
We don't want to close the sub-acromial space anymore!