This study was approved by the University of Michigan IACUC. 6-month old male Sprague Dawley rats (N=6) were used. Rats were anesthetized with 2% isoflurane, and the skin above the shoulder was shaved and scrubbed with povidone iodine. To create a full thickness supraspinatus and infraspinatus tear and prevent tendon reattachment, the right shoulders underwent a supraspinatus and infraspinatus tenectomy (19 (link)) after visualizing the tendons through a field established by a deltoid splitting incision and transacromial approach (20 (link)). The left shoulder underwent a sham surgical operation in which a transacromial approach was performed, but the tendons were left intact. The deltoid was closed using absorbable 3-0 chromic gut suture (J&J), and the skin and fascia closed using 5-0 nylon (J&J) and GLUture (Abbott). Subcutaneous buprenorphine (0.05 mg/kg) was administered for analgesia during postoperative recovery. Ad libitum weightbearing and cage activity were allowed, and rats were monitored for signs of distress or infection. 30 days after later, rats were anesthetized with pentobarbital (50 mg/kg), the supraspinatus and infraspinatus muscles were removed and their masses were recorded. The rats were euthanized by anesthetic overdose and induction of bilateral pneumothorax. Supraspinatus muscles were finely minced and prepared for RNA isolation. The distal halves of the infraspinatus muscles were used for histology, and the proximal halves were used for muscle fiber contractility.