http://AskTheTrainer.com/rotator-cuff-exercises.html Internal rotation isolates your subscapularis which is responsible for concentric internal rotation of your humerus. Since your pec muscles also help internally rotate if you have a weak back musculature you do not want to strengthen your internal rotators any more. Perform external rotation instead. Be careful with rotator cuff exercises. It is recommended to get a prescription from a qualified professional before you start making a routine of performing rotator cuff exercises. If they are not necessary there are more effective exercises to occupy your time in the gym. Personal Trainer Michael Behnken, MS, NASM-CPT-PES, CSCS