Thanks for the question. This particular question comes from EMG research conducted by Mike Reinold, Kevin Wilk and colleagues. In there studies, They demonstrated that full can scaption and sidelying ER at 0 abduction are the best option to for rotator cuff activation and decreased deltoid activity. In our question that incorrect answer should have been standing ER with towel under arm, not sidelying with towel under arm (as this exercise wasn’t tested). Sorry for that error, and making this question overly complicated. We’ll email you over the articles referenced above.