Ailee discussed hurting her knees for her "Room Shaker" choreography. 

At the showcase for her 2nd full-length album 'butterFLY' on July 2, Ailee revealed, "A lot of people acknowledge me for my vocals, and I think it'll be the same this time around. There aren't a lot of high notes. Please pay attention to the part before the chorus where the melody stacks up." 

She continued, "We put a lot of thought into our choreography and performance. I practiced while putting medicine on my bruised knees."

Ailee made her comeback with "Room Shaker" today.