I suggest you pay attention to whether the action is standardized. If the deltoid doesn't hurt, it must be borrowed from other muscles. You have to change your weight first and find yourself feeling heavy. Non-standard movements are tantamount to practicing in vain. Lift the bird sideways and pay attention to the relaxation of the forearm. The big arm has a small arm, the deltoid muscle contracts first, and the trapezius muscle (neck) is not tense. These are the muscles that deltoid loves to borrow. Pay attention to change it slowly when you do it. It was like this at first. I did it for half a year. At that time, every time the deltoid muscle didn't hurt, I couldn't lift my neck the next day. Practice oblique square, hehe. Later, I found that I felt 1 time, and suddenly I did. Take your time.