MN/PE9

I'm working on MN/PE9, and when I begin to lift my legs to complete a rep, I feel stain in my right shoulder.  The higher I lift my legs the greater the strain.  If its important, I'm doing this exercise from a pullup bar, not stall bars, so I don't have any back support.  Am I doing something incorrectly?

Yes this is not correct, on the pullup bar if you do not have a super strong shoulder flexion it is impossible to hold the shoulders open. try to arrange a stallbar or a similar support on the back.