I am trying to achieve a side star elbow plank. I am quite adept at the regular elbow plank (5 mins) and side planks (1.5 minutes). However, I can not even hold a side star plank for 10 seconds. It is physically almost impossible to keep my leg up in the air. Rather quickly I get pain on the side of my leg and the lower part of the glutes (perhaps gluteus maximus or obturator interanus) that makes it difficult to do.
I have strengthened those muscles a lot with high resistance biking and squats.
What is going on that is making it difficult for me to do a side star plank? Is this normal? How can I strengthen my leg so it can do this better?