It reminded me of when I talked about the unbendable arm with my wife. She has no problem doing it at all, and it's because she did ballet for 7-8 years.
She said relaxation and extension are the basic of ballet.
Once she had me do the "first position", where one of your foot in front of the other but facing oppsite direction and have both of your arms up like hugging a giant tree. You are not suppose to tense up to hold your arms up, your shoulder should stay level, not hunching(?) your shoulder/back at all.
That was tough, but if you relax and extends, you can do it for a long time.(which I can barely do 2 minutes)