I want to be provocative (but I don't want to step on the feet of anyone):
I feel inspired by a comment. Thank you.
Those who tell me that Ashtanga yoga does not hurt have never practiced it:
What happens when we stretch or when we build strength?
Usually we cannot go as far as we want with the body. The body simply does not allow as to do what we want. When we go further we feel something. This something that is felt is first a slight feeling which becomes stronger the further we go. This feeling in the body is called pain. It is a message from the body, I appreciate this message. It is possible to stand this pain easily when focusing on something else, i.e. the breath and being attentive nevertheless. Being relaxed despite the pain is something we learn and that brings the success (here flexibility). With time we can go further and further, we can stand more and more pain, but we know when we have to stop in order not to injure ourselves, overstretch ourselves.
Pain is a message, it can be joyful also. Positive helpful pain comes slowly. Enjoy.
In order to become more flexible and strong we must learn to handle pain. We must learn to interprete it, to be attentive to it. Pain is my fingerpost.
Becoming more flexible and stronger without any pain is not possible.