I am currently training for my Black Belt in a shotokan like style of Karate and I have been told many times by my teacher that I need to improve my back stance *Kōkutsu-dachi* as my bum sticks out and my front knee drops in (which is causing joint issues) as I am currently unable to open my hips to a 90 degree angle.

Are there any exercises that I can do in my spare time to improve my stance. I have tried to gently push my front knee into the correct position but this is quite painful. What stretches should I be doing to improve my stance?

I feel that if I am able to get my knee into the correct position, my bum will stop sticking out so fixing that is my number one priority.