Leg behind head is an asana one has to understand. Too much ambition is not needed here, but perseverance and understanding.
Eka pada sirsasana opens the hips. The leg moves backwards and backwards and backwards till one should try to bring the leg behind the head and put too much pressure on the knee.
It should be possible to bring the knees on the floor next to the body when lying on the back before one tries this pose.
Btw, have you realized already that marichyasana A is a preparation for this advanced pose. When this is seen, it's also clear that the foot of the bended leg is not close to the stretched leg, but the rim of the foot is in line with the outer body!
Understanding how important it is to move the leg backwards and not forcing the lower leg behind my sensitive neck helped me to practice safe. I don't want to hurt my knee.
It's easier to do yoga nidrasana first, then eka pada sirsasana. Yoga nidrasana becomes even a bit easier when it's practiced on the bed first.
To consider three further points improved eka pada sirsasana. I read at least 2 hints in the book by Gregor Maehle.
1. I point the foot nowadays. This makes a huge difference.
2. The chest opens, I learned.
3. The back muscles are challenged. When the back muscles are active the chest opens. It's all interconnected. Alone the awareness of these points improved the asanas.
Fact is, the further down the leg is the better it is. If it's too high it can be that too much pressure is on the neck.
Today I practiced primary. Primary has already one pose where the legs should bo posed behind the back. It's supta kurmasana. For those who have not soooooo looooooong legs it's difficult to cross the legs in front of the head. In order to practice supta kurmasana (for those with the shorter legs) it's necessary to do dwi pada sirsasana first (both legs behind the head while sitting). Then one can lower the body and the legs are crossed behind the back. The fingers can then hook behind the back, too.
This is the reason why I practice these second series poses when I practice primary at home.
Yes. I need the sofa to get into that pose. I lean against it when I take the legs in the dwi pada sirsasana position. This pose can only done when I manage it to keep one leg in position without holding it. I think I progressed here.
Yeah, Ashtanga yoga...hahahaha......:)
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