Life drawing: yoga poses with Nina

I'm still focusing on line drawing so I once again decided to leave the Conte crayons and stick to using charcoal pencils and graphite pencils.
Nina began with a series of two-minute poses in different yoga positions, which created some interesting shapes, especially with feet appearing in unexpected positions... It was a good exercise in trying to capture overall shapes very quickly.

Then came two poses of 10 minutes:

This final drawing took 25 minutes, using a 4B pencil. The model was facing away from me with here elbows leaning on a small stool (hidden from view) and her hands in a praying position. Due to restrictions of space, I had to do this one from a standing position holding the sketchpad in my hand.

I though the shape of the model's back, the visible line of her spine, and the upturned soles of her feet,  would have made this a good pose to draw, but for some reason, I don't think it really comes off at all well. I'm not sure quite why, but I just wasn't on form....