Hi Alex,
That's a great question! When you're arching back, place a ball or yoga block behind your head. If that's still not enough, place a pillow behind your upper back and a ball/block behind your head. If you do this often, you should see improvement in the flexibility of your spine. You could also help your spinal mobility by practicing spinal rotations and lateral flexion exercises and stretches. Try lying sideways on the arc with knees stacked and then reaching your top arm up and then behind you and letting your spine rotate in that direction as well, looking up or over your shoulder. Deep breaths here. This should also help "loosen" the spine up.
I hope this helps :)
Gabrielle