In general - urdhva dhanurasana versus dhanurasana.
They are different: first is to deliver more stress on spine, the second one - on knees. So try to find soft way in - open your knees, feel them strong, concentrate on your lower back. Don't forget - it is for your back. There are many asanas to deliver stress on the knee. Padmasana for example. Some beginners keep pushing the knee down, ignoring the pain. However the knee is weaker in comparison to the hip joint and gives up first. And the pain is there until hip rotates to accomodate leg position. Change body position to pin knee to the ground and you can work out hip without any pain in the knee whatsoever!
Here is my back workout:
salambasana - variations with arms pushed in the floor
sarpasana - one I referred to earlier
ustrasana without arms support (camel variation with arms folded or streched abouve head)
tadasana, from there back bend with arms down until you get grip on your heels ( note: arms down all the way, not abouve the head )