Problems with seated groin stretch

The seated groin stretch is great, but i tend to arch my back, meaning it isn't straight at all.
Is this because i lack flexibility in the legs or power in the back???
Help please....

When you are doing this are you pushing your knees to the ground with the help of your elbows or are you just pushing your legs down using your muscles in your legs?
I do it without elbows as I feel more of a stretch and I feel that it helps keep my back straight rather than when using the elbows.
The best thing to do is try both and see which works best good luck and I hope this helps