Do they help with back ache? Mines getting worse by the day
i've only known it given in the context of SPD, but you could try it! To be honest, I don't quite understand the mechanics behind it and what is actually going on with the public symphysis when you do the motion.
BUT, what kind of back pain are you having? general low back pain or SI joint pain (one-sided pain)? for the former, it would help to stretch the low back and hamstrings. for the latter, well...that's an alignment issue and tricky to fix when you've got the relaxin hormone flooding through your pelvis causing laxity in your ligaments.
and that's all assuming you directed that question to me and not a general Q about the birthing ball!!!
though now that i think about it, you probably meant it as a more general Q about the birthing ball.