If you can line up your legs to be vertical - in your plumb line, stacking your bones to balance and bear weight you will be using your body in the most efficient way, rather than having any particular group of muscles working too hard. It's much easier to do this with your feet under your hips because of how gravity pulls on your body. Here's a quick lesson to explain exactly why and how!
My friend Kate and I were talking about our dodgy sacroiliac joints the other day. The topic came up because hers is hurting right now, following a popping noise in an early morning yoga class. Popping noises, when related to SI Joints, are alarming things. Popping noises, when related to ANY joints, are alarming things. Now, for the scientific lowdown on SI Joint dysfunction, please read this excellent wikipedia article.
I will add that in yogis, SIJ dysfunction is almost always a result of acquired hyper mobility in those joints. This hyper mobility generally develops over time, by repeated insult to the ligaments that hold the joint stable.
Extreme adduction (crossing the legs over your midline) and abdcution (taking your legs out to the sides), especially with bent knees, as in some of the poses below, is fine for many people. But not all. As a result of how our bones are shaped, and the loads and forces we subject them to (think tightening your psoas muscles and other external hip rotators with years and years of sitting), not all of us can move our legs in such a big range of motion in our hip sockets, and so, when we are trying to get into poses that we don't have the mobility for, we inevitably recruit flexibility from other joints - knees and SIJ's usually.
Over time, the ligaments that stabilise these joints get stretched and damaged, and the joints get destabilised. Not pretty. Very painful. Not necessary, but once it's happened, you live with the consequences for, probably, ever. Ligaments are generally accepted as being avascular - that is, they don't have their own blood supply, and since their job is to staibilise your joints, they are not very elastic either. Once they are overstretched, they don't go back to their original length.
(Side note: I found prolotherapy (a simple injection into the ligaments which causes them to form scar tissue & therefore tighten) useful, but the ligaments in my pelvis are still less stable than those of other people.)
My early yoga teachers didn't know this stuff, or if they did, they ignored it. I didn't question their teaching: I was too green back then to know that I don't bend right for yoga.
I got stood on in Baddha Konasana to get my knees to the ground (they still don't go to the ground, but now I have an unstable pelvis. Thanks, teacher, for standing on my knees).
I got taught to yank myself into twists, even the bound ones like Marichyasana D, below. The injury to my pelvis didn't show up until years later.
Things not to do with a shonky SIJ #1. With thanks to Ursula, whose SIJ seems to be fine.
I got taught (and, full confession, I WANTED) to reach for extreme ranges of motion, and then use my arms and legs as levers, to get into Supta Kurmasana, as below. My poor lower back!
Ursula again, with her (hopefully) well-functioning SIJ and bendy body...
Now, you can tell from the photos - some people can do this stuff. Not everyone. Certainly not me. Also, I have no idea if the woman in the photos has pain. She might - I haven't asked because I found her photos by way of Google, as you do.
Kate was asking whether I still manage my SIJ every day.
Yes. Every day, I work on strengthening the muscles that stabilise my pelvis and core. Every day, I work on improving the rhythm between my pelvis, low back and legs. Every day, I work on relaxing my psoas muscles.
And some days, I work on pain management, when my SIJ's are inflamed from too much sitting (on the back of a motorbike, for example).
Kate was also asking whether hip openers help with the pain or make it worse.
Mostly? They make it worse. Especially the kind where each leg is doing a different thing. I will republish the post about the things I do for happy SIJ's, soon, but for now, if you have SIJ pain, try practicing the constructive rest position:
Or Supta Baddha Konasana, possibly with a sacro-wedgy (or a rolled up towel, which is what I use). Hang out there for a long time, at least a few minutes, and you are looking for pain-free, or at least no increase in pain! THe picture below is old, so please excuse the terrible alignment. It's better now, I promise. Also, a lot of people with SIJ issues need to place cushions or bolsters under their thighs to have any chance of doing this without having pain afterwards.
Also, my all-time favourite psoas release, which helps immensely with the sacro-lumar rhythm, is this one from Katy Says: