The first obstacle almost all meditators run into is pain while sitting. It’s the easiest obstacle to solve. The answer for most is the “kneeling chair” or “ergonomic chair” (I use the WL-SB-210-GG model on Amazon) I go into posture thoroughly in the book but the short story is: stand tall, with ears in line with the shoulders and elbows, and sit down on an angled seat. Sit comfortably, for hours, no movement, no pain, just as Patanjali advised. If you need a portable seat, use a seiza bench (the Omni bench from Dharma Crafts is well made).
What’s the first meditation instruction usually given? “Sit down and cross your legs.” Why cross your legs? “Because that’s what we do.” So you cross the legs and twist the knee joint that isn’t made to be twisted and pain starts in 15 minutes. If you are one of those with “loose joints” and your knees lie comfortably on the floor, cross-legged is ok. If the knees aren’t lower than the pelvis by a half-foot, natural curvature of the back is lost and back pain follows.
So why aren’t anatomically appropriate sitting postures used? Tradition. Buddhists use robes, yogis use shawls, Buddhists sit on zafu cushions, yogis sit on bags of buckwheat hulls. Be practical, use your own head and experiment.