I don't think yoga is good for these purposes. Yoga increases laxity of ligaments and passive support tissues and increases range of motion at the expense of stability, which will probably not improve posture and may increase your propensity for spine injury. The passive support tissues in your spine are probably already too lax, given your description, and yoga would probably just make this worse.
Yoga does not do that, unless you're doing it wrong. Of course, sometimes beginners won't know they're doing it wrong until it's too late.
From Stuart McGill: "Ultimate back fitness and performance":
Longitudinal studies have shown that the more flexibility one has in the back, the greater the risk is of having future back troubles.
Flexion and rotational stretching overloads the annulus fibers often exacerbating the spinal tissues which can occur unbeknownst to the individual. Yet they continue the practice, reporting that it "feels good". ... Generally they are ensuring that they remain chronic and will not make advances until stretching is stopped! Solomonov's group (2003) have shown that static stretching of the spine ligaments can cause muscle spasms and diminishes the stretch reflex. The reflex is a protective mechanism!
Do some mobilizing exercise approaches such as "yoga" or "pilates" work? They help some and hurt others. Flexibility without strength and motor control is useless.
Kinesiologists who specialize in spine issues do not agree on everything, but they all seem to agree on the principle of strengthening stabilizer muscles and grooving motor patterns that will keep the spine in a neutral position
before increasing flexibility, if at all, and even then, they tend to emphasize active flexibility with stable motion, as opposed to the kind of passive flexibility at the end of the range of motion emphasized by yoga. In the case of the spine, tightness of the passive tissues protect the back, and making them lax facilitates injury.
I'm sure yoga helps some people, but I still don't think the OP is presenting with the kind of problem that yoga would be much help for.
Edited by viveutvivas, 02 November 2009 - 06:13 PM.