Let's look at this from a purely logical point of view. First of all, anyone who truly understands yoga (such as a Hindu) will tell you that yoga positions were never designed to be exercises. They were designed to do one of two things - worship one of more than three million Hindu gods and/or facilitate the flow of prana (life force energy) through the body.
As Fr. Pacwa states in his book, Catholics and the New Age, " . . . (H)indus did not devise these exercises for athletic limbering or muscle building. All were meant to lead the practitioner to enlightenment and awareness of his or her inner divinity." (pg. 33).
Legendary guru B.K.S. Iyengar confirms this in his book, Light on Yoga, where he says that some yoga positions "are also called after gods of the Hindu pantheon and some recall the Avataras, or incarnation of Divine Power."
Having said all that, we come to a purely logical conclusion - it's not possible to "just do them as an exercise" when the so-called "exercises" aren't just exercises. Check Fitness About for further information.
That would be like saying the sign of the cross can be used as a triceps exercise. Sure, you can use it that way, but it's not - and never will be - a triceps exercise. Like yoga positions, it can never be a mere "physical action" or "neutral" because it has a profound spiritual meaning.
Others attempt to lend Christian names to these poses, or to pray the Rosary while practicing them; however, none of these actions negates the intrinsic Hindu meanings in these poses, at least not according to Bishop Norberto Carerra.
In his pastoral instruction on the New Age, A Call to Vigilance: Pastoral Instruction on New Age, Bishop Carerra writes: “However much proponents insist that these techniques are valuable as methods, and imply no teaching contrary to Christianity, the techniques in themselves . . . in their own context, the postures and exercises, are designed for their specific religious purpose. Even when they are carried out within a Christian atmosphere, the intrinsic meaning of these gestures remains intact.”
So even if you think you're stretching your back, if you're using the Sun Stretch to do so, whether you intend to or not, you're still posing in a position of worship to the Sun god because that's what this pose was designed to do. It was never designed as a back stretch.
It works the same way with someone who uses the sign of the cross to work out their triceps. They may indeed be working out these muscles, but regardless of their intentions, they're still working out these muscles by making a sign of profession of faith in the Triune God.
My advice is that if the idea of posing yourself in a position of worship to a Hindu god is even remotely bothersome to you, stop doing it. There are plenty of other exercises/stretches you can do that work just as well as yoga.
For further reading on this subject: