Yoga originated as a part of the Hindu philosophy, a system that is without the God of the bible, whose aim is to achieve peace without YHWH and his revelation. You worship the one whose instructions you follow. Christians do not need yoga for exercise (it's of Shiva); stretching is fine as long as you're not using positions which originated to worship another; so, not yoga. Yoga is a way to purify one's body—without relying on Jesus' sacrifice to purify you of your sins, nor on his words/revelation to purify your mind or renew your mind. People who take it upon themselves to Christianize it, are going against his commands to not worship him in pagan ways. The yoga you're speaking about is "Hatha yoga", the one used for exercise.
If you want to read more about it and reach your own conclusion:
http://en.wikipedia.org/wiki/Hatha_yogaMy method of deciding is this:
- Will I rely on "Lord Shiva's" instructions to achieve health, stress-relief, and peace?
or
- Will I rely on Lord Jesus and our Heavenly Father's instructions to achieve health, stress-relief and peace?
We can't serve two masters. You'll either hate the one and love the other. Again, you worship the one whose instructions you follow. So I don't make my lifestyle decisions based on the advice of a foreign god nor choose to engage in practices with foreign gods as their origins.