Full disclosure... I have a couple yoga DVDs (and the one that comes with the P90x system) and have enjoyed doing them over the last three years. They are quite spiritually sterile and they simply focus on the movements without importing Eastern religious messages about self-love and self-actualization.
Much like the issue of alcohol for Christians, yoga could be sinful but doesn't have to be. Dr. Mohler hints at this when he writes:
There is nothing wrong with physical exercise, and yoga positions in themselves are not the main issue. But these positions are teaching postures with a spiritual purpose. Consider this — if you have to meditate intensely in order to achieve or to maintain a physical posture, it is no longer merely a physical posture.Here is my take:
If a Christian were to go to a yoga class that was explicitly religious that would be one thing but simply following a pattern of physically strenuous movements in order to achieve greater strength and flexibility is not wrong.
It's kind of like saying drums in church are wrong because the drums have been used by drug abusing, womanizing, rock stars for much of the history of rock and roll. Guilt by association. The drums are not bad and can be used for great purposes if in the right context with the right motives.
Same with yoga. Yoga has a religious and sexual past but that doesn't mean that the movements of the body that it gives you are in and of themselves wrong. Maybe it needs a different name than "yoga" since that has so much negative baggage for some people. What if I called it my stretching and strength training but did all the same bodily movements in the privacy of my living room? Would it still be wrong? Of course not.
To be clear, I have never meditated or been to a yoga class. I have never sat cross legged with my hand on my knees while I droned "OMMMMMMMMMMMM". I have never been tempted to have yoga deviate into some sort of aberrant sexual practice. But I have greatly benefited from the flexibility and core strength that comes from these exercises and would actually commend them to you to try. It's great and will greatly improve your health.
Are Christians free to practice yoga? Maybe, maybe not. The question has more to do with the how. Once that is answered then you'll find your answer.