If by Yoga, you mean blindly embrace all tenants of Hinduism, then of course that is antithetical to Christian practice. But from what I can tell those who practice "CHRISTIAN YOGA" are not trying to promote an idolatrous synergism. A Christian approach to Yoga is not Yoga "in its true form". It's also likely that your common Yoga class at the community centre is likely not Yoga in "its true form". Doireann Fristoe explains,
Most Yoga currently practiced in [Western culture] only slightly resembles the original practice. In fact, most of what we call yoga in the West is not truly yoga at allit is only asana, the physical postures, and pranayama, the breathing exercises. There are myriad SCHOOLS OF THOUGHT in modern yoga and to sum all of them up in a few paragraphs would do them no justice. Hinduism involves yoga; all yoga is not Hinduism.
Can we incorporate asana and pranayama into the Christian's practices of prayer, contemplation and meditation?
...."Yoga's physical positions allow for the influence of the demonic." I am told that assuming the physical poses can allow demonic influence in your life. Objectors suggest that when you participate in Yoga, even Yoga that is based in Christian prayer and worship, you are unknowing worshiping demons and idols.
I am critical of the claim that a Christians can unknowingly worship a demon (idol or 'god'). It seems like a bit of stretch too me. (excuse the pun) I don't think the Apostle Paul buys this claim either as evidenced in the first letter to the Corinthians. When asked about eating meat sacrificed to idols Paul says:
"So then, about eating food sacrificed to idols: We know that An idol is nothing at all in the world and that There is no God but one. For even if there are so-called gods, whether in heaven or on earth (as INDEED there are many gods and many lords), yet for us there is but one God, the Father, from whom all things came and for whom we live; and there is but one Lord, Jesus Christ, through whom all things came and through whom we live." -1 Corinthians 8:4-6
Paul does not think eating meat that has been offered to an idol somehow defiles the Christian by consuming that meat. Paul believes this to be true because through Christ all things came and through Christ we live. (v6) God is Creator of the meat, not the idol god. So it appears that by thinking the meat is defiled might be giving CREDIT where credit is not due. How is this connected to the Yoga discussion? Let me suggest that because our bodies came from God, and thereby any physical actions necessitated with having a body (i.e. eating, stretching, sitting, laying), I am in no danger of worshiping an idol. (I am of course not including actions done with the body, such as adultery or gluttony, within this category of normal human physicality.) A Christian who does a Yoga pose (like the downward facing dog) is no more in danger of worshiping an idol (demon/god) than a non-Christian is of worshiping YHWH by raising their hands upward in a yawn or of giving a gift at Christmas time.
To sum up my answer: If I can eat meat (a physical action) that is sacrificed to idols and still be faithful to Christ; cannot I not also assume a yoga position (a physical action) in prayer and worship to Jesus without worry of unknowingly worshiping an idol?