The effects of various activities do not require one to be cognizant of them to be effectual.
What he said.
But how can any purely physical activity (stretching or some other exercise) have a spiritual or mental effect on someone?
How is this situation any different from meat offered to idols? The meat (or physical exercise in this case) is not tainted by having been associated with idolatry, if the believer uses it in an environment where the old associations are absent.
Genuine question.. I am not a yoga supporter by any means.