Assalamu Alaykum,
Yes it is allowed as a form of excercise and to have better health.
Yoga originally stems from polytheist beliefs and worship rituals. I think it is Hinduism. However, many martial arts, languages, foods and other cultural acts have some sort of background to them. This doesnt mean that they are forbidden to Muslims. Just like we use words even in the English language that originate from the Greeks or from Sanskrit (the ancient language of the Indian polytheists) i.e. the word thug, originates from words used by Hindus in Sanskrit.
Likewise Yoga is done by many people all over the world regardless of their beliefs. It is done as a form of excercise and is very beneficial for that. As long as you do not compromise any of your own beliefs and also you seperate any of the nonsensical meanings to some of the forms then it is Ok inshaAllah.
Personally there are some excercises and martial arts that i have done which are from other religions, but of course I dont believe in the meanings to them.
Wasalam.