Helpful information: http://www.thebereancall.org/topic/Yoga
Yoga in action - Japanese Gardens FtWorth TX
Yoga is the sanskrit word that means "yoking" and refers to union with Brahman, the ultimate god in Hinduism. The goal of yoga is "self realization," to realize that atman, the individual soul, is identical with Brahman, the universal soul, ie., that you and god are one; indeed, that you are god but just don't know it and need, through yoga, to discover this great "truth." Your yoga teacher will probably deny all of this, but he (or she) cannot deny that this practice comes from Hinduism. It was not invented in the West. Yoga was introduced by lord krishna in the Bhagavad Gita as the sure way to the Hindu heaven. Shiva, one of the most feared Hindu deities, known as The Destroyer, is addressed as Yoegshwara, which means "Lord of Yoga."
Yoga is the very heart of Hinduism, and is sold in the West as a form of science but it is religion. It is promoted as beneficial to health (some find the breathing and relaxation techniques to help), but in the East it is a technique for dying. The goal is to reach moksha, allegedly escaping the world of illusion (maya) of time and sense into liberation from the endless cycle of birth and death and rebirth through reincarnation. The latter is Satan's appealing lies that offers endless chances by denying God's declaration that "it is appointed unto man once to die-and after death...the judgment" (Heb. 9:27).
Hatha yoga, known as physical yoga, is allegedly devoid of the mysticism in other forms. Not so. One of the most authoritative hatha yoga texts, the fifteenth century Hathayoga-Pradipika, declares that Lord Shiva was the first hatha yoga teacher. As for the mantras, if one of them means "all is truth," that should give you the pantheistic Hindu connection. You know that all is not truth; indeed, this very idea is a satanic lie.
One can benefit physically from stretching the muscles and learning to control breathing. However, the spiritual price you pay is not worth it-if done even loosely associated with yoga. If you are interested in physical fitness then practice exercises designed for that, not those designed specifically for achieving union with Brahman!