The asanas or postures constitute the next limb of Yoga and this is the most well known limb of Yoga.

Yoga Sutra 2.46 The posture should be steady and comfortable.
2.47 In effortless relaxation, dwell mentally on the Endless with utter attention.
2.48 From that there is no disturbance from the dualities.

While it is seemingly the most innocuous limb, it is the hardest one to actually scriptually integrate into a Christian lifestyle or framework.  In fact, at one point Paul tells Timothy that physical exercise is of little profit:

8 For bodily exercise profiteth little: but godliness is profitable unto all things, having promise of the life that now is, and of that which is to come.
1 Timothy 4:8

That being said, I personally believe that our physical bodies, especially relating to our mastery of them, embody our greatest tests.  They provide one of our best chances to prove that we have the power within ourselves to overcome, to subdue our bodies without direct instruction from God, other than that we not poison ourselves.  What a simple test and yet how complex!  Ignore the physical body and prove that I can’t be trusted to care for what I have.  Yet, if I spend too much time and effort keeping in shape and I run the risk of falling prey to my own vanity and pride.  This is where the brilliance of the asanas truly shines. There is no competition (and those who would degrade Yoga by making it into a mere sport should be ashamed).  There is only the constant bending of the body to the will of the mind until one’s consciousness fills the body entirely and body and mind truly become one soul, unified in purpose and action.

Namasté