We have all heard people saying; so called health experts that. When a regular gym goer, builds up his muscles during their early ages.
A weight lifter; body builder
And now when that person stops going to gym or stops doing any strength work. Then that persons muscles will not turn into fat.
Infact, it does just because that person lifted weights.
Now, on the other hand. When a Yog practitioner does various Asans. This wholely incorporates the individuals own body strength to lift the persons own weight. And, this although will not make out any ripped muscles, etc. But, over the years it will make person much more internally and externally strong.
Note: Yog is not to be confused with the Calisthenics exercise. Though, even this is free form of exercise in which a person lifts, his/her own weight.