The US Copyright Office has issued a policy explaining that Yoga positions and poses are not covered by S102 of the US Copyright Act as they are neither dramatic works nor choreographic works - and most certainly are not musical, artistic or literary works!
The policy explains "[W]hile such a functional system or process may be aesthetically appealing, it is nevertheless uncopyrightable subject matter. A film or description of such an exercise routine or simple dance routine may be copyrightable, as may a compilation of photographs of such movements. However, such a copyright will not extend to the movements themselves, either individually or in combination, but only to the expressive description, depiction, or illustration of the routine that falls within a section 102(a) category of authorship.
You can read the Copyright Office's full Statement of Policy here and here