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As far as I have read, there is very little instructions of how to do asanas in any of the ancient texts: it was up to the guru to advise you, or not (sometimes the guru would leave you totally on your own). However, in the early 20th century Sun Salutations were melded into asana practice. This is not so different from modern exercise philosophy which advises to warm up the muscles before strong exertions. Note: the studies do not show that we need to stretch before doing sports, but we do need to warm up. I believe this is good advice before any strong yang practice as well, but it doesn’t always have to be Sun Salutations.