There is a wealth of science on stretching.

https://www.bradapp.com/docs/rec/stretching/ is (I think) an evolution of the "old" rec.stretching FAQ which was pretty good way back.

https://www.amazon.ca/Stretching-Scientifically-Guide-Flexib...

These are relatively old resources but worth reading and then updating.

At the risk of being harsh to the author, everything he knew about stretching was wrong ;)

Stretching Scientifically by Thomas Kurz is pretty much the bible on this topic.

http://www.amazon.co.uk/Stretching-Scientifically-Guide-Flex...