These images come from a manga I read called Stretch by Higashiyama Shou. It's a slice of life manga that focuses on two female roommates that used to go to high school together. I'm terrible at describing things but it's an alright read despite the horrible "yuri-baiting" that the author includes. (Which is basically ship teasing at a relationship that never happens.) Here's a translation by Dynasty Scans and here's a review that I liked.
I really liked the basic illustrations for stretching and I have these images shared on my phone for my personal use so I might as well share them here. If you follow the image and guidelines the stretches really work! All I did was crop, sharpen and edit out some text so feel free to use any images here without direct linking. Oh, and I don't think I got all the stretches that were shown in the manga, sorry!
These are done before and after the characters go for a jog.