I really think your detoid studies are getting on the right track, Legacy55, and I feel like you're getting there.
To answer your question, as far as the moving deltoids go, here is a YouTube video right down here:
https://youtu.be/tez0Byj_qQU
Would you please watch and study this video? Oh, furthermore, could you please take screen shots of this one cropped on the video's size, just for reference??
The arguement is as a result, they are totally useful for your quick sketch studies, for 10 minutes of 30 second quick studies, all flipped horizontally and/or vertically, and/or grayscale, on your computer.
The other reason why is because, you'd be able to be more faster and confident in your muscle line quality and control, and consistency.
For some extra credit, should you please be sure to look at this link of Eliot Goldfinger Human Anatomy PDF:
https://www.scribd.com/doc/106203113/Eliot-Goldfinger-Human-Anatomy-for-Artists-the-Elements-of-Form
So, good luck, and I hope you'll find these and you've found those completely and totally useful.