I would say that this is the mother of all videos of bicep girls doing their hair:
https://www.girlswithmuscle.com/1012299/
There's a follow up to Camilla521's video where she simply starts flexing:
And referring LEHCIM's post above here...Standing Abdominal Crunch pose is another good source of what you're looking for. It basically makes the biceps blow up.
:// https://www.girlswithmuscle.com/1013188/
Vids: https://www.girlswithmuscle.com/581796/ https://www.girlswithmuscle.com/547557/
I am not really sure if I can describe it correctly, but here are a bunch of examples of girls doing this. It is somehwhat like how they flex while doing their hair. If you do understand, please share some more pictures/videos like this. Thank you so much <3