

Full disclosure: am a guy.
Holding your breath allows you to tense up and clench more than you can while breathing. Taking a deep breath eliminates some “tensing” muscles from the equation. Still, with some effort (or anxiety, probably) you can still clench up while breathing.








After reading your post body, I realize this isn’t what you’re looking for, but at least for honorable mention reasons:
https://openlearning.mit.edu/courses-programs/mit-opencourseware