Using a mirror and watching how your body moves and responds to actions. I found taking photos of myself and cropping the specific parts I wanted to break down helped significantly. Also suggest building an expansive mental library, Pinterest helped with that significantly.
Doing figure drawing definitely helps with getting the gesture and fluidity of a pose down. I would also suggest doing studies on parts of the body that you find difficult.
figure study, 30 second poses line-of-action.com/practice-too...
Somebody already said in the comments but Proko's videos saved me, especially the ones on arm anatomy
understanding proportions and shape is wayyyy more important than learning things like individual muscle groups, nearly anything you can draw can be simplified into shapes. Ethan Becker has a few good videos on that.
Personally? Start with gesture drawing (only focussing on the movement of the body and not the anatomy, to draw more dynamic and less stiff), than figure drawing where you incorporate basic structure and muscles, and when you finished all that, it is time to learn the actual muscles sadly
I just use my biology textbook 10 years ago 🙃
sometimes google imaging certain muscles or groups of muscles can be helpful if you're trying to work on drawing a specific area. reference is very useful!
Tons of references for just one body part really helps HAHA
figure drawings tutorials from emiliodekureart on yt are really good! they really simplify the forms of the body in an easily digestible way