Learning anatomy is the box of pandora 
You can start with:
bones:
spine, ribcage, pelvis, radius, ulna
mucles:
pecs, abs, trapezius, glutes, biceps, deltoids
Attachement of the legs and arms to the torso
and then learn the legs and arms as shapes instead
of the muscle names, like in the attached picture.
And then draw your characters naked and use bodybuilder
references to draw it. That´s the best way to learn imo because
then you have a motivationl to understand it instead of just forcing
yourself to learn muscle names
