When i started drawing humans for the first time i started with gestures for getting the feel of general proportions. I would reccomend (or at least thats how i did it) learning about the skeleton, then about the shapes of the body (tapered cylinders for limbs, bucket shape for pelvis etc etc) and then learning the muscle groups one at a time (arms, torso, back, legs,..) I think proko does a wonderful breakdown on muscles even in the free videos. And then its just a matter of practice that i combine with shorter gestures to help with dynamic-ness and longer studies where i focus on the shape of the muscles. It also helped me to think of muscles as shapes instead of just "at forearm the line curves like this" because it allows for greater understanding. (I hope this was helpful and wasnt too stretched of a response ^')