It’s probably best to study a bit of anatomy! For arms at the very least you should be aware of the deltoid and pecs which will move with the arm 

Leg muscles go all the way up to your pelvis and there’s a good number of them so it’s easier to group everything into sections. The section between the torso and thigh I guess kinda connect the two together from an artistic standpoint. Basically a GIANT COMMA starting from yer butt and wrapping around to your groin haha. When you move your leg you’ll likely see a crease following that shape

Hope that helps!!