Excellent idea to study human muscle and bone structure. Whether the figure is clothed or not a knowledge of these subjects is necessary. The internet has a lot of free information on these subjects. Make drawings of the information you obtain and keep these drawing close at hand so you can refer to them as needed. Also, try Pinterest. It has information on many different subjects. A book you might want to check out is:
"Morpho: Anatomy for Artists" by Michel Lauricella. He has several related books.
Whatever you do, remember: Practice makes better. So, practice, practice, and then practice some more.