Your basic forms are looking good! They look rather stiff though, and a good way to fix that is to use a line of action. A line of action is a line you use when gesture drawing to show the movement (ie action) of the figure. It helps you to stay true to the refrence without losing fluidity. You have part of one in most of your figures already, because the line of action often follows the spine. Just draw a line through drawing where it feels like it is flowing, it doesn't have to catch everything :)
For practicing muscles, I would recommend looking at some anatomy books that show the muscular structure, and studying people who have very defined muscles. You should get an idea of how the muscles work with each other to move, because that will help you draw them accurately in any pose. Also, when drawing muscles, don't make them super detailed at first. Just draw the main shapes, and make sure it connects properly to the rest of the body. You can add in more detail after, the most important part is the main structure, and you wouldn't want to spend a lot of time detailing only to realize you messed up.
A good thing to note in general is that you might want to loosen up your lines. A good way to do this is to draw them very fast. Just swoosh through your lines, don't spend too much time overthinking them, it helps to make your drawing feel more natural.
