Hey Crystal, I like what your doing here, I'm seeing a lot of gesture in these poses. I agree with @Leandro Alli , you would benefit from studying anatomy as a whole, and one body part at a time. I think though, because of the kind of artist I suspect you are, you should continue drawing from your imagination. As you learn new anatomy, use it in those drawings. So, about the heads, I like to add them into my gesture drawing at the beginning. A lot of people add the head after figuring out the torso and pelvis, which is fine. One thing I keep in mind is that the body always follows the head, just like the head always follows the eyes. This goes back to my animation training, that's why I do it that way. I did a sketch to show you what I mean about making the head part of the gesture. Keep up the work, that first figure you did is pretty good, and the boxing pose is really nice, I can really feel the distribution of weight. I hope this helps :)

I clicked on your profile and saw a lot of good studies, great job! What I'm going to recommend are things that worked for me. I think it's cool that you are trying to draw from imagination but I wouldn't focus too much on it right now. I think that a larger scale study would be great for you, pick a good photo reference and a larger paper, just like Proko uses in his videos. Draw it in full grayscale and take your time with it. You will have a lot of room to work and I find it the best way to learn anatomy. Another advice would be to start isolating body parts to work on, Just a pick a body part and try to understand it's structure with a little bit more depth, Proko videos are great for that. And for my final advice, it helped me a lot to understand anatomy by trying to sculpt it. Traditional or digital. Hope it helps, cheers!