these look great! your lines are really expressive and fluid, and you've even got some solid shading in there. Tricky poses but no confusion as to where everything is which is awesome.
I would take an eraser and use it to "carve" out the shapes of the limbs to make them even clearer if you can, sometimes i find it easier to render in that kind of "backwards" way. For instance, on the second picture, if you took a thin eraser tool and hollowed out the lines you made as fingers, it would take them from skeletal to fleshed! I hope that makes sense haha...
other than that, the head shape and necks are a little off, I struggle a lot with this too, especially when there's hair obstructing everything. Unfourtunately it makes the head look disconnected from the body, so it's really worth focusing on. I would try to develop a "shape" in your mind that helps you draw skulls and necks, I try to imagine a sphere with a piece of paper taped on it sometimes, and then the neck as two triangleish shapes glued together down the middle. I would try the Faces/Expressions photos and just focus on the skull and neck!
hope that helped - I was pretty unclear on a lot of things so feel free to ask!
looks great! your poses are really fluid and energetic, which is awesome!
I would try to do longer studies, do the quick lines to make sure you know where you're going, then render render render till your arm falls off! the 30 min class type can help ease you into the longer ones. I know it's difficult, and really frustrating but the more you push through that frustration the better your art will be! Try not to skip photos you don't want to draw, and tell yourself to just get it down, doesn't matter if it's good or bad.
for perspective and crouched poses, it helps me to think that there's no "simple" or "complex" poses, just angles and shapes to copy. Everything you need is right there in the photo. Try not to "build" things in your head, just copy lines and try not to think about anything but making sure they intersect at the right angle.
sorry if that got too wordy! i hope some of it helped!
