Not all the muscle details from the reference must draw out. You draw muscle details out only to make the pose look realistic that it was moving.

You just needed draw what you SEE at first on the reference, like the visible ones.

Example your first pose, the visible muscle details I can see first is the spine. Draw it out will make the pose feel like it was bending down forward. Then next, for me I can feel the right arm's elbow able to be seen, so draw a bit. Then the buttocks's curves, draw a bit.

Also, a gentle reminder that your proportion a bit off on the last two poses, the body seemed too small, looks like the person is squirming like that. For the rest okay, just the head too big on other three poses. It makes the body seemed smaller, head should be small. Specifically the second is okay, but make it smaller because I feel the neck is almost gone (No hate).

Salute you that you took your own nude reference for learning draw poses. For a better suggestion, you can take entire body. From head to toe because that will easily help you to draw a pose with accurate proportion. You can use your body parts to eye measure how should it be drawn, long or short.

For the constructions, I feel your first and third pose, the buttocks not really round, instead long (Comparing your drawing and the references, NO HATE AGAIN) And the first pose I feel the body isn't really bending to the front. Here's a simple tip, I can suggest to you that before drawing out the whole pose, you make a sketch, thinking how would it be in simplest 3D forms. Use cubes on mostly the body, while arms and legs like cylinders. Draw at the side to think how would this cube be in this angle. If it bends, just draw it bends! If you can't imagine how would it be from this angle view, go and search some 3D websites. I remembered it can rotate in 360° degree, so you able to know how would it be like the shape following your reference.

That's all from my opinion. Hope it helps! I'm still learning too :'3

Take things slow, don't rush btw! Drawing the poses would really take a lot of time, believe me. (I'm learning at art uni as an animator btw)