You have good discipline to take the time to practice this many different poses when most like to spend their time creating a piece - efforts like this will definitely pay off to make your pieces alot better. its totally worth it. good for you for taking the time to do it. It reminds me I have to do the same thing, stop trying to create pieces always and take some time to do more studies. Also you used pen which is interesting for a study, perhaps did you want to force yourself to be very accurate the first time you make a line? My rough sketches are wild and I would not be able to come up with something this clean during a study.
My suggestion to improve your studies is to get a biology or workout book, or go to a site that shows the actual muscles of the body. Everyone's body is different and sometimes a lean persons body can be mysterious as to where exactly the muscles come together. If you learn basically where the muscles are on any body you can draw more from your imagination and without a reference and be expressive/creative with the size and shape of the body because you will know the general rules. Here it seems like you have a good eye for seeing the basic outline of the reference but if you dont study the rules of the body and the muscles, then you will need to rely on references more and it can slow you down in your art. Also, I would recommend doing studies of people with less 'ideal' figures - even if your intention is to only draw people with 'perfect' bodies - for me I find the drawing many different shape and size bodies is very rewarding with what you learn and lends more reality to your art. That's not a criticism only a suggestion. It depends on your style of art whether you will want to add shadows or not, but adding light and shadows will bring out depth and can help you out when the lines don't say enough on their own - like when things are foreshortened.
maybe try feet, hands and heads in their own separate studies if you dont want to spend alot of time on them in your whole body studies. Looks like you got the perspective right. I wouldnt say anything I've said here is to improve this practice in particular but just advice in general for yourself, me also, and anyone wanting to get better at mastering anatomy.