Well, there are a number of things you could do so by no means are my suggestions the only ways. First I’d always recommend drawing from life. Any life drawing will help you draw from imagination later on when you’re inventing poses or working out ideas. It creates a giant vault in your brain of all the things you’ve seen and worked out for yourself, things that were right in front of you. It’s always a good test to try something from imagination, it will be quick to point out your weaknesses (and there always seems to be something to understand better in my opinion). Drawing from life isn’t always an option, especially for anatomy (however, mirrors can help in that case). Draw from references, whether on Proko or other sites, I am sure there are a number of places people can refer you to. Doing wire-frame drawings can help a lot too. These will without a doubt point out how well you get the structure of something. Along with this, drop the shading if your really unsure about something. It’s really easy to add shading in and we have done it but sometimes deliberately saying “these studies are about form by NOT shading” can help you further understand the thing your trying to draw. Also, look at your muscles in the mirror. Find the skeletal points. See if you can identify one deltoid head from the others as you move your arm around. Flex, extend, rotate. Our bodies are an amazing thing to look at! Draw BIGGER. I don’t mean like gigantic or anything, but using one page for one or two bigger example instead of one page for 4 tiny drawings. Working small limits you. Can you really know where the attachments are and how the muscles lay overtop the skeleton when confined so small? Sometimes it’s nice to dedicate a whole page to a more worked out study. Last, it’s always a good idea to revisit the skeletal structure. Knowing where origins and attachments are can only help you. Plus, if you find yourself confused on how a bone actually looks or works, you’ll never be able to figure out how the muscle works on top of that...I’ve had many days where tears swelled in my eyes as I frustratingly muttered to myself over and over...HOW DOES THIS PELVIS WORK!! Sometimes it can drive you to the edge of insanity, but pushing through this to get a better idea of the skeleton will for sure help. Anyways, sorry for the lengthy reply if you were looking for short and sweet. This is all general advice as I am not specifically critiquing your drawings but just answering the question your posted. Best of luck!