Nice outlines of the figures. You need to work on filling in the outlines. Suggest you work on hands and feet (Line-of-Actions L-o-A) has a section on hands & feet. L-o-A also has a section on faces & expressions. You can also use the internet for information on these. I also recommend you check out the following sites: Pinterest and On Air Video. You should continue your practicing (Practice makes better). You need to practice every day.
good movement in the body, as others have said, you could look to movement in the outer limbs to clarify the body action
I like the one you have on the bottom, it has a nice stretch to it. You have good movement in your line, and the strokes of shadow also help you get volume in your dynamic figures.
Greatest job on your figures, Aernout, still, what is your current goal, exactly?
And do you know why???? Because you and us would and could know what you really want to aim for.
Hope it's been completely and totally sympathetic.
Stay most healthiest.