these are very nice sketches,
but you should really be pushing yourself to finish a piece.
a lot of challenges arise when creating a picture.
its more than just "do these muscles look right?"
you will learn a lot form attempting to create a believable character with a background and shadows, interacting with someone or something.
sketches are a means to an end.
they are incredibly valuable, do them, but really push yourself.
if i waited until my sketches were perfect before ever attempting a finished piece... i would still be waiting.