you have a good foundation to start with, my advise to you, draw from real life reference and pay close attention to muscle structure. take a day for yourself, and just draw hands, draw your own hand, over and over and over starting from simple angles getting more and more complex, then take a day to draw arms, with hands attached, the main thing i notice that i can critique you on is hands. hards arent the easiest thing to draw. but take a look at a hand in your drawings. ill use dead man walking as an example. notice how long the fingers are and how short the palm section is, now look at your hand, notice that the fingers and palm are roughly the same length. hands, practice practice practice, draw from life, profit