Your drawings look superb. Your proportions are good, and I generally like how you handled things. One thing that you could do to improve, however, is communicating more motion. Have you done gesture drawings yet? These look good as they are (especially the third one), but some could look more "alive".
What helps the flow of motion in gestures is the action line - basically, a long and fluid line depicting the spine. (It can be longer than the spine and head) Exaggerating this line and drawing on it is a majorly helpful tool to 'animate' the drawing more.
To give an example, a cartoonist can draw a very exaggerated line of motion and build a character on this line. Add extra lines for the arms and legs and build the limbs on these lines. Don't aim for perfection, aim for motion. Even if the proportions are a little off, it will give you a better sense of what will help improve your drawings and remove stiffness.