This is a nice sketch overall. Talking about the left side sketch you should add more line weight and line variation. And be consistent with adding shadows. The right side sketch, so whenever you draw a structure try to reduce the number of lines you use. Try to draw the structure with flowy long lines, you can learn more about this when you learn gesture drawing. You can also use simple shapes to draw structure which also doesn't require so many lines. Otherwise , good job.

I like this, especially your sketch, it has a lot of energy. I think the way you drew the arms is pretty good, they feel very dimensional and solid. The torso and lower body though looks very flat because you used too many straight, flat lines. I did a quick diagram to show you some of the things that I noticed in your drawing and what you could do to help keep things dimensional in the future. I hope it helps :)