Quick sketch seems pretty decent I think what would you might've missed in this quick sketch is the foreshortening of the forearm in that bottom figure as well as the foreshortening of their upper arm which fused with the back. Also, it may be helpful on your next quick sketch to figure out the general positioning of legs, and also you may have gotten the rhythms of the upper body on the top figure a bit wrong I think you might need to invert the S curve but keep that overall silhouette. Good luck with your future studies and I hope you improve as an artist!

I like the Ss in the lower figure and the gesture is generally there in the entire sketch. Though I think the lower figure could use more variety in terms of straights vs curves. It seems too fluid like spaghettis. If possible, I recommend Proko's figure drawing course - the free videos - on the gesture tactic CSI. Your upper figure is incorrect in that it doesn't mimic the gesture of your reference accurately. I don't know whether it is by design. I understand that it is okay to exaggerate with gesture drawing, but I think your figure doesn't reinforce the one of your reference. Your reference has force and power but yours seems to tone down on those two aspects. Also, I think it would be nice if you draw with abandon. With gesture drawing (or even quick sketches), having fun is part of the process. Your sketch indicates a lot of erasing. Practice getting it right with more confident and free form strokes. I think it is okay to draw over your lines provided you keep your strokes light. Aim to get the gesture you want. Let loose and draw. With time, it will be second nature. I'm not there yet myself but I'm working on it. You are not alone. Have fun! (please forgive my illustration, freehand drawing with a mouse is not easy. but i hope it helps to get the point across)