I do have notes on this specifically on Page 3.
1) A figure sketch in my opinion should always be timed. You should get used to drawing things quickly and with a varying degree of accuracy.
2) A 20 second sketch doesn't have to be fleshed out, it's impossible don't try.
3) The point of a figure sketch is to depict form, if that means not drawing ever component that's fine, you're only trying to show how the body is bending – the best bits where there's stretching and squashing are generally the best ones to highlight this.
4) I personally in quick sketches don't bother drawing and arms properly, I'll indicate the calf and wider parts of the arm – the shoulder and elbow area and then digress into a line towards the hand.
5) Try to use 1 strong stroke to show a line, not multiple marks.
I will post some examples from my personal visual diary, but for the time being just look at these things that I drew on my computer very uncomfortably (It's really awful drawing on a computer imo and it shows in my work which sucks.)
I used the first image to show that you can still depict form from just lines – the second to show that you can depict a form without showing everything and via the use of crosshatching to show what's not in the foreground. And the last one just to show how they match when overlayed.
The one thing I can say is to do figure sketches for 1-2 hours a day. Preferably of figures in motion. Best bet, if you're in school or something – take a small journal with, preferably blank papers and draw people you see around you – this'll help speed you up as well.
You need to train your eye some more which can be done but I won't go into that just yet, if you post more I will. One thing I can see is that your flick rate isn't high enough – you aren't looking at your model and are working off a memory. You need to learn to trust your hand some more so I would suggest doing blind contour drawings and contour drawings in general of some Still Lifes (places some objects next to eachother and draw them).