These are really good. They're very proportional and I can tell you know what each part of the body looks like. The only suggestion I would make is to use less straight lines, as straight lines limit flow and make poses look stiffer. But these figure drawings look very aesthetically pleasing:)
Wow! Nice work on all your sketches, Poyoline. Greatest job on capturing vitality on your 15 second sketches; greater job on your 30 sec doodles, and a really, totally great job on your 5m and 10 min drawings; more than that, I see a little bit of your two thirty-minute class modes. Thanks, Poyo!! Yet I've got one small favor: I love your feeling and sense of humor of your 15s and 30s scribblings, but, why don't you go for one 60s (1m) sketch, pretty please?? Because, your figure doodles will become less stiffer, and more solider and fluider, if that's your truer, realer goal. Hope it sure will help.
I think that the movement in your figures is fantastic. The figures done in red have real confidence to them, I am assuming they are the ones you spent a longer time on, just because you have less time doesn't mean that you can't have just as confident lines. Between the two sessions that you have posted though you have improved so much and you are on a great track and have an understanding of what figure drawing is. I'd say that these are great practices and keep going!
To be honest it's getting better bits by bits, which is good :)
Overall you lack of focus, what I mean by that, you seems to be panicked on your 15s sketches because you might not knowing what to do in this short amount of time.
I will suggest you to keep it simple, leave some anatomical shapes behind, like Alex Woo does, it will help you to gain confidence in your line :
For your longer sketches, you seems to have few notions of perspective BUT your drawings are clouded by that huge weakness. You could put any details, if your foundations aren't stable, everything falls apart no matter what. I'd like to share an exemple of what you could focus on for 5 mins :
Basically it's getting a mental representation, to visualize before making any mark on the paper, so you don't waste time and have fun.
Still, don't overlook perspective before studying anatomy, because it's crutial and this topic is too complex for any beginners. I made that mistake in the past :)
The proportions are good on the the 30s and above times. I can see you struggled with 15s but that's okay, you can just keep doing studies at longer times, then once you have a good grasp of anatomy/proportions you can do the shorter ones (focusing on gesture rather than anatomy). 30s to 60s are good enough times for you to break down the anatomy without rushing and it prevents you from rendering it too much. Good luck!