Here's some tip for drawing the figure. Some proportions and contours. We're going to use a manga girl. Now, as you can see, the top of her body is a little bit short, and the bottom of her body is a little bit longer. These are the idealized proportions of the figure. Now the back, crunches right here. A lot of action going on right here. Most people think the back is straight and you draw the character just like this. No the back has to be flexible, the spine is flexible. So this makes an attractive pose.
Now you can really see the back, the dynamic here, the s curve right here, how much movement there is compared to the front of the body where it's barely flat. Most people think the front is where all the dynamics happen, it's actually the back. Okay now let's draw it, give you some hints along the way. First the upper back is rounded and it's got the most mass. So it comes out to here, then go straight for a little while, but the lower back is very curved right here. Which gives her a feminine look, then she rounds out right here at the bottom and that rounding goes out beyond the top of the back.
That protrudes the most. So look at this. Now you see it's nice and fluid, nothing's choppy about that and it creates a dynamic pose.