A practice at shading
The proportions are off and also the shading as time well next try to study the shapes and size of the figure and howit interacts with light
Nice job on your back shot rendering of the shades.
I've got one tiny, littlest suggestion. I love the motion and quickness, but I'm not getting enough of the practice-by-repitition into the backsides, as gestures go. Would you please be able to do 1 hour of 29 second quick sketch of the figures in general and specifics, and particularly, the rear shots??? (3600/29=124 quicker sketches of back bodies)
The reason why you could and should do this littlest idea is because, your backs' gestures and motions will become so greatly stronger, that they'll look like you've done 'em all in literally one stroke, and to make them the lightest in strokes.
Good luck, and I hope you've found this helpful and encouraging.