Perhaps a different coloring style (not [just] line thickness) would help? The coloring looks "pillowed" as they say in pixel art, where the lighting is always central to each portion of an object and gets darker around the edges. It works and it doesn't.
There are a lot of tricks in coloring animations. Perhaps the method you have works for what its made for, especially if it's one of many frames? For instance, it would seem that the bottom of his left foot should be light to be consistent with the coloring of the rest of the image, but making it dark makes that entire leg disappear and you're only focused on the action leg. Pretty neat on that one, poot. I'd almost be inclined to do the opposite with action foot and add some light around the ball/tip to create the illusion of a streak and/or signify the edge of the hit box.