It's a decent start, but you have some fundamental flaws going on.
- In the frames 10-1-2 and 5-6-7 the forward foot is making REALLY odd stuff, jumping and snapping around.
- In frames 1-2 and 6-7 the arms stay the same, no reason for that.
- In the same frames the backwards foot/ lower leg shrinks into the knee. It's important in animation to keep the volumes consistant.
- The arms are slightly different on each of the 2 steps, but why did you just put the legs at the same place and make the back one darker. Even if you make it darker which works well in videogames, you should make them a bit different. And if you make the farther leg darker you should do the same with the arm.
not regarding the animation, the antialias on the outside of the helmet does not help.
Keep it up
