So, I've started my first running animation, mostly for practice so that I can get things right before making something more detailed. I couldn't find any good references on running, as you can with walking, but after watching some animations, I've put this together:

Now I was thinking about what was debated in the bruce lee-walking topic, about linear motion. Is this supposed to be applied in-code, or could/should it be in animation?
Alot of criticism wanted.
Cheers
Alright, starting off you can't truly just critique a part of something, though this is what I'll say. First, the animation is cycling way to quickly for a a run and it's rather hard already to even try and depict anything about the image. Though the character currently has no thighs, it just seems as it's two legs connecting to an oval base. Next, the right leg looks as if the character is simply kicking off the ground getting ready to win a sprinting race. So to have a continual looping of this will ruin the character pose no matter how great the detail, my suggestion would generally require an entire revampment of the animation. Starting out, your going to have to bring the lower right leg forward, and lift the ankle up by a margin or so, so that it doesn't seem as if the character is hyper-extending it with every step. Next, you should bring in the left leg a bit, because there are several issues with it's current stance as I issued above. Also you are going to have to redo the foot as a whole because it's highly awkward. Then in the matter of realism you are going to have to accent the natural anatomic muscle tone, that is assuming this is a human male. So instead of having flat sides try adding an oval or reclusive-type curve on either portions on both legs. Finally, there seems to be a static frame where it looks like your animation has a 3rd leg sprouting from it's right, I would suggest fixing this before doing any of the other changes.