remember when running or walking, the shoulders move reciprocatively to the legs.
as it is, the shoulders don't move at all. also, on the extremes of the arm movement, you've got one fram for the forward, one frame for the back an what appears to be two frames that are almost identical in the middle. i would give them a little bit more difference, because as it is it looks like there's only three frames when there's four.
0Well. that didn't make any sense. i'd do an edit but i can't at the moment.
so;
1. Move the shoulders back and forth
2. differentiate between the "middle" frames a bit more to reduce the 'popping'