To me, the main issue seems to be that the upper ends of the upper leg bones are fixed to a static pixel. That makes it seem like the whole pelvis is fixed in position but that is not what is happening when a person runs. As a person straightens a leg, it pushes the whole upper body upwards starting from the pelvis.
As a general piece of advice, note that whichever pose you are trying to draw a figure in, think about the limbs on which it rests or hangs because those will determine the positions of everything else.
In your animation, your upper leg bone shortens when the leg straightens below the pelvis. It should not. The length of the bones should instead stay the same and push up the pelvis and everything that rests on it (spine, head, shoulders and then the arms which hang from the shoulders will naturally rise up as well).