I know it's likely already been said, but here goes:
Zombies can't run, because running/strenuous exercise damages the muscles slightly. This is most seen when athletes over-train for a competition, and end up tearing a muscle. The reason that we can run is because the muscle damage is repaired fairly quickly, so we're fine. Zombies are dead, so there is no repairing of cells. This means that once a zombie runs and tears it's muscles, it's stuck like that.
Zombies can't run, because running/strenuous exercise damages the muscles slightly. This is most seen when athletes over-train for a competition, and end up tearing a muscle. The reason that we can run is because the muscle damage is repaired fairly quickly, so we're fine. Zombies are dead, so there is no repairing of cells. This means that once a zombie runs and tears it's muscles, it's stuck like that.