Well, Michonne seemed like she had no problem slicing off the head or killing the zombie that was about to get Andrea. Also, you have to consider the fact that these Walkers have bodies that have decayed to some degree and thier bone structure, tendons, and muscules will not be as fortified and dense as a normal human.
That's the point. It's a bridge too far for me to suspend disbelief. Yeah, it looks cool in the comics,movies and on TV. But I just don't buy it. Like I said, even in the books it seems like a sop to the ninja fanboys.