I've read a lot about heel striking being a big no-no if your heel is hitting way out in front of you - which apparently distributes shock up your leg, into your lower back, etc. Bad news. But what if you heel strike beneath your center of gravity? Like, your foot never goes out in front of you, but you're hitting with your heel first. Would this just be considered "inefficient" but not likely to cause injury? I'm not a heel striker, but think about this from time to time.