Sweet Spot

I consider myself to be an eternal student. I think it is wise to keep in mind that there is always something more to learn on any topic. It was in this vein that I enrolled in a coaching certification course put on by USATF (USA Track and Field). I had the opportunity to learn and discuss running biomechanics with people who have coached for decades. The biggest question on my mind was foot strike. So I asked several people how do they teach new distance runners to run? What footstrike has given athletes maximum efficiency and minimal injury? The most experienced coach in the group told me about the "sweet spot". This spot is "just behind the forefoot".
Just to back up a minute here, I have heard and read many different interpretations as to what the best running foot strike is. Some say that the forefoot is best because the big calf muscles absorb the shock of impact, and not the heel. Others say that the human body has evolved a physiological heel pad under the calcaneus bone to absorb this shock, so we should use it. A few say runners should run in whatever style feels natural. Most would say that they strike with their "midfoot". This last description never sounded plausible to me. I always thought this was impossible because you can't land on the concave part of your foot. The "midfoot" is the arch and is concave. But the way he described the "sweet spot" made me realize something. That is, when your foot has already started the motion of moving front to back, then it is possible to strike on the backside of the convexity. The convexity is the ball of the foot (see diagram). I don't think I would call this "midfoot", but maybe since that term has no real anatomical definition, this is what "midfoot strikers" are talking about.

By describing the sweet spot as he did, this gave the new distance runners a cue to start the foot moving backwards before the foot lands. Why is this helpful? A good analogy would be a canoe. A moving canoe has momentum, just as you do when you run. If you stuck out the paddle straight into the water it would slow you you down. Starting the paddle moving back first and then dipping it in the water helps maintain momentum. Conversely, heel strikers will typically fully straighten their leg when they land. This is akin to sticking the paddle in the water and slowing down the canoe.
This is not to say that this is the best for everyone. I still believe that if you are a slow runner, and are experiencing no problems, heel striking may be alright for you. There are other factors involved that may predispose you to injury if you do anything else. However, if you are concerned about performance, you should find your "sweet spot".