Where your foot is landing
When running, foot strike location in relation to the body position is a major factor in efficiency. If foot contact with the ground is made in front of the line of the body, regardless of where on the foot the contact happens, the foot will act as a break in motion. Ideal foot contact would be under the body to allow forward momentum to continue unimpeded.
What does this mean for the average runner? Think more about where your foot is landing and less about which part of your foot lands first.