Being balanced is of utmost importance when running. Imbalance leads to injury. Proper posture helps keep us balanced.
Ideally, our bodies are in a sort of neutral position when we have good posture. By this, I mean that the head is balanced atop the neck and shoulders in such a way that we aren’t fighting gravity or straining in any way. Over time, gravity and old age conspire to make our posture less than ideal. The head is thrust out a bit on an outstretched neck, and the back is hunched.
Proper posture is achieved by having a poised sort of positioning of the head and neck. To practice this, it is a good idea to first take off your shoes to eliminate the possibility of interference by footwear. While standing, pick a point on the ground about 15 to 20 ahead of you and relax you eyes and fix your gaze on that spot. This should have the effect of bringing your head back on your shoulders and lowering your chin a bit.
This sort of posture is clearly evident in the lead runner from the last video, Haile Gebrselassie. We also see this posture in children, as you’ll see in this video: