The relationship between the two legs in running (Part 1 of 2)

Demonstration/Experiment:
Stand up straight and disect your body down the side to divide the front half of your body from the back half. This imaginary division should run perpendicular to the ground (straight vertical) and travel through your body's actual center of mass. Exactly half of your body's mass is behind this plane, and the other half is in front.

In the figure to the left, the red dots are joints and the yellow dot is the flexible portion of the thoracic spine just under the rib cage. I've made the stomach and chest cavity a bit thicker than the arms and legs to represent the fact that they contain more mass relative to their length.

When you lift the knee (like the knee drive in running) your whole leg moves in front of that plane (figure left). When it does, the mass of that leg changes your whole body's center of mass (from the line under the blue dot, to the line under the yellow dot) so that you fall forward. If you don't want to fall forward, then you are forced move an equal amount of mass backwards to act as a counterweight. If you remain balanced above the same point over the foot, that mass has to come from your torso. The leg moved forward, part of the torso moved backward, and the balance point remained the same.

This is what happens when the body is balanced ABOVE a ground-based"anchor" (the body's source of stability). But when you are flying through space, as in running, there is no ground based anchor. The "anchor" becomes the body's center of mass and it is located just below the ribcage when standing straight.

So let's go through the above experiment again, but this time, instead of standing on the ground, anchor yourself near the center of mass just below the ribs. If you have a roman chair (for abdominal exercises) you can suspend yourself on the elbows while your feet dangle below.

In this model (figure left), the axis point is not the foot, but it is the center of the body. So when the knee is lifted and the mass of the leg moves forward, the whole body rotates forward. Everything below the axis of rotation moves backwards (hips, legs) while everything above the axis moves forward (chest, shoulders, arms, head). For every action there is an equal and opposite reaction.

From this position (one knee lifted and the other leg straight in line with the body while anchored by the body's center of mass), there is only one way to get the torso to be straight up and down again. That is to move the mass of the other leg an equal distance behind the torso as the first leg is in front.

In this last picture, the line under the blue dot is the center of mass for teh whole body while the lines under the yellow dots represent the center of mass of each leg (from the foot to the hip on each leg). Notice how the torso is vertical and the centers of mass of each leg are equidistant from the center of mass for the whole body. Since the figure is anchored at his body's center of mass (yellow dot under the elbow), the vertical position of the torso is completely dependent upon the centers of mass of each leg being equal distances from the vertical line under the blue dot.

Stay tuned to find out how this relationship applies to running.