Wheel Analogy

Ever heard the expression “It looks like the wheels have come off.” used by sports commentators once the form of a runner deteriorated? It couldn’t be more perfect of an analogy.

The wheel has several properties that make it such a good horizontal mover. First, it always has its general center of mass (GCM) above its support. Second, it constantly changes its support when moving. And lastly, its support is always at exactly the same distance from its general center of mass. Let’s look more closely at each one of these.

The general center of mass is the point where one can balance an object. For a circle this is its center point. In a concrete case like the wheel, this is where the axle goes.

The change of support can easily be observed by the spokes of a wheel. When the wheel is moving, there is a constant shift from one spoke to the next. There is also a constant feed of new material that is contacting the ground, namely the tire. This constant switching is reducing wear. This in contrast with a fixed support that you have to drag to move.

A circle by definition, and by extension a wheel, has all the points of its edge at the same distance from the center. This offers a smooth horizontal movement. There is no up and down bouncing, or vertical oscillation. No energy is wasted in counteracting gravity. The movement is both smooth and efficient.

Let’s apply these concepts to running. Of course, humans are bipeds and don’t have wheels. We won’t be as efficient as with wheels but we can be as efficient as possible. We’ll use the three factors as mentioned above: GCM above support, constant change of support, and no vertical oscillation. A human’s general center of mass is located a bit below the navel. Every time we are supported, we want our body’s weight over this support as perfectly as possible. The supports we use during running are our two legs and feet. With every stride we use one to balance our body against the surface. As with the wheel, no matter the speed with which we move forward, a new support is constantly available. The faster we go, the faster a new support should come. Bounce should be minimal. Bouncing is observable by someone watching our head move next to a fence or any other horizontal edge. Or from the runner’s perspective, if you keep an eye on the horizon or a fixed object, it shouldn’t move up and down a lot. A fine example of this is Alberto Salazar, a top marathoner in his time. During the 1981 New York City Marathon there is a shot of the leading runners crossing the Queensborough Bridge. Here we see them behind the railing with their head and shoulders sticking out above. An excellent reference frame to observe vertical movement. Salazar is hardly moving up and down. There is no energy wasted in lifting the body. The “Salazar Shuffle” is indeed an efficient way of moving forward. Here Channel 6 provides the full broadcast. Around the 1h10:50 mark the broadcast switches to the bridge crossing.