Most of us know that when we run (or just walk around), our weight should be on our hips. This allows us to move faster and more powerfully, and to prevent injury. It’s also often said that the hips are the “center of gravity.”

All this is completely true. But what does “center of gravity” mean, anyway?

The “center of gravity” is the point in a body around which the resultant torque (or “resultant force“) due to gravity and other sources of mechanical energy vanishes. In other words, all of the forces that are generated by the body, as well as their interactions with the earth’s gravitational field, all get canceled out at the center of gravity.

The resultant force. This isn’t a commonly used term, but it’s one whose implications we should understand if we want to become safe, effective runners.

At the center of gravity, there is absolutely no movement. There is nothing pushing or pulling; no forces in motion. In that way, the center of gravity almost perfectly resembles the eye of a storm. Circling around it, defining it, is an incredible amount of mechanical energy coursing through the body in straight lines called vectors.

The body is built to handle vectors in particular ways, just like a car. A car is built to handle vectors of energy in a straight line: driving forwards. It is also designed to handle vectors that are coming at it sideways. Think of when you make a turn. If you would keep making that turn you’d eventually do a full circle. Centrifugal force goes in a vector from the center of the circle to the edge, tracing the radius of that circle. Apply too much centrifugal force for the amount of traction, and the car will flip.

The same sort of thing happens with the body. Experts in Jujitsu and Judo are trained to apply mechanical energy in vectors that the opponent’s body is not equipped to handle, in order to create pain to force a shutdown, or to break the machine apart if necessary.

Because the body is a very complex machine, it can manipulate and control many different vectors of energy. However, it is astonishingly better at controlling some vectors more than others (which is why you damage it if you apply force in the wrong vector). The body is best at controlling a vector that is pointed directly down at the ground from the center of our hip area. And the  body accomplishes this by pushing against the ground with the legs:

This makes sense: it’s how we propel ourselves through the world. That is also why skill in all sports ultimately comes down to hip mobility. In running, we see that the difference between elite and world-class athletes still comes down to hip mobility.

Remember that vectors are straight lines. If we have perfect hip mobility—no problem. The mechanical energy produced by our hips will travel in a straight line into the ground and also through our body (Newton’s Third Law), and it will go directly where it’s supposed to.

All the muscles and bones in our body are configured to deal with mechanical energy in certain vectors, and those vectors all end up at the same place: the center of gravity, where all the forces cancel out.

This is exactly what we see: the biggest muscles in the body (which channel the most mechanical energy) must, structurally, converge at the hips. And just look at the body’s structure: the iliopsoas group, the gluteus maximus, the quadriceps, the hamstrings, the latissimus dorsi—they all attach in extremely close proximity to the hips. The body is designed, in effect, to cancel out all of the mechanical energy coursing through it—all of the resultant force—at the hip complex. This is what it means, mechanically, to say that the hips are the “center of gravity.”

Because the body is configured for the center of gravity to be relative to the hips, any problems with hip mobility will change the center of gravity to some less-desired place. At first blush, this might not seem like a problem, but look at what changing the center of gravity does to the car: If one wheel stops spinning during a turn, the center of gravity shifts and it might send the car into a tailspin—resulting in severe damage.

For the body, this means that if one of those big muscles can’t handle the vectors correctly, the center of gravity will move to a mechanically unnatural/incorrect place, and the resultant torque will affect the body in ways that the body was never configured to handle. If the center of gravity is too high and too far forwards, you get lower back pain. If it’s off to one side, you’ll usually get lateral (sideways) knee pain and shoulder pain (probably with a whole bunch of compensation patterns at the hips).

There’s not one of us where the center of gravity resides in the mechanically ideal center. But again, this is not because “we got a bad center of gravity.” It’s just where the resultant torque vanishes. Change the strength and function of a muscle—especially at the hips—and the center of gravity will also change location.

Remember, even at the elite level, hip mobility still matters. One of the most important reasons that it still matters is because the actual center of gravity of elite runners has an extreme amount of overlap with its ideal location, meaning that the mechanical energy is moving through the body in all the vectors that the body is best equipped to handle.

Not only does this produce greater speed—since the muscles can pull in the same vector that forces are moving—but it also results in safer running.

When we run, our center of gravity moves slightly forwards.

Because the forces affecting the body always converge at the center of gravity, the most powerful muscles have to be there. Therefore, runners need highly-developed, highly capable frontal leg muscles (hip flexors and quadriceps), in order to interact with a center of gravity that is chronically moved forwards; the frontal muscles need to be able to interact with that resultant torque.

For that reason (and others), it’s very important for runners to be quite dedicated to training their frontal muscle groups. This means training everything from the quadriceps to the frontal calf muscles to the hip flexors. If the frontal muscle groups are strong, the body will feel safer and safer in moving the center of gravity farther forwards. And “strong” means that the frontal muscles must be more mobile and more dynamic. In other words, that the rear muscles, which oppose them, must also be strong.

The farther forwards the center of gravity is, the greater the speed we can achieve. If the body feels safe in maintaining the center of gravity very far forwards (thanks to very strong frontal muscles opposed by very mobile real muscles), we will become very, very fast runners.