The “hip complex:” The body’s differential.

The “hip complex”—the intricate arrangement of bone, muscle, nerve, and connective tissue that makes up the human hip—is one of the most sophisticated pieces of machinery in nature. As runners, it behooves us to get to know it intimately, because it is the center of athletic power. When the hips don’t function correctly, the body is not capable of dealing with the majority of the resultant torque (from forces produced during walking and running). This is the source of many common running injuries.

Addressing problems with the hip allows the resultant torque to be properly channeled and allocated to the center of gravity, which, during standing, lies squarely within the hips. Therefore, most interventions into the mechanics of the hip complex have to do with maintaining and facilitating the proper flow of mechanical energy throughout the body.

In Donella Meadows’ list of “leverage points” into a system, changes to hip mechanics are characteristic of place # 10:

10: The structure of material stocks and flows.

In this case “materials” refers primarily to the forces that the body generates and interacts with.

It’s important to discuss the hip complex from a few different perspectives. The technical details of how it functions are extremely important. However, even more important is to understand why it operates as it does: If we understand the proper function that it was evolutionarily designed for, and why it is so important to maintain it in correct working order, we’ll be able to divine many of the details of its mechanical function as necessary side-effects of our journey of athletic development.

Because the leverage points are ordered in terms of effectiveness, intervening in a more powerful leverage point (a lower number) will change the behavior of less powerful places in the system (a higher number). That’s why we don’t need to know the mechanical details (as specifically) if we focus on # 10; the mechanical details and the training of particular muscles refer to point #11 (The sizes of buffers and stabilizing stock, relative to their flows).

Arguably the most basic function of the hips is to provide a vertex around which the legs and the torso can counterrotate and oppose each other—in other words, to allow us to stand up. But the task of allowing an “unstable” tower like the human body to maintain standing is no easy feat. And it gets no easier when you consider that this tower has to move, in an extraordinarily agile manner, across variable terrain.

The hip accomplishes this due to its function as body’s differential. In the same way that a car’s differential allows each tire to exert a different amount of force into the road, (keeping the car traveling in the same direction),  the hip complex allows the legs to push against the ground with different amounts of force, while allowing the body’s balance and direction of travel to remain stable.

In other words, the function of the hips is exactly analogous to a car’s differential—and its shape is not that different, either. Both mechanisms consist of a series of components connecting three drive shafts in a T-structure (the left leg isn’t shown):


As mentioned above, both mechanisms (technological as well as biological), join three different shafts: a main drive shaft and two axle shafts. I use the example of a car differential because in that case, it is quite transparent that the primary purpose of the differential is not to join the axle shafts and the drive shafts structurally, but to join them in such a way that mechanical energy becomes transferred cleanly from the main drive shaft to the axle shafts. (Such a complex mechanism would not be necessary, if the hips were about pure “structure” or pure “movement.”) The hip complex is that sophisticated only because it must manage complex forces without compromising stability.

In a car’s differential, the drive shaft turns one way, and one axle moves in the same direction, while the other moves in the opposite direction. During a running stride, when our left arm swings forwards, the spine twists to the right—forwards—but the left leg swings back (and the right leg swings forwards). The hips, just like the differential, must negotiate these asymmetrical forces, in order to maintain the whole machine moving in the correct direction.

If only it were the case that we all possess perfectly-working differentials. But for most of us western runners, a lifetime of rest—in addition to the burden of lateral dominance—means that one side of the body has been growing slightly more, slightly faster, without environmental pressures, such as the need to move and run efficiently, forcing the correct development of the hip complex.

Because it is one of the most highly complex systems of muscles in the body (and it handles the most mechanical energy), misusing it and neglecting it—as most of us have, by virtue of our urban lifestyles—means that it is one of the first mechanisms to go awry.

That, by the way, is exactly what we see. Most of the great pathologies of movement for westerners, heel-striking, and bending from the waist to pick up heavy objects, are both symptoms of a poorly-functioning hip complex. I’ll discuss them accordingly in the future.

Because the hips, like the car’s differential, negotiate asymmetrical forces, we get hurt when they’re not functioning right. That’s why runners usually get knee pain and lower back pain: those two places in the body are the closest joints to the hip (that aren’t part of the hip itself).

For runners, knee pain is more common than lower-back pain because the lower back can rotate side to side, dissipating a small portion (but not all) of the forces that an incorrectly-functioning hip is not capable of handling. On the other hand, the knees are much less capable of handling the side-to-side torque (caused by the twisting of the spine during walking and running gait), that the hips are supposed to take care of.

Therefore, they are much more susceptible to damage—but damage only (typically) occurs when the hips are functioning incorrectly.

It is a testament to how badly we take care of the body’s differential, that most of our knees get hurt as we run.

For the purpose of injury prevention (and all-around athletic power), the most important place we can focus our attention as beginner runners is on the hips. It is far, far more important to develop the hips than to develop the quads, or the calfs, or to “work on speed.” If we develop the hips, speed will come—and it will be safe and sustainable. There are no guarantees if we come at it from the direction of speed.

The goal of increasing hip function will necessitate the increase of the function of the rest of the muscles. Because the hips are the nexus of the flow of mechanical energy, intervening at point # 10 (the structure of stocks and flows) will mean that point # 11 (the size of buffers relative to their flows)—i.e. the peripheral muscles, such as the quadriceps and hamstrings—will also necessarily increase their correct function, relative to the flow of energy.

If the hips start asking the quads for more power, the quads will grow accordingly. But it only works that way because the hips are the nexus of these flows (and so changes to their structure will be easier and more far-reaching if they occur at the hips). But this cannot occur the other way around.

The center of our athletic expression resides in the hips. Ensuring correct and complete hip function is the first important move we should take in developing our athletic potential. There are other characteristics of the hips, which I will discuss in the future, that also place them front and center in our athletic journeys.

Just like every single joule of force that goes from the car’s engine into the ground must pass through the differential, so must every bit of energy that goes through our body pass through the hips. Without a correctly-functioning hip complex, any of the power that the rest of our muscles can generate will amount to very little. In view of that, let’s attend to the hips.

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