To say “stand up straight” is a bit of a misnomer; the body is made up of a series of curvatures.

When you look from the side at someone standing up straight, you’ll see that their body actually follows a curve which begins at their heels and ends at the top of their head. Think of a hunting bow: when a bow is strung, the bowstring holds both ends of the bow together, allowing the structure to store a magnificent amount of potential energy, to be released when an arrow is loosed. The extensor muscles of the body (hamstrings, glutes, and back muscles) perform largely the same function as the bowstring:

The quintessential proud or dominant posture is achieved by tensing the extensor muscles to align the majority of the bones in the body to create a series of arches, or bows. By maintaining tension in this way, the body creates a firm (yet dynamic) structure. This isn’t a spurious analysis: throughout the history of architecture, arches have been the quintessential support structure. For these same reasons, suspension bridges are built in high-wind and earthquake-prone areas: because using tense cables to support the structure not only maintains the bridge’s shape very well, but does so despite the power that wind or seismic events can exert on it. By contrast, a bridge that is completely rigid through and through would be far less resilient. 

Because the body is a moving system (as opposed to a rigid system), it creates these arches in order to more easily engage with the forces that routinely interact with the body, such as the force of gravity and the kinetic energy generated during exercise.

As you can see in the above image (and likely recognize from your own experience), the greater and more tense the curvature in all of the arches of the body, the more emphasized this proud posture becomes. When you see two people puffing their chests out at each other in a clear contest of dominance, they are not only trying to make their bodies look larger to their perceived opponent.

In a very real way, there is a much deeper, parallel game going on: their bodies are competing as mechanical systems. The extent of the curvature and alignment of all of these different arches shows the opposing system just how much potential energy this system is able to actively store in its structures. This hasn’t gone unnoticed throughout the ages: according to the Online Etymology Dictionary, the Modern Greek word kamari, from the ancient Greek kamarou “to furnish with a vault or arched cover,” is used to describe someone puffing up their chest to appear dominant.

When two people puff up their chest to try and intimidate the other, each underlying mechanical system is telling the other: “Look at all this potential energy. This is how much kinetic energy I can deliver. You think you can match me?” The firmness and resiliency of this “system of suspension bridges” also contributes directly to our athletic achievement. Simply stated, creating arched structures is exactly how the body expresses its strength.

In order to run powerfully, the human body contralaterally alternates the expression of these arched structures during the gait cycle. In simpler terms, this means that when you toe off with your right leg, the body tenses the extensor muscles (red) to exert power against the ground by creating a firm arch that begins at the base of the head, and ends at the foot. At the same time, the same arch on the left side is collapsed; the flexor muscles (depicted in blue) are pulling at that side of the body—basically crumpling it forwards—in order to move the center of gravity forwards, creating distance between the back and the front foot, and therefore produce forwards motion.

These actions of synchronized flexion and extension occur most powerfully from the hip, the leg, and the lower leg—which is where the majority of the body’s power is generated. They are referred to as “triple flexion” and “triple extension,” respectively.

This full-body arch can be seen in the most powerful runners, from Meb Keflezighi to Shalane Flanagan to Usain Bolt:

By understanding the body on a much deeper (and more abstract) level, we can glean a lot of information that can help us in our athletic development. In other words, it’s not about muscle strength or about aerobic power. Muscle strength is for creating the arch (and collapsing the other one) and aerobic power is about being able to repeat this process over the course of 1000 meters, or the marathon.

That’s the point of strength, or endurance. If, during our training, we’re developing our aerobic engine or our muscle strength, but we’re not teaching our body to recreate and strengthen these arches, we’re not going to have a chance at becoming the fastest version of ourselves. The fastest version of ourselves will, inevitably be found within these geometric principles.

A word of caution now: none of this means that you should just go and try to arch your back as far as possible when you run. The key is to develop the muscle power necessary for your body to express these arches, and the aerobic power necessary to repeat them ad infinitum. Think of it as a question: what needs to happen for my body to want to create these arches, and to maintain them?

For a lot of us, the journey of athletic achievement needs to be reframed: it shouldn’t be about increasing something, be it speed power of endurance. (They will increase, but not because we directly push at it). Instead, it should be a process of inquiry, to get our body to tend towards simplicity: in our training, what are the necessary steps—A, B, and C—to take, in order to get our body to express the smoothest, most parsimonious curve, (and then to get it to do that over and over)?

The body’s speed, power, and endurance will increase in service of finding the answer to this question.

However, next time you go run, don’t try to “arch” anything. That will leave you with a contorted back, tight hamstrings, aching calves (and possibly plantar fasciitis).

If you must use this as a training idea, think of it in a more abstract way: string the bow (from the base of your head to the heel) on the side that’s contacting the ground. Because the body moves contralaterally (the right arm and left foot move back at the same time, while the others move forwards), your upper body should freely rotate opposite to the leg that’s stringing the bow.

Relax the body: remember, this process of stringing the bow will alternate with each step (some 160-180 ish times per minute), so the muscles need to remain taut but supple to achieve this. It isn’t going to be easy: as I mentioned above, properly maintaining this bow structure takes more strength than not doing so. But the upshot is that the body is made for it. The better formed (smoother, firmer, more dynamic) the arches of your body become, the more resilient to stress your body will be.

The very same thing that produces speed—the correct geometry—will also protect you against injury.