Flat Feet and Foot Pain: The Pull Door Model of the Midfoot and Rearfoot

One of the big reasons why flat feet can hurt so much is because when they flatten out they can actually increase the amount of stress applied to them throughout the day. Here we will look at the structures involved, a bit of math, and what a pedorthist can do to help.

The rearfoot, midfoot, and forefoot of a right foot viewed from above.

The foot has a few different parts. The rearfoot (red) connects the shin to the ground. The midfoot (blue) is essentially the back part of the arch of the foot. The forefoot (green) includes the toes, the ball of the foot, and the front half the arch.

The bones of the rearfoot in a neutral (rectus) alignment.

The bones of the rearfoot in a flat (pes planus) aligned foot.

The rearfoot is made up of two bones, the calcaneus or heel (red) and the talus or ankle (green). The talus sits between the long bones of the lower leg and acts like a hinge. The calcaneus forms the back portion of the arch of the foot and connects the rearfoot to the ground. It has a funny motion where it kind of topples over when weight is placed on the foot. Typically the bottom of the heel shifts outward while the top leans more towards the midline of the body. Keep this motion in mind.

The midfoot consists of five bones that are all vaguely cube shaped. The one on the middle of the foot just in front of the talus is called the navicular. When you are looking at the inside edge of your foot, you can probably see it as a bit of a bulge near or at the highest point of your arch. As the foot takes weight the midfoot tends to splay a little bit with the middle portions drifting towards the midline of the body and downwards. This sideways shift is also a motion to keep in mind.

Ok. Now for the wild bit. A touch of math.

In mechanics there is a concept called a moment. It is essentially leverage, completely unrelated to a moment of time. Forget clocks entirely for these kinds of moments.

A moment is a tendency for something to rotate. Torque. Leverage. It consists of a force, an object for that force to act on, a point of rotation, and a peculiar distance. I find it easiest to visualize this with a door. Just a standard door with hinges on one side and a handle on the other. For this example it will be a pull door, remember that muscles only pull, they cannot apply a push force.

A standard setup for a pull door that easily creates a strong moment.

A nonstandard pull door that would not work very well at all.

So we have our door. When we pull on it the door will swing open. Excellent. A moment in action. We applied a force on the object and made it rotate about a point. Standard operating procedure. But what if we moved the handle so that it is right in line with the hinge. That's where our peculiar distance comes in.

If you've ever run smack into a door because you tried pushing on the wrong end of it (like me) you've encountered this scenario. It is very difficult to open a door if you are applying a force, a push or a pull, too close to the center of rotation at the hinge. The final aspect of a moment, a tendency to rotate, is that the applied force has to be a perpendicular distance away from the point of rotation. Perpendicular just means that the more complicated math we don't need to go into likes to be broken down into right angles.

So the equation looks like this: M = f x d

Moment equals force multiplied by distance. So if we want to rotate something more we need to either increase the force or apply it further away from the center of rotation. Likewise, if we want to stop something from rotating, or reduce its tendency to rotate, we can either lessen the force acting on it or move that force closer to the point of rotation.

A neutral (rectus) rearfoot does not allow for much of a moment to form at the ankle.

A flat foot allows for a much larger moment that can put a lot of strain on the ankle.

Remember how the base of the heel tends to shift towards the outside edge of the body? And when the arch flattens out the midfoot drifts towards the midline? A large part of this (not the ONLY part, foot mechanics are complex) is because the joint between the ankle and the heel allows for rotation. Where those two bones meet is a centre of rotation. As the bones rotate, as a flat foot gets more and more flat, the forces acting on this joint move farther away from the center of rotation.

As you can imagine, having a really big rotational force in the middle of your foot can cause discomfort. In this case all of the ligaments holding the bones together on the inside of the foot are being stretched out a lot as the heel rotates around the ankle and the high part of the arch tries to drop down to the floor.

How a pair of custom orthotics can help is amusingly straightforward given the length of this article.

A pes planus foot with a supportive orthotic that places the rearfoot in a more neutral position.

A firm material is placed around the heel bone and underneath the arch to support them and prevent them from moving too much. This keeps the force of a person's body weight lined up nicely through the center of the ankle. We can't change the amount of force being applied, that's your body weight and the amount of time you need to be on your feet each day, but we can move the door handle so that it is right in line with the hinge. True it makes for a very inefficient door, but we'd rather keep the entrance to foot pain shut.