Shock Absorption and Foot Type

The structures in the foot aren’t static. They move and flex as we walk: the heel contacts the ground, body weight is transferred to the foot, our body travels over the foot, the heel lifts off the ground and then our weight is removed from the front of the foot. There are upwards of thirty joints in the foot and all of them move when we stand and walk. These motions are important because they are what provides shock absorption while we are standing and walking. The difference between a smooth comfortable ride on squishy suspension and an uncomfortable, jostling, jarring ride in a steel wagon. 

Of particular note is the joint in the rear of the foot that connects the ankle to the heel. This joint consists of the ankle bone, or talus (in green) and its connection to the heel bone or calcaneus (in red). It is often referred to as the subtalar joint because it is the joint on the bottom of, or sub to, the talus bone. In other fancy terms this is classified as a synovial plane joint, which essentially means that it lets your heel bone kind of slide across the bottom of the ankle bone and pivot a bit as shown below. 

Bones of the rearfoot with the unusual motion of the subtalar joint highlighted.

So even though this subtalar joint includes the ‘ankle’ bone, the talus, it probably doesn’t do what you are expecting the ankle to do. That big gas pedal motion where you pivot your whole foot up and down does also include the talus, but it actually happens at its upper interface with the shine bones (in black).

Instead, the subtalar joint controls how we interface with the ground when we walk with its side-to-side motion of the heel. When taking a step, we will ideally hit the ground heel first. Its even more specific than that in fact, we want to hit the ground the outside part of our heel and then have it roll through it’s range of motion until we are resting on the middle or even the somewhat inside portion of it.

This resisted motion acts just like the springs in a set of shocks that make going over a speedbump in your car more comfortable. It is a touch harder to conceptualize because while the springs simply compress up and down, linearly cushioning the motion of the car above the road, the subtalar joint kind of twists to cushion this motion. Actually, with driving on the mind, the rearfoot acts a bit like how the crumple zone on the bumper of a car converts a harsh sudden impact into a much more comfortable, more drawn out interaction, rather than imparting all that force in one sudden BAM.

When this motion goes too far, the bottom portion of the heel shifting too far towards the outside edge of the foot, it is called rearfoot eversion. It is essentially the rearfoot component of a flat foot and all that extra sliding and rotation can create pain in the foot on its own. Take a look at Moments and Doors for a closer look at that.  A flat, or pes planus, foot has very soft suspension that can cause some problems.

The rearfoot bones of a flat foot highlighting excessive motion at the subtalar joint.

But what about high arches?

A high arched foot will naturally have the rearfoot tend more towards inversion, where the bottom of the heel tends towards the midline of the body. Feet with high arches tend to be less flexible on average (it’s a consequence of how all the bones fit together) so there tends to be less rearfoot motion available for folks with higher arches. On average this motion is also less fluid, like being on stiffer shocks, and this also contributes to their being less shock absorption.

A lack of shock absorption can cause pain not only in the heel, right at the base where it touches the ground, but also further up the ankle and into the leg. Every time the heel hits the ground the impact will send a shockwave up the leg. Stiffer suspension means a sharper, more powerful shockwave that can irritate the back of the ankle, muscles of the calf, the hips, lower back or frankly pretty much anything else connected to any of those systems.

The rearfoot bones of a high arched foot highlighting the limited motion at the subtalar joint.

In a case like this I would use very squishy, but long lasting, materials to create what is essentially a crash pad underneath the heel to give it some shock absorption. It ends up being kind of like one of those fatigue mats in factories which work off a similar principle. However, in this case it is small enough to fit inside your shoes and it follows you around wherever you go.