Understanding the purpose in the design helps us appreciate the importance in the full mobility of the foot joints. For example a foot that is confined by restrictive or what we sometimes call “supportive” footware is less likely to have the mobility required to act to its full pumping and cushioning potential. I have also worked with joints that were jammed or out of place. Luckily we seem to have come to an era where the footware manufacturers are beginning to understand the mechanics of the healthy foot and are seeing more shoes that can help us keep it in a healthy, happy, pumping shape. Check out 5 finger shoes or minimalist shoes if you’re interested in this type of footware.
In my continuous quest to study the human body I get to find the engineering genius hidden within it. If you have a bit of engineer in you, I hope you will appreciate the intricate design behind mechanisms that may be less known yet profoundly important for structurally integrated and pumping body.
Tarsal Sinus pump and shock absorption - how it works
Tarsal sinus is a wide opening between the two bones on the outside of the foot. If you lift the foot off the ground the gravity draws the calcaneous from talus and the calcaneous drops towards the ground. This action opens the tarsal sinus wider. The veins in the leg have one-way valves and as the space in the tarsal sinus becames available, the blood can only go one way and it rushes in to fill it. Then, when the foot comes in contact with the ground the weight of the body brings the calcaneus and talus closer together and compresses the tarsal sinus. This forces the blood through the little vein on the inside of the ankle. The blood is effectively pumped up the leg and towards the heart.
The action of tarsal sinus filling and emptying also acts as a shock absorber for the foot. The outgoing vein carrying the blood up the leg towards the heart is of smaller diameter then the vein in the tarsal sinus, and therefore not all of the blood in the tarsal sinus can depart all at once, it takes time to push it through the small output vein, this slows down the closing of the talus and calcaneous on each other. Maximum flow rate in small diameter tubes is determined almost entirely by tube diameter, beyond a fairly low limit additional pressure produces little if any additional acceleration of the flow. This hydrolic deceleration of the approximation of the talus and calcaneous is a shock absorber. Some automotive shock absorbers work on the same principal forcing fluid through a small aperture.
And this is how it works when the foot is healthy, free and strong.