A small, flat shelf of bone called the Sustenaculum Tali extends from the inside of the calcaneus and enables the foot’s whole elastic and weight bearing system to function efficiently in stance and gait. The 180-degree position of our own sustentaculum tali may well be the single most distinguishing feature of the human foot. The gorilla foot is angulated downward at approximately a 45 degree angle. Because of this the gorilla cannot maintain an erect posture in walking for more than a minute or so, at which point it must assume a quadruped gait. The straight-ahead hallux and the ground-touching heel are all distinctively human too. Evidence of the vital arch supporting role of the sustentaculum tali is demonstrated by the perpendicular line of weight falling from the body column to the foot passing exactly through the center of the sustentaculum tali. Any habitual or chronic shift in this line, such as caused by the elevated shoe heel plus the crooked shoe last, affects the sustentaculum tali and the long arch.
Conventional arch supports focus on an orthotic to “lift” or “support” the depressed or strained arch. The support is mainly under the center of the arch curve (navicular-first cuneiform joint). The orthotic is doing what deceptively appears to be the logical course; following the anatomical contour of the arch instead of providing the prime support where it is most needed— under the falling line of body weight through the sustentaculum tali.
So what about flat feet and fallen (failing) arches?
There is no conclusive single cause when the sustaniculum tali tilts and loses its natural support. So a series of rationales must be voiced. First, we can assume that if the sustentaculum tali is normally angled at 180 degrees it will maintain its support function. Second, if any chronic imbalances are imposed beneath the sustentaculum tali, such as with an elevated shoe heel, then there will be a corresponding shift in the falling line of body weight, affecting the angle of the sustentaculum tali, and with consequent effects on the rearfoot structural and functional mechanisms.
Hence the domino sequence; tilted body column and shifting of the gravity center, resulting in a shift in the falling line of body weight, followed by redistribution of weight stress paths through the foot, with corresponding arch strain and rearfoot dislocations. Any combination of variables could become involved. The pivotal element is the sustentaculum tali. It is so small and seemingly an obscure part of the foot anatomy. Yet its influence on the structural integrity of the rearfoot, as well as the midfoot and forefoot, is enormous. And in turn, it is vulnerable and influenced by the biomechanics of the shoes beneath it.
So why does this matter for running?
Proper running form involves landing in a perfect midfoot landing directly under your center of mass (and the sustentaculum tali). Se the picture below of Danny Dreyer from Chi Running and notice his column alignment while running. In this position your weight is supported on the structures designed for this impact. Any shift in this column either by heel striking in front of your center of mass and/or a shift in this column with elevated heel shoes will lead to unnatural stress in areas not designed for this.
How do you move forward then? You lean from the ankles and let gravity assist you as you liftyour legs. Sound too simple to be true? As one young Jedi said to Yoda…”I do not believe it”. Yoda’s reply: “That is why you fail”.
For a great videos showing some of these angles and corrections you can make standing and running go to this link from Terra Plana EVO , Newton Running instructional videos, and this inclusive set of videos from Chi Running.
Critical Role of sustentaculum tali. Center of mass passes perpendicular through this structure and the shelf needs to be maintained at 180 degrees to maintain its integrity. This is the position on the figure on the left with a flat shelf. The angled shelf does not allow proper biped gait.
Rearfoot Anatomy 101- This is the Left Foot from the Rear
13- Tibia, 14-Fibula, 16-Talus, 17-sustentaculum tali (correct alignment), 18-Calcaneus
Chi Running with the Center of Mass over the Sustenaculum Tali- this is where the body is designed to bear the load. It is no different than standing position in the proper posture with feet flat on the ground.