For many years, podiatric biomechanics has revolved around the subtalar joint axis. Indeed, the “neutral” in Neutral Position Theory, still the dominant biomechanical paradigm today, refers to the near midpoint position of motion about this axis. More recently, the “Subtalar Joint Axis Location/Rotational Equilibrium” or (SALRE) Theory of Foot Function has argued that STJ axis orientation in relaxed bipedal standing is the critical factor for determining corrective orthotic forces. This historical emphasis is understandable, given the locking/unlocking role of the subtalar joint with respect to tarsal integrity against weight-bearing forces. However, just because it is historically prominent, understandable and predictable does not necessarily make it of primary importance in terms of improving foot function. This is because a foot’s overall postural set relative to the ground, not the end-point orientation of the subtalar joint axis, has the most influence on function. Think orthopedically.

The most pertinent question to ask about any corrected over-pronating foot in the gait cycle is this: at the end of midstance, prior to heel lift and forefoot loading, is the posture of the foot supinated enough to maintain a stable, suitably plantarflexed position of the first metatarsal against ground reaction forces (GRF)? Because if that is not the case, it doesn’t matter whether the subtalar joint is in neutral or where its axis lies relative to the foot. A collapsed tarsus will perform just as poorly regardless of where the wedges, skives, bumps or contours are placed according to popular theories. The first metatarsal will fail to bear the majority share of weight with forefoot loading, shifting the load to the lesser met heads. The first metatarsophalangeal joint will struggle to dorsiflex against the restraint of fascial windlass tension (aka functional hallux limitus). The forefoot will splay excessively, inviting bunion and hammertoe deformities. If you have not transformed the operative posture of a foot on the ground, you are at best practicing “smoke & mirror” medicine, possibly hiding symptoms for awhile but missing the goal of good orthopedics. The sine qua non of orthopedic practice, regardless of profession, is restoring good musculoskeletal structure for improved function and health.

Common orthotic with low arch relative to foot in correct posture (left); unsupported zone in red (center); foot fully supported in corrected posture (right).”

When we consider the entire phenomenon of over-pronation, from heel strike to mid-stance, the vast majority of joint motion occurs external to the subtalar joint. After heel strike the calcaneus rolls anteriorly primarily in the sagittal plane, carrying the STJ along for the ride. If, as in most cases, the tarsus too readily unlocks due to insufficient supination, the calcaneus will undergo abnormal plantarflexion and eversion, again carrying the STJ along for the ride. Depending on the integrity of the spring ligament and other ligaments of the foot, some feet will have tarsal travel or splay that is only arrested by contact of the navicular with the floor. In even more extreme cases, the instability will include the talocrural joint and the patient will be walking on their medial malleoli. Regardless of the extent of tarsal instability, since most people have some degree of arch insufficiency, the relatively small range of the STJ will be dwarfed by the combined motions of the other tarsal joints, especially the motion about the heel rocker axis. Putting such a foot in subtalar neutral will not address the dysfunction of general tarsal collapse. Declaring the self-evident truth that an over-pronated foot has a medially deviated axis, then rearranging the usual bumps on the orthotic to theoretically assist supination around that axis, cannot mold the foot into the necessary amount of supinated posture. That requires a full contact device in the custom shape of the individual foot that was captured with the right amount of supination.

The amount of tarsal supination at HS is a major determining factor for the speed and extent of ensuing pronation, so an orthotic that can help mold the foot in supination can be helpful even before the foot strikes the ground. At ground contact, the orthotic will have much greater compressive influence on the plantar foot inherited from GRFs. Therefore, if the orthotic was made to capture optimal supination posture during casting, it can now act as an effective postural template for foot correction when the rubber hits the road. When the entire postural set of the foot is controlled by the orthotic both STJ range of motion and all other tarsal motions are controlled simultaneously.

As in any complex system, the whole of foot biomechanics is more than the mere sum of its parts. Historically, the podiatric profession has focused understandably but myopically on the role of the STJ. The result has been an intervention strategy that misses the forest for the tree. I propose that tarsal posture is the missing “whole” in foot biomechanics.