A podiatric introduction to Biomechanics of the Foot

There are three main theories that help determine the cause of the foot pathology from a biomechanical standpoint:

  1. Sagittal plane facilitation (SPF)
  2. Subtalar joint neutral (SJN) or Foot Morphology theory
  3. Tissue stress theory (TS)


STJN was initially described by Merton L. Root, and therefore is  also known as “Rootian” biomechanics. STJN occurs at heel strike, and at the end of midstance. Foot morphology is currently used term to describe abnormal or normal foot function as it relates to STJN. For example, a foot that has FM that deviates from “normal” such as forefoot varus, would need to an orthotic device to return the foot to normal. If the foot remains with a forefoot varus,  pronatory compensatory motion would occur at the STJ thereby causing an abnormal STJN.

How is the orthosis made to return the abnormal FM to normal?

Negative Casting

  1. Casting the foot in NWB neutral position
  2. Lock the midfoot i.e. end point so that arch isn’t “wobbly”
  3. Ensure the forefoot to rearfoot alignment is correctly captured
  4. Ensure the cast has a proper arch, and the lateral edge of the foot is captured

Positive Cast

  1. After pouring

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