Normal Foot Pronation is defined in terms of hip mechanics: internal hip rotation drives the same side foot to pronate, external hip rotation drives the same side foot to supinate.

Normal Foot Motion (See model left):  At late mid-stance the right hip is externally rotating, which drives the right subtalar joint into supination.  Subtalar joint supination locks the midtarsal and forefoot  The foot is stable as the body's weight travels over the inner longitudinal arch.  (Zitzlesperger S 1960.The mechanics of the foot based on the concept of the skeleton as a statically indetermined space framework. Clinical Orthopedics, 16:47-63)

Click here to view a closeup model of Hip Drive Pronation.

Abnormal Foot Motion:  At late mid-stance, if external right hip does not drive the right subtalar joint into supination, the midtarsal and forefoot remain unlocked and unstable as the body's weight travels over the inner longitudinal arch (Elftman H 1960. The transverse tarsal joint and its control. Clinical Orthopedics, 16:41).  Forefoot shearing occurs (the loose bag of bones syndrome). This commonly results in foot pathology, e.g., bunions, hammertoes, plantar calluses, and heel pain as well as postural distortions.

Embyological Unwinding of the Foot