There remains a great deal of confusion in the literature as it exactly what is normal pronation and what is abnormal pronation.
However, when one understands the normal coupling between the pronation/supination motion in the subtalar joint to the horizontal plane oscillation of the pelvis, this confusion is quickly dissipated.
Hip Drive: Defined as Normal pronation
The understanding of normal pronation is based on the work of Inman (Inman VT. The Joints of the Ankle. Williams and Wilkins, 1976) in which he couples subtalar pronation/supination to the horizontal plane oscillation of the pelvis:
- Ambulation is an internal/external rotation of the pelvis around the weight bearing axis of the body. The total amount of this rotation varies from individual to individual. As the pelvis swings forward, the femur and tibia synchronously rotate internally. It is this internal shank rotation that generates rearfoot pronation at heel contact.
- As the pelvis swings backwards, the femur and tibia synchronously rotate externally. It is this external shank rotation that generates rearfoot supination, initiating at flatfoot stance in gait.
- This coupling of pronation/supination to internal/external horizontal plane rotations of the pelvis is referred to as Hip Drive (Rothbart) Transverse rotation of the innominates pronates and supinates the feet. Counter Clockwise l rotation of the right innominate, pronates the right foot. Clockwise rotation of the left innominate pronates the left foot.
Gravity Drive : Defined as Abnormal pronation
If the subtalar joint's pronation/supination is decoupled from the horizontal plane rotations of the pelvis, it is referred to as Gravity Drive.
A common cause of gravity driven pronation is a result from structural weaknesses within the foot itself (e.g., Rothbarts Foot and/or PreClinical Clubfoot Deformity).
In Rothbarts Foot, at flatfoot (stance phase), the first metatarsal and hallux are not touching the ground.
As the forefoot is weighted, gravity forces the first metatarsal and hallux downwards until they rest on the ground (referred to as Gravity Drive Pronation)
This downward rotation of the first metatarsal and hallux forces the subtalar joint to pronate when it should be supinating (See Figure below)
Rothbart BA 2010. The Primus Metatarsus Supinatus (Rothbarts) Foot and the PreClinical Clubfoot Deformity.Podiatry Review, Vol. 67(1):