Ivo Waerlop DC DABCN, Shawn Allen DC DABCO, Robert Lardner PT
A lot of folks seem to be on a mission to eliminate pronation, considering it the scourge of humanity and source of most human ailment. While we agree that overpronation causes biomechanical faults in the lower kinetic chain, so does under pronation, and some pronation through the various axes of the foot is necessary and required for normal locomotion. This month we would like to deepen your understanding and appreciation of pronation and its potential impact on the musculoskeletal practice.
When most people think of pronation, they think of midfoot pronation, or pronation about the subtalar or transverse tarsal joints. Pronation can actually occur about any articulation or bone, but with respect to the foot, we like to generalize and think of rearfoot (ie. talo-calcaneal), midfoot (talo-navicular) and forefoot (transverse tarsal) pronation.
Pronation, with respect to the foot, is loosely defined as a combination of movements of eversion, abduction and dorsiflexion which results in flattening of the planter vault encompassing the medial and lateral longitudinal arches 1. In a normal gait cycle, this begins at initial contact (heel strike) and terminates at midstance, lasting no more than 25% of the gait cycle. 1,2
In a perfect biomechanical world, shortly following initial lateral heel contact with the ground, the calcaneus should evert 4-8 degrees 3,4 because the body of the calcaneus is lateral to the longitudinal axis of the tibia 5. This results in plantar flexion, adduction and eversion of the talus on the calcaneus, as it slides anteriorly. At this point, there should be dorsiflexion of the transverse tarsal (calcaneo-cuboid and talo-navicular) joints. Due to the tight fit of the ankle mortise and its unique shape, the tibial is driven into internal rotation (medial rotation)6. This translates up the kinetic chain and causes internal rotation of the femur, which causes subsequent nutation (anterior tilt) of the pelvis and extension of the lumbar spine 7-9. This should all occur in the lower kinetic chain through the 1st half of stance phase. The sequence should reverse after the midpoint of midstance, causing supination and creating a rigid lever for forward propulsion 10,11.
Pronation, along with knee and hip flexion, allows for crucial shock absorption throughout the 1st half of stance phase 12. Pronation allows for the calcaneo-cuboid and talo-navicular joint axes to be parallel making the foot into a mobile adaptor so it can contour to irregular surfaces 13,14. Problems seem to arise when the foot either under pronates (7 degrees rearfoot valgus results in internal tibial rotation), or over pronates (> 8 degrees or remains in pronation for greater than 50% of stance phase) resulting in poor shock absorption11.
This all being well established, what about asymmetrical pronation? It is rare that people over or under pronate exactly the same amount on each side. These asymmetries are usually driven by weakness, such as from old injuries, surgeries or from compensations. Excess midfoot pronation on the right causes more internal rotation at the right knee, and an increased valgus stress at that joint. This puts the quadriceps at a mechanical disadvantage and places an adverse load on the adductor group, often making them stretch weak (unable to develop necessary eccentric loading), and thus reflexively and protectively alters the function of the abductors, glutes and others which often become weak from repeated eccentric loading in an attempt to help limit the degree and rate of internal limb rotation. The right foot, since it is now a poor lever in its more pronated state, will often be externally rotated with toes clawed or hammered (intrinsic muscle imbalance), because the center of gravity has moved medially and they are trying to make that limb and foot stable to bear weight so they can progress forward. They will often toe off ineffectively from the inside of the great toe (as is often evidenced by a pinch callus medially) and will do so with an unbalanced and uncoordinated effort of the hallucis flexors, abductors, adductors and extensors causing functional instability of the medial foot tripod. The medial rotation of the lower leg (relative because of the externally rotated foot) causes internal rotation of the thigh and anterior nutation of the pelvis on that side, both which now put the gluteus maximus and abdominal wall anchors at a mechanical disadvantage thus limiting hip extension on that side. In this scenario, the sagittal plane extension usually expected at the hip has to occur somewhere and is frequently passed either into the knee (hyperextension) or into the lumbar spine, along with rotation and lateral bending to that side possibly increasing compression on the right facets. From a neurological perspective, the vestibular system now kicks in to level the head the result being contraction of the left paraspinals, sometimes inadequately so. Arm swing usually increases on the contralateral side to assist in propulsion forward all in the ongoing environment of unequal stride lengths from the asymmetries described above. Perhaps one question that should be entertained is this: What effect could this have on spinal mechanics over 10 thousand steps a day in the average person or even more devastatingly into an aggressive athlete ? What effects are we placing on the nervous system and what neuroplastic changes are occurring? As you can see, gait is terribly difficult and complex with little room for error and with compensation possibilities almost countless in number. Given time, the body will usually find a compensation pattern that provides an adequate degree of stability to ambulate, however not without a price and not without pain if the system is overburdened long enough.
The consequences of under or over pronation, regardless of cause, ultimately means other articulations, including the spine, will have to attenuate more shock 7, 15-18. Over time, this may lead to back pain 7, 12, 15, 17, 19, knee pain 16,17, 19-23, articular cartilage degeneration 19-21 or ligamentous laxity 16, 21, 22 due to repetitive stresses.
As a chiropractor or other manual medicine practitioner, understanding the mechanics of pronation and its effects on the kinetic chain should alert you to other possible causes of a patient’s presenting symptoms or mechanics observed during the physical examination. Some pronation, just as some supination, is necessary for normal ambulation. When there is excessive or deficient pronation, the body must compensate in some other manner, which often results in a symptom which sometimes can be the reason the person sought your care in the first place. Recognizing normal mechanics will help you to decide where the fault in the biomechanical chain is and which action: manipulation, exercise, stretching, prescription of an orthotic etc. would be appropriate.
Dr. Waerlop, Dr. Allen and Robert Lardner lecture and write on gait mechanics and rehabilitation. They are biomechanical consultants for Vibram USA. They can be reached at drivo@netzero.com , doc@doctorallen.com, and rlpt@sbcglobal.net
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