Some thoughts on gait and foot pronation…
It is our perception that alot lot of folks seem to be on a mission to eliminate pronation, calling it the scourge of humanity and source of human ailment. While we agree that overpronation causes biomechanical faults in the lower kinetic chain, so does under pronation, and some pronation is necessary and required for normal locomotion. Today we would like to deepen your understanding and appreciation of pronation and it’s potential impact on the foot and lower limb. Today’s post will seem pretty technical for some readers. But it is merely a language issue, one that if worked at will in time become second nature. This is language that we are putting into our Shoe Fit DVD 3 part program, and language that we used at our program launch in Austin TX last week. So if you are thinking this stuff is too difficult, you will be shocked that in a few months many shoe stores around the country will have staff that will be familiar with these terms and biomechanics. Shoe stores that will be able to communicate with doctors and therapists on a common level, because that is what is necessary to improve shoe fit and client outcomes.
When most people think of pronation, they think of midfoot pronation, or pronation about the subtalar or transverse tarsal joints. In lay terms this means collapse through the arch of the foot. Pronation can actually occur about any articulation or bone, but with respect to the foot, we like to think of rearfoot (ie. talo-calcaneal), midfoot (talo-navicular) and forefoot (transverse tarsal) pronation.
Pronation, with respect to the foot, is globally defined as a combination of eversion, abduction and dorsiflexion which results in flattening of the main arch (aka. plantar vault) encompassing the medial and lateral longitudinal arches. 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.
In a perfect biomechanical world, shortly following initial contact with the ground, the calcaneus should evert 4-8 degrees because the body of the calcaneus is lateral to the longitudinal axis of the tibia. 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 tibia rotates internally (medially). This translates up the kinetic chain and causes internal rotation of the femur, which causes subsequent nutation (anterior tilt) of the pelvis (see picture above) and extension of the lumbar spine. 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. So in summary, what we have said here is that when the foot hits the ground there is a process of unlocking of the foot structures to allow the foot to splay and flatten to a degree for shock absorption and adaptation to variable surfaces. And when this splay and flattening (pronation) occurs, the leg spins inwards to allow other normal biomechanical events to occur higher up in the kinetic chain.
Pronation, along with knee and hip flexion, allow for shock absorption throughout the 1st half of stance phase. 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. 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 absorption.
The consequence of under or over pronation ultimately means other articulations, including the spine, will have to attenuate more shock. Over time, this may lead to articular cartilage degeneration or ligamentous laxity due to repetitive stresses.
What about asymmetrical pronation? It is rare that people over or under pronate the same amount on each side. 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 stretches the hip adductor group, often making them stretch weak, and shortens the hip abductors, especially the gluteus medius, which often becomes short weak. Frequently the IT Band will shorten in this scenario. The right foot, since it is now a poor lever, will often be externally rotated and toes will claw, because the center of gravity has moved medially and they are trying to make that limb stable to bear weight on so they can progress forward. They will often toe off from the inside of the great toe (as is often evidenced by a pinch callus medially ) and this medial foot tripod challenge can lead to factors causing bunion formation. The medial rotation of the lower leg (relative, because of the externally rotated foot) causes internal rotation of the thigh and anterior nutation (tipping) of the pelvis on that side, both which now put the gluteus maximus at a mechanical disadvantage thus limiting hip extension on that side. Now the extension has to occur to somewhere, so it often occurs in the lumbar spine, along with rotation and lateral bending to that side, increasing compression on the right spinal facet joints. From a neurological perspective, the vestibular system now kicks in to level the head, the result being contraction of the left paraspinal muscles. Arm swing usually increases on the contralateral side to assist in propulsion forward. What effect do you think THAT has on spinal mechanics over 10 thousand steps a day? What effect are we having on the nervous system and what neuroplastic changes are occurring?
Having both feet planted on solid ground, or rather having both feet planted solidly on the ground should concern you. As you can see, knowing about pronation and its effects on the entire kinetic chain is paramount. The effects reach far beyond the foot and can often be the root of recurrent biomechanical faults in the human frame. Think about this next time you buy a shoe, add an insert into a shoe or get fitted for orthotics. There are many things you are impacting if the recipe or prescription is not perfect.
Yup, we are the Gait Guys (two aging bald guys promoting gait literacy) and yes, we have the references to back us up. No fluff, just the facts…..