custom footbeds

THE DREADED DIAGNOSIS: PRONATION

You may have been told by a boot-fitter or even a ski pro or coach that your feet pronate and that this will make it difficult, if not impossible, for you to hold an edge and/or control your skis. Or you may have overheard a boot-fitter run through a pronation diagnostic drill with a customer having a boot fit and/or alignment session. It goes something like this.

Boot-fitter to customer: Now stand with the weight even on both feet and flex your boots. Oh yes, I see your problem. Look down at your knees when you flex forward. See what happens. Your knees move towards each other. This explains why you are having so much trouble skiing. Your feet are pronating.

Customer, fearing their situation is terminal: Is this bad? Is there any hope? Can you help me?

Boot-fitter:  Yes, pronation is very bad. But don’t worry. I can help you. A pair of custom footbeds and an alignment job and you will be a World Cup star.

Customer: Wow! Sign me up!

It all sounds good. But there’s one small problem……. OK, it’s a big problem. Standing on two feet and flexing ski boots has nothing to do with skiing. In fact, it has nothing to do with the normal function of the lower limbs (ergo – the legs). The boot-fitter may not realize it, but the boot-flex procedure is something of a con, a sleight of hand. The basic balance strategy of the body when standing erect on two feet is to maintain position Centre of Mass slightly in front of the ankle joint so that gravity tends to disturb balance and cause a forward fall by causing the ankle to flex forward. This tendency is opposed by muscles in back of the leg starting with the soleus.

The soleus is an extensor muscle. It’s job is to extend or plantarflex the ankle so that the forefoot is pushed down; towards the floor. This pushes Centre of Mass backward, against the force of gravity to the isometric contraction position.

The foot is configured for standing on one foot so that gravity will tend to cause the foot to evert or pronate. Eversion is the turning of the sole of the foot away from the centre of the body. The tendency of gravity to cause the foot to evert or pronate is opposed by a group of muscles that I refer to as the inverter sling. Here’s the kicker. The inverter muscles of the everter sling are also extensors.

Think of the front of the boot cuff as a resister of forward ankle flexion. Guess what happens when the resistance of the boot cuff starts to support your weight when you flex forward and your shins press against the front of the shaft? Your hard working extensor muscles start to go on vacation because there is less and less for work for them to do. Since the inverters have also turned off, guess what happens next? The feet relax and fall inward in a quasi-pronation movement. It’s not the same as functional pronation. The knees turn inward towards each other more than they would when the extensors and inverters are in isometric contraction. There’s nothing wrong with your feet. It is just that muscles only do their job when they have something to do. Things are not really what they appear to be. But the illusion sells footbeds and alignment programs.

 

PRONATION: FORCES IN THE FEET – PART 2

Before I can discuss the role of pronation in enabling a skier to develop a dynamically tensioned base of support, I need to clarify the implications of a neutral foot.

The 2 big buzz words in boot-fitting are neutral foot and neutral alignment meaning that the knees track straight forward during ankle flexion.

The ankle complex consists of two major joints. The base of the tibia forms a joint with a bone called the talus. The resulting joint is called the tibial-talar joint, commonly referred to simply as the ankle joint. A second joint below the talus is the sub-talar joint. This joint underlies the tibial-talar joint. It allows the foot to rotate about its long axis in eversion and inversion. The tibial-talar joint is the joint that flexes the foot in plantar-flexion (toes move away from the shin) and dorsi-flexion (toes move closer to the shin).

TT -STJ

Due to the confusion and misinformation that has arisen surrounding pronation, a common perception exists that anything other than flexion of the ankle is abnormal and problematic. The solution is footbeds that are claimed to maintain the foot in a neutral axis and alignment procedures typically follow that are claimed to correct abnormal tracking of the knees and ensure that they move straight ahead on a neutral axis when the ankle joint is flexed.

None of these positions are supported in sound principles of biomechanics. But I will save further discussion of this issue for a future post. The image below shows the same neutral configuration of the feet in bipedal neutral stance as my last post. In this image I show the straight ahead (neutral) excursion trajectories of Centre of Mass and Centre of Pressure in each foot. I also show the axis of the ankle joint as square (at right angles) to the excursion trajectories. This does not happen in reality. But in order to differentiate between neutral Sub-Talar Joint and pronation I need to indulge the neutral camp.

Neutral axis

The limits of the Base of Support that define what is called the sway zone is shown in grey. Because of the large area of the Base of Support at one time or another some formulators of ski methodologies have advocated a wide track stance with  weight and steering on both skis because it is more stable than a narrow stance where the weight is predominantly on the outside ski of a turn. In fact, as I will show in future posts, a wide track stance is not only highly unstable, it precludes the ability to develop a dynamically tensioned base of support on which to move from one foot to another foot.

PRONATION AND THE BIOMECHANICS OF EDGE CONTROL

The stick man sketch below are Figures 23 A and 23 B from US Patent No. 5,265,350 (expired) awarded to the writer.  The stick man in FIG. 23 A is engaged in quiet standing with the weight equally distributed between the left and right feet. This is called Bipedal (two-footed) Stance. The force vector W emanating from the Centre of Mass or CoM  is the ‘disturbing force’ of gravity.  W is called a disturbing force because it is tending to disturb the equilibrium of the stick man and cause him to topple.

Bi-Monopedal

Gravity is an ‘attractive force’ like magnetism. CoM is where you are in relation to the supporting surface. In this case, ground. But W is not the force applied to ground by the stick man. The applied force occurs at the contact points of the foot or feet with ground.  In FIG 23 A,  W lies equidistant between the two feet in the transverse plane.

In normal Bipedal Stance, each foot supports equal proportions of the bodyʼs weight W, assuming equal leg lengths. Approximately 50 percent of the load is borne by the heel. The remaining 50 percent is borne by the heads of the long metatarsal bones. The load on the head of the first metatarsal (aka the ‘ball of the foot’) is twice that of each of the heads of the other four metatarsals. The anteroposterior (ergo, front to back) distribution  of the load through the foot is due to the position of the CoM of the body above.  The point on the foot where the centre of the applied force appears to act is called the Centre of Pressure or CoP. I say ‘appears to act’ because CoP could lie somewhere in the vault of the arches of the foot. In Bipedal Stance CoP lies on an axis that runs through the proximate centre of the heel and the head (ball) of the 2nd metatarsal. In ice skates, this is the ‘balance point’ where the ice blade is mounted. The forces shown as w2 are the centres of the ground reaction force or  that opposes CoP.

The footwear industry’s dirty little secret is that shoes are made on lasts that approximate the shape of the human foot in Bipedal Stance; standing on two feet and not moving. When you start to walk in a shoe, the structures deform and distort to accommodate changes in the architecture of the foot. Ski boots are worse. Not only are they built on lasts that are the approximate the shape of your feet and legs in quiet Bipedal Stance, the structures impede or even prevent the user from attaining a dynamically balanced base of support on one foot. Claims in relation to skiing are made that the human foot functions best in skiing when its joints are immobilized, preferably in a neutral position. In a neutral position, joint actions of the foot and knee and hip are limited to flexion and extension with transverse and orbital movement of the leg in hip joint within its normal range of motion.

A whole industry has been established on methods of immobilizing the foot and stabilizing it in a neutral position with custom formed boot shells, custom formed liners and custom formed footbeds and orthotics that significantly restrict or prevent pronation. The indirect effect of preventing pronation is that the position of CoP on the axis running through the proximate centre of the heel and head of the 2nd metatarsal becomes fixed.  For reasons that will be explained in future posts, this can have the effect of preventing the user from being able to establish the over-centre edge control mechanics that the best skiers use and especially an inability to establish a dynamically stable base of support on which to move from ski to ski.

THE MYTH OF PRONATION AND THE COLLAPSING ARCH

If you purchased a pair of ski boots or had work done by a boot-fitter you may have been told that your feet pronate and that this will cause your arch leg to collapse creating a myriad of problems including a lack of control and especially an inability to effectively transfer energy. You may have also been told that your foot functions best in skiing when it is immobilized, preferably in a neutral position, one that completely prevents pronation. All of the preceding are myths. But more than simply being untrue, they are potentially dangerous. ‘Pronation is bad’ is nothing more than a good story; one that most people buy into simply because they have no idea of what pronation is. So they tend to assume that the store clerk or boot-fitter actually knows what they are talking about.

When I first started modifying ski boots in 1973 in an attempt to help my own skiing I bought into the pronation is bad story after reading an article in a running magazine about how over-pronation (an invented term) was causing injuries in runners. The article implied that the human feet, far from being a marvel of complex engineering, are riff with problems like ‘pronation‘  that require corrective devices such as footbeds or prescription orthotics to make feet function properly. This made sense to me at the time, but only because of what I didn’t know. After reading the story in the running magazine, I had what I thought was an epiphany; ‘If foot defects can cause a myriad problems in running, imagine what havoc they must be wreaking in skiing’. Thinking I was on to something, I had prescription orthotics made for my wife and myself by a sports podiatrist. Later, I began making custom footbeds for ski boots. In the 1970s, I was one of the few boot-fitters in world making custom footbeds. And while the response of the majority of skiers I made them for was overwhelmingly positive there was one problem, my own custom footbeds didn’t work for me. In fact, footbeds and prescription orthotics made skiing much more difficult for me than insoles with no arch support. Unfortunately, the pronation is bad story caught on. Soon the evils of over-pronation morphed into the evils of pronation period; any and all pronation was bad and should be stopped or at least greatly restricted.

As Mark Twain quipped, “It ain’t what you know that gets you into trouble. It’s what you know for sure that just ain’t so”. For the past 30 years most of the world has known for sure that pronation is bad. Now a study published this month (1) confirms what I and others have maintained for years, not only is pronation not a predisposition to injury but there are significantly less injuries in those whose feet pronate.

Far from being a bad thing in skiing, pronation is the key to setting up the over-centre mechanism that makes the forces of skiing work for you instead of against you. So what is pronation?

In future posts I will explain what pronation is and why it is essential to sound ski technique and the ability to balance on the outside ski.


1.  British Journal of Sports Medicine (http://bjsm.bmj.com/content/early/2013/06/12/bjsports-2013-092202.abstract?sid=3e90eba8-f3ca-46f7-88e0-d8a28f83b729)

Sources of the story on the Danish study include:

A Popular Myth About Running Injuries – New York Times
http://well.blogs.nytimes.com/2013/06/26/the-myth-of-pronation-and-running-injuries/?_r=1

Speciality running shoes may not reduce injuries – Health – CBC
http://www.cbc.ca/news/health/story/2013/06/27/running-shoes-injuries.html