Part 1: Background
The lower limbs of the human being comprise one of the most complex systems to emerge out of the process of evolution, authorities say, because of the animated manner in which its elements articulate with each other in three-dimensional space. The most complex of the lower limbs is the human foot, say these same authorities, because it is one of the most dynamic structures in the human system. The processes of walking and other forms of human locomotion have been studied widely and thoroughly for centuries. Yet despite the existence of vast amounts of scientific knowledge, the ski industry is awash in misinformation about the function of the lower limbs, particularly the function of the foot and what its role should be in skiing. The majority of this misinformation is based on nothing more than casual observation and uninformed opinion.
The problem with unstructured tinkering with the mechanism of the lower limbs and foot is that their function has a profound influence on human posture (or stance, as it is referred to in skiing). In order to maximize the chances of survival the human system is extremely adaptive in a sense that there are back up systems for back up systems. Since mobility was (and still largely is) key to survival the lower limbs will continue to provide some level of function even when their integrity has been severely compromised by injury or some form of constraint. Perhaps no better example of this adaptive capability exists than in skiing.
Skiing is a complex activity that severely challenges the human balance system. The primary interface of the balance system with the ground or snow is the human foot. Although not formally stated, the objective of the ski boot is to constrain the three dimensional movements of the elements of the foot that is essential for balance in an unspecified and unpredictable manner. This is accomplished by encasing the foot and lower leg within a rigid padded structure. In short, the intent of the ski boot and many forms of boot modification are to render the foot substantially dysfunctional. This form of constraint not only interferes with the movements of the elements of the foot essential to balance, it also introduces foreign forces that pollute nerve messages (proprioception) originating in the muscles and joints that give the balance system vital information. In a very real sense the ski boot mimics a form of pathology or injury artificially grafted onto the exterior of the foot and leg. The fact that the majority of those who try to ski can actually make it down a ski slope without incurring a fall serves as mute testimony to the sophistication of the human balance system and the process of adaptation. In view of this the potential for enhancing balance in skiing by providing a functional environment for the foot and leg becomes obvious.
Part 2: Base of Support
The human system is a model of efficiency in that it only does something when it needs to and then it only does the minimum. As bipeds, our center of mass (CM) is stacked on top of the base of support instead of suspended between it as it is with quadrupeds. This places unique demands on the lower limbs that require it to perform a number of distinct functions specific to the activity. In quiet standing the lower limbs must provide a base of support for CM. But in walking and other forms of locomotion the lower limbs must:
1. Adapt to the supporting surface,
2. Absorb the energy of impact,
3. Provide a base of support, and
4. Provide a structure to support propulsion.
In skiing we are mostly concerned with the function of the lower limbs as a base of support. With respect to the model of efficiency the lower limbs will only function as a base of support when they need to. This function is triggered by the presence of CM acting through the feet against a ground reaction force (GRF). Here the weight of the body pushes down on the top of the foot and the ground pushes back against the sole of the foot. This compresses or squashes the arches of the foot causing them to become tight. This is the principle of compression/tension that makes a truss or bow work. How tight do the arches get? As tight as they need to be to support the loads imposed on them. The redeeming feature of this principle is that the tension in the plantar vault will increase in proportion to the weight imposed on it by CM. In other words, external forces will tend to make the foot and lower limbs stronger as the external loads increase in magnitude. The limiting factor in skiing for this mechanism is the availability of a (dynamically) stable source of GRF to compress the vault against.
The foot uses the principle of the tripod to support the weight of CM on three points. This is the most stable and the most efficient configuration in terms of the ability to adapt to irregular surfaces and serve as a system of abutments for muscles to pull against. This system is called the plantar vault. Its three support points are: 1) the heel, 2) the ball of the big toe, and 3) the ball of the small toe. In skiing, an environment must be created in the equipment that permits the foot to produce as much tension in the plantar vault as it needs to allow the muscles of the legs to resist the external forces. In this capacity the support points of the plantar vault serve as abutments for the muscles of the lower limbs that control the position of its joints. The premise that supporting the arches of the foot is both beneficial and necessary in skiing has been widely promoted and accepted. However, structures placed indiscriminately under the sole of the foot can greatly diminish and even effectively prevent the tensioning process.
How the Foot Develops a Competent Base of Support
Figure 1 of the drawings below shows the plantar vault of the foot supported on the bases of its three arches (A and B represent the balls of the feet and C the heel). Figure 2 shows a plan view (looking down) of the three components of the vault with bases AB, BC and CA. Figure 3 shows how force that tensions the arches at points A and B creates abutments that become foundations for the muscles of the legs and foot to pull against. Figure 4 shows how the arches relate to the foot and how the weight of CM is transferred to the arches at the ankle joint (P).
In the next part I will use a three dimensional model to show the sequence of events that allow the arches to produce tension that muscles can use to develop a base of support to erect a structural column with which to support CM. As you begin to relate these mechanisms to the pressure readings obtained from the Birdcage experiments [see supplementary article--ed] it will become obvious that the joints of the foot can direct forces to the ball of the foot in response to external forces that challenge balance. It is this mechanism that we will focus on since it is responsible for maintaining a stable base of support on the stance leg in skiing.
Part 3: Points of Support
As noted previously, the foot is a tripod-like arrangement that supports the weight of CM on three points: the heel, the ball of the large toe and the ball of the small toe. Of these three points the heel (1) is the largest bone and support point in the foot while the bone that forms part of the ball of the large toe (the 1st metatarsal) is the largest of the five metatarsals (2) with the 5th metatarsal being the smallest (3). In terms of normal loading in quiet standing with the weight equally distributed on both feet the heel carries approximately 50% of the weight of CM borne on that foot, the ball of the large toe 35% and the ball of the small toe 15%.
Although not obvious in the figure below the vault of the arch of the foot is comprised of two components with different functions. Metatarsals 1-3 are directly connected with the heel. This group forms the structural part of the vault. Metatarsals 4-5 connect to the heel via an intermediate bone. This part of the vault underlies the structural part. The mechanism by which it connects to the heel allows it to act as a steadying mechanism for the structural part of the vault. In this capacity it is very much like an outrigger on a canoe. The outer or 5th metatarsal also acts as something of a landing gear at foot strike in assisting the foot in adapting to the ground, rolling the foot into pronation and directing the movement of force (or center of pressure, COP) towards the ball of the large toe.
The tripod-like arrangement is the most efficient for adapting to irregular surfaces. It also permits the load on the foot to be directed towards either the ball of the large toe or ball of the small toe in order to counter the forces of a surface sloped across the long axis of the foot. If the foot rolls about its long axis towards point 2 the load will increase on the ball of the large toe. If the foot rolls about its long axis towards point 3 the load will increase on the ball of the small toe. In conjunction with this loading the axis of the ankle (and leg) will rotate towards the load through the action of the sub talar joint. The grey bar on the hindfoot represents the axis of the ankle.
Part 4. Alignment of Forces
In the drawing above the line running through the heel and the ball of the second metatarsal relate to what is called the ‘mechanical line’ of the lower limbs. When standing on two feet in the anatomical reference position the center of the ankle is aligned directly under the head of the femur in the hip socket. A plumb line dropped down from the center of the head of the femur would fall through the center of the ankle and heel bone and run on a long axis through the ball of the second toe. This is called the ‘natural balance point’ of the foot because the distribution of load across points 2 and 3 of the tripod is equal across this axis. This is the reason why skate blades and inline wheels are always located on this axis. However, it is important to recognise that this is a quasi-static state that is not applicable to activities like walking that involve alternating single limb support. Unfortunately, this model has been used inappropriately for skiing by some that have suggested that the ideal location of a ski edge should be on the natural balance point. For reasons that will become obvious as we move forward this concept is flawed. For now we will focus on how the force vector responds to increased loading on the ball of the foot. The drawings below are a very simplistic representation of the mechanics. In reality that loading mechanism in the foot is far more complex in terms of the forces applied to the ball of the large toe. The sequence from left to right show the effect of increased loading of the ball of the foot.
We have established that the foot is a tripod-like arrangement with well-defined points that distribute the skier’s load through the vault of the arches of the foot to the ground. We have also established that the force acting on these points can and does vary and that the distribution of load at the three points determine whether the force applied to the foot by CM acts to the inside of a turn or to the outside of a turn.
The next thing we will consider is how the three load carrying points of the foot relate to the ground through the interface of the stack of ski equipment that resides between the sole of the foot and the snow (ground).
It is commonly believed in the ski industry that the weight of CM should be equally distributed over the entire sole of the foot. This would mean 1/3 of the weight would be carried on the heel, 1/3 on the ball of the large toe and 1/3 on the ball of the small toe. From both a theoretical and practical perspective this perception is incorrect. The foot is not designed to function in this manner. As we have seen even in normal quiet standing on two feet 50% of the weight of CM on each foot is carried on the heel with the remaining 50% distributed unequally on the balls of the foot.
It is also commonly believed in the ski industry that the force applied by the skier should be aligned with the long axis of the ski and that the knee should track on this line. We should be very concerned about the affect of the alignment of the three points of the foot with the mechanical references of the ski, specifically the alignment of these points with the transverse running center of the ski, the inside edge at the waist and how these issues relate to sidecut.
At this point we can make the following general statements about the foot:
1.The heel will always be capable of bearing a portion or even all the weight of CM when the foot is in a ski boot because the leg connects to the top of the heel.
2.The ability to apply force to the ball of the large toe is dependent on the capability of the ‘bridge’ between the heel and the ball of the foot, in other words, how efficiently the 1st metatarsal can be loaded.
What factors affect the intensity or magnitude of the force that can be directed to the inside of a turn? The heel will always be located in the proximate transverse center of the ski. But the orientation of the ball of the foot depends on a number of factors.
1. The foot can either be abducted (turned out at the forefoot – common) or adducted (turned in at the forefoot – rare, but a slight amount is preferable) in relation to the transverse center of the ski.
2. The size of the foot. The center of the foot will always be on the proximate transverse center of the ski. But as the foot shrinks in size, it shrinks in width from the center of the foot.
3. Sidecut width at the waist of the ski.
There appears to be widespread belief (blind faith?) in the ski industry that clamping a ski boot about the foot somehow directs the force of the weight of the skier to the edges of the ski or skis (doesn't seem to matter whether it is one or two skis) while at the same time miraculously optimizing 'balance'. While I may have believed this for a very brief period of time I quickly saw through it as an outrageous marketing story with no substance as it relates to support based in sound principle. Even if the human leg were a solid, rigid member like a steel tube it could still not behave in this manner.
The human lower limb system is comprised of a complex series of joints that allow the foot to adapt to any surface and then position CM in balance over the base of support resulting from the adaptive process. The weight of the body is transferred to the tripod-like structure of the foot through the two joints that make up the ankle complex. These joints allow the foot to rotate about its transverse horizontal axis in what we call 'ankle flexion' while simultaneously rotating about its long axis. This latter rotation is coupled to rotation of the leg. This joint is called the subtalar joint. It works together with the ankle joint in a co-dependent relationship which means that you can not separate them from a point of function.
If you look at the drawings of the foot in Part 2, you will note that a force applied by the body to the ankle that is primarily transferred to what amounts to a bridge between the heel (1) and ball of the foot (2) will tend to rotate the ball of the foot downward about the transverse axis of the foot at a point under the heel bone. At the same time the force will act in a line towards the front of the foot and into the turn (draw a line between the base of the heel bone and the ball of the great toe and this should become obvious). There are other forces as well. But they all have one thing in common; they all act towards the inside of the turn. How these forces act on the snow depends on their (mechanical) relationship with the ski which in turn relates to ski geometry and orientation of the mechanical points of the foot.
I see the issues described above as having a huge impact on skier performance. In fact they are so obvious to me that they literally jump up and kick my front teeth out. But my experience to date with ski pros and coaches even the highest levels has been that this issue has nothing to do with skiing.
In our online conversation Rick Schnellmann remarked, "I have always speculated at the end of the day when I would take off my ski boots and put on my running shoes how nice it would be to be able to ski in a boot that would offer me the same balance and agility potential as my sneakers and still offer support. I knew that the restrictive nature of conventional ski boots require us to sacrifice some of the balance potential of the foot and forces us to allocate the responsibility to other areas of the body."
On the contrary, I do not believe that a ski boot needs to offer the same freedom as a running shoe. Indeed, I believe that such freedom is not only unnecessary but would be detrimental to skiing. What I do believe is that the forces of skiing (CM vs GRF) must be directed through the outside or stance foot of the turn. There will be situations where balance is disrupted and restoring balance will often require rescue responses that involve the use of other areas of the body particularly the arms. However this should be the exception not the rule. A quiet directed upper body characterized by neutral muscle positions of the torso (pelvis, spine and shoulder girdle) are essential to effective skiing.
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The Birdcage Boot Experiment By David Macphail
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Alphabetical Article List
The Foot And Its Role In Skiing By David MacphailThe Foot and its Role in Skiingby David MacPhail
Part 1: Background
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