PUTTING THE MECHANICS AND PHYSICS IN THE BIOMECHANICS OF SKIING


Balance is an internal response to an external disturbing force

Recently, I came across an excellent article by Ian Griffiths titled, ‘Putting the mechanics back into ‘biomechanics‘. Ian is a Sports Podiatrist in the UK. His article was published on http://www.running-phyiso.com/mechanics/ on February 4, 2015.

According to the introduction by PhysioTom, Griffiths has been challenging misconceptions around pronation and foot function for some time. Griffiths states that the aim of writing his blog is to try and put some mechanics back into discussions of biomechanics. According to Griffiths if those discussing biomechanics, “don’t do physics”, then it may be time to choose another speciality. I whole-heartedly agree. From what I have read in the literature over several decades, it is not so much a case of putting the biomechanics back into discussions of the biomechanics of skiing, but more a case of putting the mechanics into discussions of the biomechanics of skiing where little or no component of mechanics currently exists. It is not possible to engage in a meaningful discussion of the biomechanics of ski technique, especially at the World Cup level, without including the components of mechanics and physics and especially the opposing forces acting across the inside edge of the outside ski of a turn.

Here is an article that I wrote on skier balance in February of 2002.

Good balance is everything in skiing. Few would argue the point. So why is it is that no one seems to be able to describe what good balance is? It’s pretty hard to know when a skier has good balance if no one knows what good balance is. It’s even harder to learn good balance for the same reason. If you watch skiers coming down a hill it’s obvious that some have better balance than others. How can you tell? They are typically much quieter and more fluid in their movements than less balanced skiers. And they don’t get tossed around as much in the bumps. More important, they determine where and how they want to move. Skiers who have good balance, are in control.

The ability to stand upright without falling over is a form of balance. But good ski technique is more than simply not falling over. Good balance allows a skier to resist the external forces of skiing as efficiently as possible by controlling their position on their skis. The muscle effort required to do so with good balance is a fraction of that required when balance is poor. The stress on a skiers’ body is equally reduced. If you want to become a good skier, then you have to know what good balance is and what you have to do to get it.

Webster’s dictionary defines balance as equilibrium or to remain in equilibrium. Not much help here. No wonder the term balance is nebulous in skiing.

The definition of equilibrium is more involved. But it provides clues as to what balance is (or should be). Webster’s defines equilibrium as: a state of balance between opposing forces or effects, the system involved (that’s the skier) undergoing no total change. This provides a better picture of balance because equilibrium in this context means that a skier can initiate a movement pattern and then return to the same body position from where the movement started. This implies the ability of the body’s balance system to maintain control of the movement of the joint system.

From a perspective of physics, Webster’s defines equilibrium as the state of a system in which the net force is zero and which may be either static or dynamic. Skiing is a dynamic activity because the external forces acting on a skier are constantly changing. Here, balance is a process and not a static condition because the body must change in response to changes in external force. Balance in this situation is the equivalent of a stalemate in a tug-of-war. Both sides are pulling at each other. But neither side is moving. In the human system, balance is an internal response to an external force that challenges equilibrium because the body has no direct control over the external force. But in the balance tug-of-war scenario, the balance system must have the reserve power to give up or take territory at will in order maintain equilibrium.

Balance is controlled by the balance system of the CNS. Its job is to maintain an upright position and prevent falls that could cause injury, especially to the brain. It does this by gathering information from a wide variety of sources distributed throughout our body. Vision is important to the process. So are the tensions sensed in the muscles and joints. But in standing upright, some of the most critical information about balance comes from the feet. In skiing, the feet are where everything happens.

Since walking and running are a series of intentional falls and recoveries, the balance system has a complex job. It has to continuously analyze the flood of information it receives and then compare it to movement patterns stored in its memory bank. In fractions of a second it has to decide if movements are putting the body in danger of falling. If so, it must respond with appropriate action.

The problem with standing erect (Figure 1) is that humans are vulnerable to toppling. This makes balance strategies intended to prevent falls extremely complex. The brain tells us the body that we are standing perfectly vertical in a static position. But this is an illusion. The human system incorporates an ingenious strategy to address the problem of standing upright. Instead of trying to maintain a perfectly vertical position, the body is configured with our weight slightly in front of our ankle (Figure 2). Although we don’t sense it, we are in effect leaning slightly forward. In this position, there is a constant tendency for gravity to cause us to fall on our face. The balance system counters this tendency with the muscles in the back of our leg that push down on our forefoot. This action pushes us backwards; just enough to prevent a forward fall. But not so much that we would fall over backward. Staying upright involves a constant cycling of this back and forth movement called sway.

Activities like skiing make the job of the balance system even more complicated because it has to use movement patterns from hard-wired activities like walking and running to assess the risk of falling on skis. The important thing to know about the balance system is that it will always produce the best balance solution it can for a given situation. So if a skiers’ balance is poor on skis then the balance system is probably doing the best it can with what it has to work with. Simply trying harder to have better balance will usually make things worse. The solution is to find out what is causing problems for the balance system and take steps to correct the situation.

In Figure 1 the ankle is shown with cross hairs through a circle. This is the main joint the body rotates about in an upright posture. The ankle is the key joint in balance in skiing since this is the point where the most significant external forces act.

Figure 1Figure 2 below shows the ankle extended up to the center of mass (black and red circle). The center of mass or COM represents the net position of the weight of your body. It is where you are in relation to the ground.

Figure 2

Figure 3 below shows a simple model of the foot with a strut extending upward from the ankle to the center of mass. This model is similar to the one the human system uses to maintain an upright position. If we were to turn the model upside down it would become apparent that our body is configured like an upside down pendulum hinged at its base at our feet. This arrangement was first suggested by Dr. David Winter at the University of Waterloo in Canada.

Figure 3

An external force W (gravity) pulls the weight of the COM forward and downward, towards the ground. In effect, gravity is trying to topple COM. An internal force M (muscle) pulls against the rotation of the body caused by gravity W. The opposing arrows through the center of the COM show the direction of forces and relative strength. The external force W tries to rotate the body clockwise. It is shown as a negative (-) force. The action of these forces is shown at the ankle. Force W is said to tend to disturb equilibrium. The internal force M is controlled by the balance system. It acts to oppose force W by pulling COM counterclockwise. It is shown as a positive (+) force.  Equilibrium exists when M – W = 0. Therefor M =W. This is called the balance equation.

When an external force acts on our body it tends to cause rotation at the joints of the foot, knee and hip. Our balance system senses the direction and strength of the external force and signals muscles to pull in the opposite direction with equal force. This prevents the joints from rotating. If the external force changes in any way, the balance system responds to match the change. The important point is that for balance to exist in this relationship, the balance system must be in control of the relationship of the 2 opposing forces. This requires that the balance system sense the slightest change in the direction and strength of an external force and respond immediately with opposing muscle action. One factor that helps balance is that the pull of an external force usually tends to stretch a muscle as it is contracting. This is called eccentric contraction or what I wrefer to it as elastic tension. The reason it is so important to good balance is that the harder and faster an external force pulls against muscle in eccentric contraction the harder the muscle pulls back. In other words the pull of an external force can actually make muscle stronger and faster. In eccentric contraction muscle can produce up to 8 times as much power as it can in concentric contraction.

The important point in the balance process shown in Figure 3 is that force M represents an internal response to an external force W. If the balance system can successfully oppose an external force it can usually protect the affected part from injury in addition to maintaining balance. Ski equipment, but especially ski boots can disrupt the balance system and cause a serious loss of muscle power. When the balance system controls the balance equation (equilibrium), it has the ability to adjust the position of the COM in relation to the base of support at the feet. It does this by either increasing or decreasing muscle force so as to create movement at a joint. When the intended position of the body has been attained, the balance system adjusts the muscle force to maintain the balance equation. When external forces exceed the opposing internal forces of the skier, equilibrium is lost.

13 comments

  1. Hello David, this is my first comment here but it is more of a question: you discuss load transfer, pronation, grf, etc all of which seem to be framed within the context of the outside/load bearing foot/ankle/leg. My question is what, in your view, is the role of the inside foot/ankle leg in guiding or initiating this load transfer, especially as it pertains to the tri-planar complex of both feet and the alignment of the cm/first met head of the outside, aka stacking?

    1. The inside leg is used to create stability across the pelvis so the outside leg can be maximally internally rotated and abducted. Stand on one foot and touch one finger of the opposite arm (left foot, right hand) on an adjacent vertical surface. Notice how little influence it takes from that finger to steady the position of COM on the ball of the stance foot. Try the same thing except lift the opposite foot a foot or so off the floor and touch it gently against an adjacent vertical surface. This is how the inside leg should be used to help maintain the position of COM on the ball of the outside foot. There is an excellent article on this in the December 2014 issue of Ski Canada called Four Common Myths of Skiing by Martin Olson. In Myth 2, Wight the Downhill Ski, Olson states, “in the photo sequence, Anne has most of the weight on the downhill ski. But believe it or not, she isn’t trying to do that. She is controlling the pressure entirely with her right (inside) ski. By keeping the stance narrow and relaxing the right (inside) leg, it’s much easier to balance and the hips will naturally move inside to achieve advanced skier angles…. Don’t think about pushing against the snow — think about the snow pushing against you as you relax into angles and the (outside)ski bites”.

      I have known Martin for a long time. He is a very smart ski pro. There are minor points of disagreement between us on some issues. But he nails the issues associated with the Four Myths. Get a copy of the article, frame it, put it your wall, study it and apply it.

  2. When aligning the boot cuff I always have an issue with my left leg because it is bent outside the range of the normal cuff adjustment. Do you do this only by realigning the cuff pivot point or can one use shims between the cuff and liner? If realigning the pivot point does one put a new hole in the cuff or does one put a new hole in the lower shell? The last question should rock the Titanic once again; do you ever need to use canting of the boot sole ( whether shims with binding or worse) or can you align a person well enough that that is unnecessary?

    I’m going to take a guess at the answer to the canting question, you never need sole canting. The reason I theorize that is because originally I thought that anything over a 3 degree cant was prohibitive and meant there was an unaddressed alignment issue within the boot. As I became more advanced in my in-boot work I found that I could drop that cant angle to nothing above 1 degree generally and an absolute limit of 2 degrees. Ever since I learned alignment techniques from a group of ‘canters’ canting helped my skiing but now I realize I was “balanced” but in an immobilized fashion. I have worked down from an original setting of ~3 degrees. Now with no foot bed/orthotic and using your recommendation on cuff alignment I’m skiing with no cants and minimal stress on the body and loving it. So I’m thinking that boot sole canting is simply another band aid for misalignment and no one should ever need it except for extreme exceptions because if the ‘load transfer stance’ position is proper then we can anatomically align through our natural skeletal design. I use the term ‘extreme exceptions’ simply because of the many years it has taken me to get to this point; watching skiers with great mobility and not having been able to accomplish it myself. I’m not as seriously handicapped as less fortunate people but my ski equipment was. I truly feel every one can get better performance which is why I’m so interested in learning as much as I can on the subject.

    I’ve got to say, this ‘art’ of yours is so much fun because it works!!!!!!!!!!

    1. This is an issue that I should address in my blog. Please post this as a comment.

      I went through an evolution in boot modification where I had a ‘brief’ love affair with under-binding and later under boot sole cants. Over the years, I bought every book ever written on ski technique and boot modification. Most of them went in the recycle bin. When I experimented with cants, I found through anecdotal experiments, that it makes a big difference whether the cant is on the boot sole or under the binding. I know why now. Once I got the boots sorted out so that a skier could stand properly in the boot shell without interference I found that cants no longer did anything useful. I am not saying that cants do not affect a skier. Clearly they do. I firmly believe that Stenmark’s secrete was that he used 2 degree cants under his boots, high side out. This literally gave him a huge edge over his competition by improving what I call the inside edge-load transfer axis. I will post on this subject soon. I agree than when used to achieve a flat ski, cants are band-aids that don’t address the real issue.

      Whether to put a new hole in the cuff or shell depends on the cant mechanism and how much I have to move it. If the cuff has a cant slot that has reinforcement around it, I just grind the slot a bit longer with a Dremel type tool (I use a Foredom grinder). If I move the cant mechanism in the shell bottom, I make sure I move it enough so that the new hole is clean and separated from the old hole which I plastic weld closed using a strip of plastic cut from the top of the shell lower that sits behind the cuff. I start with the easiest option first.

  3. greetings David:-) Nationals are over here at the ‘loaf…great fun to watch Shiffrin!!! Not only amazing touch on snow but she skis with such ‘minimums’…such a tall stance never spread out against the snow unless she needs it! Such a contrast to the rest of the field that was here.
    sooo what’s next for your blog..are we going to get outside the boot +discuss how stance contributes(or doesn’t) to skiing at this level? Looking forward to discussion. Colorado blue here today…Easten FIRM:-)

    1. Now that ski racing has pretty much wound down for the season I am going to do a series of posts where I compare the techniques of those such as Shiffrin, Hirscher, Fenniger and Maze to other racers and then reboot and start from square on with basics and move forward in a logical progression. I believe that after a break of a month or so the spring is the time to make any changes to boots so they can be methodically tested well before the start of the new World Cup Season.

      Shiffrin gave me a few anxious moments in the beginning of the season when it became obvious to me that she had made some changes to her equipment in the fall, in particular, her ski boots. Fortunately, she got things sorted out over Christmas. In slalom, she is now in a class by herself. Why she isn’t winning in GS is a subject I will delve into on my next post on what I call THE AUSTRIAN MOVE. This is what Hirscher and Fenniger use to advantage through an often aggressive impact move to the tail of the inside ski. While I am speculating because I don’t have access to Shiffrin’s boots, I believe she has close to the optimal set up for slalom but not GS. I believe ski equipment is a like a race car where the suspension (the boot) has the suspension set up for the track in combination with the right tires (skis). A stiffer boot flex works to advantage in slalom. Stance will be the subject of several posts.

  4. Further adventures; I discovered that the removal of a piece of boot board about the size of an ‘oval’ half dollar piece increased the lateral mobility of my quirky left foot even more. Note that I didn’t do the ‘transfer load test’ at this point. The first day I was able to test it on slope was with about 2 inches of very sticky fresh snow and initial results didn’t seem positive but “I didn’t have the right wax’ either. I dropped my ramp/delta a couple of times and things appeared a bit better. I wrote it off as a failed test and went home. The following day my knee was sore and as that remained I began to suspect that the ‘minor’ adjustment was to blame. I liked the added mobility but knew that this wasn’t enough to out weigh the knee situation because it had really become fun to ski without knee pain! Today I had (made) some time and went through a thorough ‘transfer load’ test. I found that the Nordica stock insole (which has the slightest bit of shape and arch) was hitting the arch of my left foot ever so slightly and I recognized I had felt that the day I’d skied this configuration. My left foot pronates more than my right foot so the right foot which I had adjusted also seemed fine so I stayed with the Nordica. I found a flat insole and placed that one in the left boot which tested infinitely better, found it necessary to readjust the cuff slightly, geared up, went out into the rain and skied off into the woods. Skiing felt pretty good but there was a slight something missing in the ski’s response time. I pealed off the half degree of cant under the inside of the left boot, looked down the 40 foot ravine with a 40+ degree. pitch and figured worst case I crash into the creek at the bottom. The result was best case scenario because it worked great!! Despite all the hiking I had to do to get up out of various ravines and back to the house there was no knee discomfiture which makes me feel really great about this setting, just need to ski some ‘real’ slopes with some speed and bumps to fully confirm this.

    I already knew that a tiny piece of foam added or removed can change everything. Not really sure why I thought I could cut corners in the testing of each change though. I’m glad I did because it really reaffirmed how valuable these indoor tests are. My logic tells me that my left foot was benefitting from that slight arch support because it couldn’t possibly hurt!! Well, it not only hurt function, it was killing my knee. I’m sure I’ve improved function once again and can’t wait to get on a real bump run!! Thanks again David!!

    1. Over a number of years, I developed a very specific approach to testing and assessing interventions that affect function of the lower limbs and by extension, the entire body. As described in my post on out of the boot exercises, I like to start by developing a kinaesthetic awareness of the reference shank angle then experimenting with different ramp angles of the boot board-binding delta in a boot shell with no liner. Then I add the liner and experiment with insole and other components one at a time. This only gets one into the ballpark. Experimentation in a structured manner during actual skiing is the only way to confirm the optimal set up. Most skiers tend to discount the affect of seemingly insignificant sources of interference to foot function. As you have discovered, even a small degree of impingement on the arch can have a huge negative effect on lower limb function. This will not become obvious until the boot is very close to optimal. When I got my boot close to optimal, I sensed a very noticeable impingement of the arch of my left foot in high load GS turns even though the insole was flat. The source of the impingement turned out to be a detail of the rubber sole of the liner that wrapped up the inner aspect. When the arch of my foot compressed the inner aspect contacted the liner detail. Cutting the detail away solved the problem.

      Experimentation with results assessed by actual skiing is the only method I believe will result in the optimal set up.

      1. Actually, you discovered that minor impingements effect the arch greatly, I only confirmed it!

        Switching gears, I got a chance to run around a track today with my ‘toe spreadable’ sandals and discovered that the zone on the bottom of my foot that I’m trying to land on when running is your ‘load transfer’ point. That was a really cool discovery!

      2. I referenced an excellent study in my post on footbeds and arch supports that found that the hot buttons for balance are the heads of metatarsals 1-2-3 and to a lesser extent, the associated toes. It didn’t matter whether the biomechanics of the hindfoot were impeded. So long as the load could be transferred to the heads of MTs 1-2-3, the effect on balance was unaffected. The same study also found that what I referred to years ago as ‘dynamic tension’ in the arches of the foot potentiates neural activity associated with balance. As tension in the arch changes, the height of the arch changes in lockstep. As the height of the arch changes, the angle of the shank changes independent of changes in flexion angle of the ankle joint.

  5. Excellent simple description of complex subject. I am confused however by clockwise/counterclockwise movement description. Seem to be opposite in direction…..where is the clock? In MW , M moves counterclockwise and W moves clockwise. Explain? Thanks.

    1. I like to think in terms of tendencies of moment about a joint. Griffiths talks about this on page 3 of his article under ‘External Vs Internal joint moments’ in reference to the STJ, “So, for any given weight-bearing activity in any given individual, there will always be external pronation moments, internal pronation moments, external supination moments and internal supination moments acting across the STJ. It is the interplay of these forces that dictate both the motion around the STJ axis (if any) and, therefor the subsequent stress applied to various tissues”. In the sketch in my post, if W were to increase in magnitude due to a perturbation or a deviation from the fall line that causes an unopposed transient spike in G force, then dorsiflexion will occur unless the muscle force M increases proportionally. Just because there appears to be a static position of the shank in weight-bearing does not mean there are no forces at play. It is the sudden spike in the load W when it is offset laterally from the inside edge that applies inversion stress to the lower limb.

      The use of + and – was arbitrary. Flexion and extension would have been less confusing.

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