ADDENDUM TO THE ORIGINS OF KNEE ANGULATION


The intent of my last post was to create an awareness of the lower limb alignment indicative of stability and how a lack of stability, whether intrinsic or caused by footwear, especially ski boots, will cause a skier to default to the use of knee angulation in what will be a failed attempt to hold the edge of the outside ski.

A skier will be unable to develop the requisite biomechanics to balance on their outside ski if they lack stability in barefoot monopedal stance under the minimal challenges associated with a flat, level unperturbed surface. If they lack lower limb/pelvic stability, there could endless combinations of causes which is why I listed a number of resources to help address this deficiency.

If a skier/racer exhibits good to excellent  stability under this basic test and they become unstable with the addition of any form of footwear, it suggests, but does not unequivocally prove, that the footwear is the cause. In more 4 decades of working with skiers and racers at all levels, I have consistently found that I can turn monopedal stability off and on at will. That I can do this without limitation, is indicative of cause and effect. In the 2 world class racers I am presently working with, even a small change in a liner or the over-tensioning of a shaft buckle or power strap has an immediate and noticeable effect on outside limb/pelvic stability and balance.

A key exercise I like to use with racers and elite skies I am working with is the vertical stacking exercise shown in the graphic below. This exercise is performed by starting from bipedal stance with the feet stacked under the heads of the femurs and the head and torso vertical and then making fluid arcing movement of the COM over the ball of the big toe while keeping the torso and head stacked vertically and the pelvis and shoulders horizontal as indicated by orange vertical and horizontal references in the graphic below. The torso should be aligned with the transverse or frontal plane, square with the foot.

A lack of stability in the biokinetic chain is typically evidenced by a drop of the opposite side of the pelvis and a leaning in the opposite direction of the torso and/or the head or both. While this reduces the load on the pelvis side of the  leg it creates a myriad of issues. Inside hip drop will cause the inside leg of a turn to assume the load as the skier inclines thus creating further instability on the outside leg.

Elite skiers and racers like Shiffrin are able to get over it (find stability on their outside foot and ski) in milliseconds. This enables them to retract the inside foot and ski with knee flexion as they incline into a turn similar to the mechanics cyclists use when they corner; outside leg extends, inside leg retracts.

The vertical stacking exercise is best performed in front of a mirror.

7 comments

  1. Dear David,

    Sounds as if you are off hiatus or is the snow simply calling?

    Yes this thing of an unstable knee of great interest. Actually I teach this and use a series of drills aiming at the same things you talk about here. And have been working on a dry land training routine to bolster the strength of the foot to carry the load on the inside edge of the outside ski. Foot stability i.e. the short foot (Dr. Vladimir Janda ET all) is of vital importance. Dr. Emily Splichal’s work is based on his and hers is right on the money. Without the foot being strong the entire kinetic chain seems to collapse which to me explains why we as Mike Richardson (https://skimoves.me/2017/07/29/shoeliner-hacks/comment-page-1/#comment-2048) pointed out find it is hard to get gripping even with a high edge angle.

    Additionally being able to control the counter balance begun in transition and carried to the extreme in the apex of the turn not only depends on the foot but the gluteus medius and quadratus lumborum working in concert to articulate the pelvis. That brings me to the drawing in this post it does not to my eye at least show a counter balance.

    There are two options for the figure to avoid falling over; 1) the entire body has to lean laterally to maintain balance (inclination without angulation) or 2) counter balance where the pelvis moves latterly and the upper body stays centered. Being able to counter balance in combination with a short foot cements the kinetic chain that increases the likelihood of monopedal skiing being effortless and possible. Actually there is at least a third strategy as you note here of ‘knee angulation’. Counter balancing would be the better choice while ‘knee angulation’ as you point out is disruptive to ski-locomotion. (Picture show the postural distortion well https://skimoves.me/2017/08/06/the-origins-of-knee-angulation/)

    I have students try to trick their neuralgic systems standing monopedal perfectly vertically supporting themselves with poles lift there poles. If they are honest and trick it they fall over. This brings awareness to the autonomic qualities of gait. An understanding of the applied physics and new awareness of their body, makes for a learning dialogue.

    I was greatly interested that you can turn monopedal stability on and off at will by tinkering with the boots. I have had similar experience in the field (on snow) but often I can’t get enough rocker angle (shank angle) to get the foot to function (foot stability). When I can; strength, stability and athleticism seem to improve instantaneously. thomas

    1. The balance issue is counterintuitive. It is counter torque acting on a over-centre mechanism. I will post on this issue. Work with me to help me explain so you understand it. I struggled for years to figure it out. Typically, once someone understands a complex issue it becomes simple. Then they forget how hard the process was.

  2. Is there any video showing a positive and a negative response to those stimili above so that we can better understand (view) the body movements you are explaining ?

    1. Not that I know of. In order for a video to effectively communicate an issue like this, it has to have scenes with very specific view angles. At some point I will try and create one using Poser. But it is a long process.

      It would be helpful if you could please give me more details of what you would like to see.

      1. Well If possible a video showing the exercise starting with bipedal stance and maybe with lines and arrows showing the expected movements and the correct / incorrect position of pelvis and torso… It is hard for me to figure out the exact exercise and the different movement.

  3. “If a skier/racer exhibits good to excellent stability under this basic test and they become unstable with the addition of any form of footwear, it suggests, but does not unequivocally prove, that the footwear is the cause. In more 4 decades of working with skiers and racers at all levels, I have consistently found that I can turn monopedal stability off and on at will. That I can do this without limitation, is indicative of cause and effect. In the 2 world class racers I am presently working with,

    {even a small change in a liner or the over-tensioning of a shaft buckle or power strap has an immediate and noticeable effect on outside limb/pelvic stability and balance.”}

    I tried to make that last partial sentence red, underlined, italicized, bold print because it may seem ‘over reaction’ to those who haven’t worked with David’s methods of testing/fixing the issues but no truer or important words could be added. Balance can be correct and perfect until any of the last top three buckles of a four buckle boot is tightened incorrectly and that alone screws up the balance which is easy enough to prove on snow by simply unbuckling/loosening the buckles and comparing the results. (But that simple thing took me a long time to figure out that such a ‘miniscule’ thing could have such a huge effect:) Most overlap ski boots have a longer ‘radius/bend’ of the outside overlap than the inside overlap so when one tightens the second from the bottom buckle can visibly see the inside travels further than the outside when the buckle is fastened. This may block the needed inward rotation of the lower leg. My most recent solution is to fasten a ‘stop’ (piece of plastic, metal angle stock, etc) to the ‘under-lap’ where the skier can stand perfectly balanced on the floor so that the two sections can’t be pulled beyond that point:)

    1. “This may block the needed inward rotation of the lower leg.”

      It usually also blocks the throat of the boot lower/shaft interface so that when the lower aspect of the shank (tibia) attempts to move forward on the gliding hinge of the ankle joint it is obstructed. This drops center of load down the shaft causing what I refer to as the ‘trip effect’ that is similar to catching the toes on the nose of a stair tread as one is ascending. The foot of the swing leg gets left behind usually causing a forward fall.

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