INVERSION STRESS: THE MOVIE


Here is a video clip that shows what Inversion Stress looks like. Inversion Stress with its co-star, Varus Thrust, makes a regular appearance at all levels of skiing. It is a star performer in World Cup Alpine events and even in the Olympics. The sweeping in-out pumping arc of the knee of the outside leg is the classic signature of Inversion Stress.

14 comments

  1. It is also patently absurd to suggest that a conscious mobilizing of muscle activity could possibly equate with the processes of balance resulting out of postural responses which are processed in milliseconds at a subconscious level

    As you analyzed beautifully above Ligety (and Hirscher) are doing the blatantly absurd.

    1. Actually they aren’t doing the blatantly absurd. To understand what is actually happening requires the requisite knowledge. A good read is Principles of Neural Science Third Edition by Kandel, Schwartz and Jessel – a 1135 pages.

  2. I actually like his body position; but as Hans points out, he is super static (50/50 weighting) and never gets to real carving. The major movement creating the edge angles is eccentric (negative) outside-leg abduction and inside leg adduction. He could WaistSteer from this position, but no one taught him the movement pattern, so it doesn’t occur to him. Instead, he finds this body position and waits for the turn to happen, with direction change aided by low-radius carving skis and “steering” (which is weak hip rotation). Then he just stands up with his butt sticking out. Dave is right that torque applied to the outside knee is always bad. However, torque on the inside knee can be easily managed by the strong iliotibial band.

    1. Actually, the problem is that the skier is using the outside leg in concentric contraction in an attempt to make the outside leg into what amounts to a rigid strut. Concentric contraction by its very nature tends to stiffen the lower limbs. There are numerous examples in YouTube videos that advocate skiing on two feet and using the legs like levers to hold the skis on edge and use let them self-carve turns to the radius of the sidecut. Some of these videos have several hundred thousand views or more. Some have over a half a million views. So it seems that this simplistic concept resonates with a large audience. However, it is not an effective ski technique because concentric muscle activity on the outside leg precludes the use of the stretch reflex (more on this in a future post). Waist steering (actually pelvic driven top-down torque) is what differentiates Ligety from his competitors and even from Shiffrin. Both make what I call the skimove to set up a platform under the outside foot by transferring W to the snow within the sidecut zone. This engages the external forces to drive torques in multiple planes into the turn.

      The platform enables the CNS to modulate forces and torques into the turn with postural responses. Once a platform has been set up and the external forces engaged, Ligety uses global pelvic rotation to wind the outside and inside legs and skis into the turn with top down torque conversion through the subtalar joint. This rotates both skis into the turn. The inside leg is used to direct and align R from COM to the ball of the outside foot and create tension across the pelvis to the outside leg with both legs in closed kinetic chains created by the edge locked skis.

      Pelvic torque (aka – waist steering) can torque both skis into a turn without a platform on the outside ski. But the torques on the outside leg will be in opposition to each. This will create stress in the outside leg, especially in the knee. All torques on the outside leg should be in phase.

    2. I have rethought the pelvic torque issue (aka as waist steering) and agree with your position that the muscles of the outside leg are in eccentric contraction. My reason for this conclusion is that it is only possible to internally rotate the leg in a top down fashion from the pelvis in a closed kinetic chain and use it to effectively drive the inner aspect of the boot cuff into the turn when the leg and ankle joints are extending against EC. The problem with using global pelvic torque without first inducing pronation with load transfer is that the load transfer point will be under the heel and the ankle is plantarflexed. Load transfer will induce inversion opposed by internal rotation of the outside leg driven by global pelvic rotation. The implications are that perturbations in GRF (shocks) cannot be effectively dissipated in such a configuration. The other obvious danger is that pelvic torque can be used to lever wide skis onto their edges. This runs the risk of exacerbating Fat Ski Syndrome.

      1. Both the angulation-leverage turn (with the weight on the outside ski) and the WaistSteering turn (inside ski) use the IT Band to reinforce the “skeletal stack” or kinetic chain. Both ways have to deal with the “open” nature of alpine skiing, in that the surfaces are extremely variable. In the former, the sagital balance attempts to keep the weight centered over the sweet spot of the outside ski, but the ski hinges and bucks as it encounters variable terrain. In WaistSteering, the outside ski becomes an outrigger, similar to an outrigger canoe: the “double-hull creates stability as a result of the distance between the hulls rather than due to the shape of each individual hull.” In the WS turn, the two skis harmonize with the body and create stability in a different way. With the weight on the outside ski, there is no way to harmonize the two skis together, so they go their separate ways. WaistSteering is a bit like Theory of Relativity; it takes some time to grow accustomed to it. Thanks for taking some time.

  3. On second thought: This guy has other problems. Because he is completly locked in his hips(waist) and shoulders he is not able to waisteer.. He simply loads his ski and prays for it to come around. If he would waisteer he would apply torque to his skis and actively steer them through the turn. Plus he has no fore aft movement which would also allow him to actively guide (not a good word) his skis through the turn.

    1. A couple of points. This guy is a Austrian Interski demo skier. He is locked because he has no other option with which to oppose the external forces that are driving load transfer. The central issue is that he has unbalanced moments of force on both skis. I am about to show the serious consequences of having an unbalanced moment of force (torque) on the outside foot driven by an unsuccessful attempt by the central load-bearing axis to transfer the load of COM through the outside foot to ground. Countering these unbalanced moments of force requires active recruitment of the leg muscles that rotate the foot (ski) and leg into the turn against the load driven external forces acting in the opposite direction. Guess where the opposing forces meet? At the knee where the opposing forces are manifested in the brittle knee ligaments. Using waist-steering in this situation will not relieve the stress. The current focus in skiing seems to do whatever seems to work to get a ski on edge. The liability imperative is to minimize the risk of injury. In view of this, the focus needs to shift to finding the best way to apply force to the ski while minimizing the risk of injury for a given set of conditions.

  4. This guy is completely locked top to bottom. It starts with his right ankle. His boot does not allow him to roll his ankle inside (no Biomech here- just the bare facts)). He cant get a solid edge set and has to fight his way through the turn, desperately trying to look pretty. If he was properly aligned he would be smiling all the way. But that requires more than a few cants . How do you do it Dave?

    1. In simple terms, the central load-bearing axis is attempting to transfer the load W of his COM to the snow under the outside ski. But the attempt is unsuccessful because the load transfer axis is on the outside turn aspect of the inside edge. This is rudimentary level lever science. In fact, the unstated objective of most ski boot fitting intervention is to prevent load transfer to the snow. The skier in the video has no platform underfoot. So there is no support for the foot regardless of what has been done to his boot. In effect, the skier is without a leg to stand on. This is not an isolated case. Years ago this was called edge-chatter. I can easily find inversion oscillation at all levels of skiing. The only way to correct this is to have sufficient freedom within the boot for load transfer induced pronation. Even then, this can only happen in the very brief window during ski flat during edge change. In the next week or so I will begin posting video sequences that illustrate the key moves a skier must make to make the collective skimove.

    2. The ankle does NOT roll inside. It only dorsiflexes and plantarflexes as a hinge joint. It has nothing to do biomechanically with this situation. Problem here is alternating attempts at foot eversion and inversion at the subtalar joint but his foot is captured by what we call inversion stress and he never gets a platform under his outside edge.

      1. Tricky terrain. I am here to improve my understanding of ski technique and understand that Bio mechanics is an important aspect and a science.Bio mechanically it might be incorrect to say the ankle rolls inside but ski racers including WC skiers talk about it all the time and do what they think they are doing And it works for them. Michaela Shiffrin starts her turns by rolling her uphill ankle into the hill (the phantom move I dare to say). .In BM terms she does something else.
        The guy in the video has as we all agree equipment problems. But his technique is obsolete..Whether he could waisteer with his kind of equipment or not is debatable.

      2. The reality is that it is not tricky, let alone even remotely difficult to understand and correctly articulate the mechanics, biomechanics and physics of skiing. The relevant information has existed for decades. What is missing is a lack of initiative that seems to be rooted in a lack of desire or perhaps a perceived lack of need to become educated when uninformed opinion seems to be sufficient to exert influence. The concept of rolling the ankles to hold an edge or initiate a turn is patently absurd. It is well established that the muscles that would serve to evert or invert the foot lack sufficient power. This issue aside, the term ‘rolling the ankle’ is inaccurate as a biomechanical entity. It is also patently absurd to suggest that a conscious mobilizing of muscle activity could possibly equate with the processes of balance resulting out of postural responses which are processed in milliseconds at a subconscious level. It is this sort of archaic thinking or none thinking that relegates skiing to one of the early stone ages in this age of readily accessible information.

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