OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP


In view of the positive response to my recent posts and comments I have received, I have decided to take a step-by-step approach to explaining the mechanics and biomechanics of balance on the outside ski.

I am going to start the process by comparing balance on one foot to balance on two feet. I refer to balance on one foot as monopedal stance (one foot) and balance on two feet as bipedal stance (two foot). The graphics are for illustrating general principles only.

The graphic below shows monopedal stance on the left and bipedal stance on the right. Orange hash marks delineate the alignment of major body segments. Black reference lines on the right leg of both figures show the angle of the leg in relation to the ground.

In order to transition from a balanced position in bipedal stance to a balanced position in monopedal stance, either the foot must move towards the L-R center of the torso or the torso must move towards the foot that will become the stance foot, or a combination of the 2 movements must occur. The central issue is the amount of inertia acting on the torso. In skiing, due to the degree of inertia, the new outside foot of a turn is normally guided into position under the torso as the skier or racer approaches the fall line in the top of a turn.

Moving the foot into position under the Centre of Mass so it stacks in line with the ball of the foot usually takes an inward movement (adduction) of the leg from the pelvis of 6 to 7 degrees. In the upper left figure in monopedal stance, the leg is adducted 6.5 degrees and has formed a varus or outward leaning angle with the ground.

If the leg only adducted, then the sole of the foot would end up at an angle of 6.5 degrees with the ground and the figure would end up on the outer edge of the foot; on the little toe side. In order for the torso and Center of Mass to stack vertically over the ball of the foot, the sole of the foot must turn outward, away from the center the the body. This is called eversion. It is enabled by the joint that lies below the ankle called the sub-talar joint. The sub-talar joint is tied to the tibia where it acts as a torque converter. When the foot everts or inverts, the sub-talar joint translates this on an approximately 1:1 ratio into internal or external vertical axial rotation of the leg.

When the foot everts, the subtalar joint rotates the vertical axis of the leg towards the center of the body an equivalent amount; in the subject case, 6.5 degrees.

The combination of eversion/internal vertical axial rotation of the leg is called pronation. If either of these actions is interfered with, or worse, prevented, it is impossible to create the alignment necessary to stack the torso and Center of Mass over the ball of the support foot.

The consistently stated objective of footbeds is either to limit or even prevent pronation. Put another way, the whole idea of footbeds is to make it difficult or even impossible to balance on the outside foot and ski.

If this issue is not crystal clear, please post comments as to what is needed.

7 comments

  1. Based on all my experiments I would have to say no to the outside of foot posting. With all my experiments of using wedges to tip the foot inside the ski boot or only the fore foot in relation to the heel; all would be deemed pretty much failures to be polite. (DISCLAIMER: not saying that applies to everyone:) As David has stated in the past, the best thing to do with the surface under the foot is to have it flat in all directions thus allowing the foot to ‘self’ adjust in all directions as it sees need. Too much pronation isn’t any better than too little pronation so the best we can do in all of our foot wear is to have there be the minimum of angles in relation to the bottom of the foot wear as possible. I’m only leaving a small window here for the need in certain sports; skiing and possibly skating for the need of net ramp angle but it would be easiest to attain if one could start flat on the ski for example and then systematically move up IF needed. Much easier than starting at an artificial angle and then trying to figure out if one needs to move up or down:)

  2. When Mikaela Shiffrin describes garlands in her video is that the movement pattern you are talking about.

    1. The ‘Get Over It’ video with Shiffrin shows the movement. When Shiffrin talks about getting forward and Over It, she means moving forward in the hips in relation to the new outside ski and getting the weight onto the base of the ball of the foot.

      Shiffrin also talks about the need to be patient. She is referring to the need to let the ski to roll over onto its new inside edge before inclining into the turn. Watch carefully and you will see the demonstrators both make an impulse movement as the ski goes flat between edge change. That the second demonstrator has obvious boot problems suggests that the mechanics and biomechanics may not be fully understood by Shiffrin and/or Burke Mountain. The consistent forward angle of the torso with the snow suggests that Burke understands the effect of the angle of the torso in maximizing hamstring tension as it pertains to Achilles forefoot load transfer.

  3. Another thing… regardless of where the centre of mass is located relative to the centre of pressure in the above-described mechanism, when you go into a stable monopedal stance, as you would when you are in a turn, the ankle is dorsiflexed forward and as this occurs the tibia rotates internally several degrees. This means that the main muscle forces acting across the ankle (the plantarflexors) are no longer acting along the long axis of the foot, but rather partly across it, medially toward the big toe. So, the beneficial effect of that muscle force is to force the base of the big toe into the ground, and that becomes the centre of the turn (centre of pressure). In the absence of this internal rotation movement, the center of pressure remains somewhere in the middle of the forefoot, which is some distance from the medial edge of the ski, where it is needed.

    1. Yes. Bear with me as I think we are headed into new territory or, perhaps, territory that as far as I know has not been explored. Achilles forefoot load transfer is maximal in late stance just prior to terminal stance. As the speed of loading increases, muscles are brought into play that are not active in mid stance.

      The loading under the base of the big toe is heel-toe rocker in which the base of the big is pressed downward in relation to the heel and laters-medially across the bases of the toes. Once the base of the big toe is loaded, internally rotating the leg from the pelvis will have the effect of pressing the base of the ski under the inside (medial) edge downward. I will produce sketches and write a post explaining how the rotational force is transfer to the lateral aspect of the heel piece of the binding and the medial aspect of the toe piece about a proximate center of rotation under the distal tibia.

  4. Although the ski boot minimizes the full range of motion of the ankle and sub-talar, especially given the time-in-boot factor, does it make any sense to have the foot beds reversed so that the sub-talar is “automatically” pronating the sole of the foot to the inside edge of the outside ski?

    Deanski

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    1. Based on my present knowledge, no. I am taking my posts towards a discussion of the dynamics of the pre-activation loading sequence that starts in the transition phase and culminates in rocker impulse loading that starts at ski flat between edge changen and sets up a balance platform as the ski rolls over onto its’ new outside edge.

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