EXTENSION/ROTATION EVENT TIMING


Here are two sequences of Mikaela Shiffrin from an older post that show the timing of internal rotation and extension of the outside foot and leg in a slalom turn. The sequences are the same. The  annotations are slightly different.

Slalom does not allow the luxury of the perfect execution of events that GS does. But it is the timing, especially the rotation of the foot and ski onto its new inside edge and into the new turn by the powerful hip rotators, that is the key event.

Shiffrin

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Shiffrin

 Click on the image to enlarge it

4 comments

    1. Tyler, actually it is related. I will be posting soon on the importance of shear forces to the function of the lower limbs in general and to the global 3 dimensional control of the outside ski in particular. In the unweighted (unloaded) state, the human foot equates to a sack of bones. This loose or lax state facilitates the dissipation of ground reaction force at foot strike in locomotion. It also facilitates the adaptation of the load transfer points of the foot to irregular surfaces. As COM moves in front of the ankle and advances towards the balls of the feet, physiologic processes in the feet and leg progressively tighten the arches of the foot to the point where it becomes a rigid lever for propulsion. The problem is that the shear forces that cause the foot to elongate and the metatarsals to spread are very low. How much force does it take to reduce or prevent elongation and spreading? Next to zero. Once I got my boots to the point where my foot could maximally elongate and spread, I was surprised (how about shocked?) to find that even a tight sock could cause significant problems. Until I see studies that compare a maximally loaded, fully elongated, fully spread, unrestrained bare foot against the same foot in a ski boot such as the Atomic Waymaker I will regard the feature as purely aesthetic.

  1. Go David! It’s great to hear someone else also talking the power of the hips in turning! I can’t tell you how many PSIA examiners have frowned on me when they hear I teach my clients how to pronate and use their hips…. Even my beginners!

    1. Cookie, it is not so much a case of using the hips to create turning force as some claim but using the hips and especially the inside leg to create a foundation in the hips to direct force to the outside ski. The objective of the transition phase is to neutralize the torsion across the pelvis or ‘reset’ it so it is square to the skier’s trajectory. At the end of the transition phase there is no centrifugal force (which is not really a force), only gravity and momentum. In the transition phase, momentum makes it possible for the skier to create a force that is normal to the slope of the hill by extending. Ligety takes about creating pressure in the NY Times video. The concept of crossing over into the new turn as still seen in racers at a World Cup level leaves the new inside ski behind and with it any possibility of setting up a vertical force over the inside edge in opposition to ground (snow) reaction force which is essential to make the edge grip and act as a pivot.

      As the new outside ski rotates onto its inside edge the skier rotates the outside leg internally reaching a maximum of approximately 40 degrees. As internal rotation approaches maximum the inside leg is used to apply a countering force that prevents the pelvis from rotating away from the outside leg which has the same effect as rotating the outside leg externally. The key to the use of the hips is the transition phase where the skier moves to the inside ski while it is still on its inside (current) edge.

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