Asides

COUPLED FORCE WHOLE LEG ROTATION

The most important event in a turn is whole leg internal rotation (Event 7) following ski flat (Event 3). But the mechanism by which whole leg internal rotation is applied to the ski is as important, if not more important, than the actual whole leg rotation.

As the outside ski changes to its new inside edge, the racer rotates the whole leg internally using top down rotation from the pelvis. The purpose of ski flat at the conclusion of the transition (Event 1) phase, is to neutralize torsion across the pelvis so it is square to the trajectory of the racer. In order to use whole leg internal rotation, the COM of the racer must be positioned on the new outside foot at ski flat in what I call monopedal stance. Monopedal stance (aka monopedal function) is a physiologic state wherein balance is achieved with the weight of the body borne on the medial plantar aspect of a fully pronated foot.

The graphic below is Figure 23 from my US Patent No. 5,265,350.

Bi-MonopedalFigure 23 A depicts bipedal stance. The points of the central load-bearing axis are stacked vertically over top of each other in the frontal plane. The load W from COM is centred between the feet with each foot carrying half the load (W2).

Figure 23 A depicts monopedal stance. In monopedal stance, the load W from COM is aligned over the proximate centre of the head of the first metatarsal in both the frontal plane (across the racer) and saggital plane (front to back). Monopedal stance at ski flat is eloquently demonstrated by Bridget Currier  in the Burke Mountain Academy YouTube video, Get Over It with commentary by Mikaela Shiffrin – (http://youtu.be/Bh7KF49GzOc).

The opening graphics advise the racer to Get Over It and Stay Over It, meaning maintain the alignment of W from COM, over the proximate centre of the head of the first metatarsal in the frontal and saggital planes throughout the entire turn. But few racers can Get Over It, let alone Stay Over It, because the structures of their ski boots prevent them from assuming monopedal stance. This is especially true of racers whose boots are closely formed to the shape of their foot and leg in what amounts to perfect envelopment.

The graphic below is a re-creation of the stick person in Figure 23 above. The notations have been revised to reflect the terminology used in blog posts. The left stick person is depicted in bipedal stance. The centre stick person is depicted in monopedal stance. The right stick person is depicted in fixed neutral stance. When the foot is fixed in neutral, pronation is not possible and the foot is prevented from everting (the sole turns outward). In order for W emanating from COM to be positioned over the proximate centre of the head of the first metatarsal, the foot must evert approximately 7 to 8 degrees as depicted in the centre stick person.

The graphic below shows the effect of fixing the foot in neutral. When a racer attempts to balance on the new outside limb at ski flat, the inability to align W with GRF at the inside edge of the outside ski will cause the racer to fall into the new turn or consciously move away from the outside ski. .

Falls into turnPreventing the foot from pronating within a ski boot causes other problems. When the leg is rotated internally relative to the foot by the hip rotators, a torsional load is applied to the foot. Conventional ski boots do not provide surfaces for the foot to transfer biomechanically generated forces such as torque to. In addition, the structures of conventional ski boots present sources of resistance which interfere with the movements necessary to establish force transfer connections of discrete aspects of the foot with the boot shell.

Figures 22 A through 22 D below are from US350. Figures 22 A and 22 B depict the architecture of a foot in bipedal stance. Figures 22 C and 22 D depict the architecture of a foot in monopedal stance. Changes in the length of the foot in bipedal and monopedal stances are annotated as  L1 (bipedal) and L2 (monopedal). Changes in the angle of dorsiflexion of the ankle joint in bipedal and monopedal stances are annotated as  A1 (bipedal) and A2 (monopedal). Changes in the height of the arch in bipedal and monopedal stances are annotated as H1 (bipedal) and H2 (monopedal). Internal rotation of the leg in monopedal stance is annotated at 6. Changes in the length of the foot in bipedal and monopedal stances are annotated as  L1 (bipedal) and L2 (monopedal).  Changes in the position of the head of the first metatarsal in bipedal and monopedal stances are annotated as  2. Changes in the position of the medial tarsal bone in bipedal and monopedal stances are annotated as  3. Changes in the width across the heads of the metatarsals in bipedal and monopedal stances are annotated as  4. Shear forces, which will be the subject of a future post, are shown in Figure 22 D.

Screen Shot 2015-01-08 at 2.12.34 PMIn order to apply top down internal rotation, the racer has to move the load W to the ball of the foot as shown in the graphic below.

IdealThe short video clip below shows how the foot must be able to pronate within the confines of the ski boot without interference in order to set up the force couple required to transfer whole leg internal rotation to the new outside ski. The typical most significant source of interference is the structures of the ski boot in front of the ankle joint on the inner aspect of the boot.

 

The red bars in the BIPEDAL foot define common sources of interference created by structures of the ski boot that prevent the foot from pronating and establishing force transfer connections with the shell as shown in the MONOPEDAL foot. While the connection of the two transfer points suggests that the centre of rotation lies within the confines of the foot its true centre is not intuitive. This will be the subject of the next post.

BIO-MEDICAL PERSPECTIVE

Normal medial STJ movement of the talus is followed by a mandatory normal 1:1 coupling of the tibia to encourage normal internal leg rotation and normal dorsiflexion of the ankle. This normal coupling mechanism produces a synergistic postural response enhancing internal rotation of the entire leg.  Pelvic counter ensures hip capsule tightening which stabilizes the hip joint during the turn.

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Dr.Kim Hewson is an Orthopaedic Surgeon and former Director of Orthopaedic Sports Medicine  at the University of Arizona.  He is currently a veteran Telluride Ski School Alpine Instructor and Staff Trainer in the Biomechanics of Alpine Skiing.