Mikaela Shiffrin

HEEL PRESSURE VS. HEEL/FIRST METATARSAL PRESSURE DIFFERENTIAL

The Balance Axis

The two University of Ottawa pressure studies that used elite ski instructors found that, without exception, every ski instructor started their turns with pressure under the head of the first metatarsal (ball of the foot) and that in some, but not all turns, the pressure moved back to the heel in the last part of the turn. What the studies did not make clear is that when the Center of Pressure (COP) is under either the heel or the head of the first metatarsal, there will always be a secondary center of pressure under some other aspect of the foot. When COP is under the heel, there will be a diffused, but weak, area of pressure in the forefoot across the heads of metatarsals 1-2-3. But when COP is under the head of the first metatarsal, there will always be a well-defined, localized secondary area of pressure under the heel. The relationship of the two COPs (heel or head of the first metatarsal), affects the relationship of the balance axis of the foot and the balance axis of the ski.

The ski has two balance axes. These are the theoretical points where a ski would balance on the head of a pin if the mass of the ski were perfectly distributed. The graphic below shows the two balance axes of a ski. A section below the top view of the ski shows the minimum profile width or waist of the ski in black. The limits of the sidecut are shown in grey. A triangle indicates the Balance Axis.

Balance Axis

(click on graphics to enlarge them)

Heel COP

When COP is under the heel, the load or pressure, will lie on the transverse Balance Axis of the ski where it will be distributed outward from its center as shown in the graphic below. Because of the weak, diffused secondary area of pressure across metatarsals 1-2-3, pressure under the heel will extend more forward, towards the shovel, than rearward, towards the tail. The load on the heel lies on the anatomical center axis of the foot that runs through center of the heel and the head of the second metatarsal. As with the Balance Axis of the ski, the anatomical center axis of the foot is the theoretical transverse balance point in bipedal stance. This is the reason why skate blades are mounted on this axis. LeMaster and others have used the skate example to explain why a ski is hard to hold on edge compared to an ice skate. But the balanced load across the transverse aspect of the ski will keep the ski level on flat terrain and resist forces that attempt to tip it onto one of its edges. Heel COP

What has not been recognised in skiing, is that the transverse balance axis of foot becomes re-aligned in monopedal stance so that it runs through the center of the heel and the center of the head of the first metatarsal as shown in the graphic below. By placing the highest load on the ski under the head of the first metatarsal, the re-alignment of the anatomic balance axis that occurs in monopedal stance, unbalances the load on the outside ski causing it to tip on edge. This unbalanced load causes the limits of the ski at the shovel and tail to displace away from the center of the load (COP) on the portion of the inside edge under the head of the first metatarsal. The diagonal load axis across the Balance Axis of the ski sets up components of force that are perpendicular and parallel to the Balance Axis of the ski.

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The re-alignment of the balance axis of the foot that occurs in monopedal stance can be clearly seen in images below of speed skaters and Mikaela Shiffrin balancing on a slack line.

Speed skaters

 

Shiffrin Slack line 1

The re-alignment of the anatomical balance axis of the foot is the end result of the transition to the inside ski while it is still on its current edge and the creation of pressure from extending the leg. This mechanism enables racers such as Shiffrin to Get Over it at ski flat and initiate edge and turning effects simply by balancing on the new outside ski and managing the heel/first metatarsal pressure differential.

The actual mechanics are more complicated than what I have described in this post. But as the saying goes, “One step at a time.”

THE AUSTRIAN MOVE

The high loads in GS turns make it the acid test of the ability to control forces across what I call the Inside Edge-Load Transfer Axis of the outside ski. The rapid timing of slalom make it the acid test of the ability to maintain and control forces across the Inside Edge-Load Transfer Axis of the outside ski while rapidly applying whole leg internal rotation to the foot and ski. After struggling early in the season due to pre-season changes in equipment, in particular, her ski boots, Shiffrin has emerged as the preeminent female slalom technician. I believe that there are two reasons why Shiffrin has not dominated GS, 1) a less than optimal boot/ski setup for GS and 2) the failure to make effective use of her inside ski in appropriate turns using what I call the Austrian Move as exemplified by Marcel Hirscher and Ana Fenniger.

In my post VONN VS FENNINGER ( https://skimoves.me/2015/01/24/vonn-vs-fenninger/), I criticized Fenninger for using a dominant position on her inside in gates where Vonn was quicker with the use of a dominant position on the outside ski. Vonn flowed seamlessly from turn-to-turn while Fenninger often appeared choppy. When used inappropriately the Austrian Move can actually slow a racer. But when used appropriately, a race can be won with the effective use of the the Austrian Move in only a few turns.

Here’s the video clip where I compare Vonn to Fenninger. Note how Fenninger lifts the tail of her outside ski so she can drop in the hips and create an impact load on the tail of her inside ski. This move is most effective when it is done before the force exerted by the snow on the sidecut of the outside ski exceeds the load transferred by skier across the Inside Edge-Load Transfer Axis.

 

The Austrian Move is not skiing on two skis or skiing with all the weight on the inside ski during a turn. Nor, is it the same as the transition move associated with the Ski Move, although the Austrian Move often evolves out of a turn that starts with the Ski Move. The Austrian Move is often a divergent move of the inside ski away from a dominant ‘over it’, position on the outside ski.

In the photo sequence below,  Marcel Hirscher makes a very rapid Austrian Move in about a tenth of a second from a dominant position on his outside ski (left leg) to a dominant position on his inside ski (right leg). Hash marks overlaid on Hirscher’s skis make the Austrian Move easier to see.

Austrian Move

Hirscher makes this move so quickly that it is not easily seen at anything less than frame-by-frame.

In the clip below, Hirscher uses what I call the Sudden Impact Austrian Move where he comes down hard on the inside edge of the tail of this inside ski. High impact loading is not possible on the outside ski which must be progressively pressured with leg extension in order to prevent the edge from being overloaded across the Inside Edge-Load Transfer Axis.

 

The clip below shows Fenninger using the Austrian Move.

 

The problem with the use of a dominant inside ski in a turn is that it is risky. Once a racer commits to the inside ski, the mechanics of the Austrian Move severely limit the ability to make directional changes. So if a racer’s line is off, especially if it is too high, they usually need to make an athletic move to correct it. In turns where the load is not great, a dominant position on the inside ski can actually be slower that turns made with a dominant outside ski. In the video clip below, Fenninger  shows in the last sequence what happens when the Austrian Move goes wrong.

 

Racers with small feet typically have an Inside Edge-Load Transfer Axis of the outside ski that is less than optimal because it is dependent on the position of the proximate centre of the head of the first metatarsal being aligned over the inside edge. The Inside Edge-Load Transfer Axis is the point where the vector of the load W emanating from CoM intersects a vertical line emanating from the inside edge perpendicular to the transverse aspect of the base of the outside ski. As it becomes increasingly offset to the outside turn aspect of the inside edge, the Inside Edge-Load Transfer Axis becomes increasingly unfavourable. The Inside Edge-Load Transfer Axis is optimized by aligning the proximate center of the head of the first metatarsal directly over the inside edge of the outside ski. While FIS regulations appear to allow the use of skis with the appropriate Minimum Profile Width underfoot required to align the head of the first metatarsal over the inside edge, female World Cup racers do not seem to be using GS skis with less than a 64 mm Minimum Ski Profile Width.

Until such time as female racers use skis with a Minimum Profile Width that will allow the Inside Edge-Load Transfer Axis to be optimized, the Austrian Move may be the only option by which to be competitive in GS.

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.

 

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

Click on the image to enlarge it

Shiffrin

 Click on the image to enlarge it