OVER THE EDGE…… OR NOT


Correct positioning of the foot (in relation to the ski) is more important than forced constraint and squeezing the foot. (1)

In upright postures and situations where the superincumbent body must balance against external forces like gravity or a resultant force such as exists in a dynamic physical environment like skiing, the skeletal structure applies the force that opposes ground reaction force. When the load-bearing elements of the foot are in contact with the supporting surface, the point centre of GRF is called COP. But while the skeletal structure applies the force, it is coordinated muscle activity orchestrated by the CNS that creates the dynamic alignment of the joints of the skeletal structure required to effectively apply the force. Towards this end, the skeletal structures of the foot have key  load-bearing elements through which forces are applied. The major load-bearing elements are the calcaneus and the head of the first metatarsal. The minor load-bearing elements are the heads of metatarsals 2 through 5 and the tuberosity of the 5th metatarsal. The sketch below shows the 2 major load-bearing elements of the foot and the force vector between the two elements when COP is under the head of the first metatarsal. There are no load-bearing elements within the vault of the longitudinal arch.

Major Load PointsIn the skier/ski equipment interface, the orientation of the key load-bearing elements of the foot in relation to the mechanical aspects of the ski is crucial to the ability of the skier to establish a supportive platform under foot  at ski flat between edge change and maintain it in a turn. Six factors affect the forces acting at the inside edge of the outside ski of a turn. The following four factors are in the horizontal plane.

1. The orientation of the anatomical centreline of the foot in relation to the running centreline of the ski.

2. The dimension between the proximate centres of the heads of the first and second metatarsals.

3. The location of the Centre of Force or Centre of Pressure under the outside foot when the ski is on edge.

4. The width of the ski under foot.

The fifth and sixth factors; the effect of the height of the base of the boot board underfoot above the base of the ski and the angle of the vector of R are in the frontal plane. They will be discussed in the next post.

Ideally, the orientation of the anatomical centrelines of the foot and ski should be congruent as shown in the sketch below.

1. alignedIn the sketch below, the anatomical centreline of the foot is offset from the running centreline of the ski. Since the anatomical centreline of the foot is the natural balance axis, in situations like the one shown, the skis will tend to tip onto edge when running straight on two skis.

1 offset

In the sketch below, the foot is skewed in relation to the running centreline of the ski. This will affect the relationship of the key load-bearing elements of the foot with the mechanical points of the ski.

1.3The sketch below, arrows on either side of the dashed lines indicate the dimension between the proximate centres of the heads of the first and second metatarsals with arrows on either side of the dashed lines.

2This dimension will increase or decrease in proportion to foot size. Because ski boot moulds are usually scaled up and down from a US men’s size 9, the centre of the head of  the first metatarsal will become positioned further inboard in relation to the inside edge of the outside ski as the foot size increases and further become further outboard positioned in relation to the inside edge of the outside ski as the foot size decreases as shown in the sketch below.

foot smallerIn the sketch below, the Centre of Pressure is under the head of the first metatarsal. When the outside ski is on edge in a turn, the Centre of Pressure can only be located under the head of the first metatarsal if the skier has established a platform at ski flat between edge change as described in PLATFORM.

3In the sketch below, the Centre of Force lies on the anatomical centre axis of the foot. When the outside ski is on edge in turn and  the skier was unable to establish a platform at ski flat between edge change as described in PLATFORM, the Centre of Force will lie on the anatomical centre axis of the foot. When this happens, an inversion moment arm will set up between the Centre of Force and the inside edge of the outside ski.

3.2

In the sketch below, the width of the ski underfoot aligns the inside edge with the centre of the head of the first metatarsal.

 

4.1

As the width of the ski underfoot increases, the inside edge underfoot will increasingly move inboard (into the turn aspect) in relation to the centre of the head of the first metatarsal. As ski width underfoot increases it becomes quickly impossible to maintain a platform under foot even if the foot was able to pronate at ski flat between edge change.

4.2

The width underfoot of fat skis today that are increasingly being skied on groomed is approaching 150 mm. This virtually ensures the presence of an inversion moment on the outside foot that will rotate the leg externally. The reaction force under the foot will translate inversion into a varus thrust on the leg at the upper aspect of the inside of the boot cuff.

4.3

The optimal relationship of the 4 factors in horizontal plane is shown in the sketch below. When these factors are optimal for a skier, but especially a racer, the superior performance is not derived from magic but from good mechanics.

Ideal


  1. M. Pfeiffer (Institute for Athletic Sciences, University of Salzburg) – Kinematics of the Foot in the ski Boot: The Shoe in Sport/The Ski Boot.

 

4 comments

  1. Confirms my clinical and demographic findings. Excellent prelude to Fat ski injury predisposition discussion.

    1. Dr Hewson, We have know this for years, we have done over 10,000 biomechanics assessments and have practical experience on snow with 4000 skiers in a formal evaluation, in the last ten seasons. respectfully, Harald Harb

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