EDGING MECHANICS/SKI CONTROL: THE FOOT WIDTH FACTOR


Without hard data on the Ski Profile Width of the ski in combination with morphological data on the foot, it is not possible to arrive at definitive conclusions in regard to the position of the proximate center of the head of the first metatarsal in relation to the inside edge of the outside ski for a specific racer. However, from a visual assessment, it appears as if ladies and men’s Ski Profile Widths are the same or very close to the same for the same events.

  • The ability of a skier to apply a force to the outside ski that is perpendicular to the transverse aspect of the base and aligned in opposition to the Snow Reaction Force acting at the inside edge is fundamental to edge control, balance and control of the skis.
  • The presence of an unbalanced moment of force across the inside edge of the outside ski degrades or precludes the normally effective postural responses that balance moments of force across the joints of the lower limb.

For decades, 70 mm underfoot was the standard Ski Profile Width of most skis. Accordingly, ski boot sole binding interface width was standardized in the order of 68 mm.  In addition, ski boot shells for females were, and as far as I know, still are, scaled from men’s shells, typically a US Men’s size 9.

The 2014-2015 Edition of the FIS Specifications for Competition Equipment specifies Ski Profile Widths for both men’s and ladies World Cup and Entry League racers at 65 mm or less for all skis except slalom which is 63 mm or greater.  The Ski Profile Width for U18 is a fixed at 60 mm underfoot. Of interest, is the fact that while the Minimum Ski Profiles are the same for both men and ladies, Super-G  and GS skis for ladies have smaller radius (greater sidecut) than men’s skis (Super-G: 40m for ladies vs. 45m for men, GS: 30m for ladies vs. 35m for men). Ski Profile Width is especially critical  for female racers and male racers with small feet because the width of the foot determines where the proximate centre of the head of the first metatarsal will be positioned in relation to the inside edge of the outside ski.

The center-to-center dimension between the heads of the first and second metatarsals of a US Size Men’s size 9 foot is in the order of 31 to 33 mm. Ladies with small feet can have a center-to-center dimension between the heads of the first and second metatarsals of 25 mm or less. The implications are that ladies and men with small feet with center-t0-center dimensions between the heads of the first and second metatarsals of 25 mm or less on a ski with Minimum Ski Profile Width of 60 mm or greater will have the proximate center of the head of the first metatarsal as the absolute inner limit of the center of load W, 5 mm or greater outboard of the inside edge in combination with the higher torque loads arising from greater sidecut than men’s ski for the same discipline.

Although many shoes for women are simply scaled down versions of the same shoe for men (Frey, C., 2000), previous studies describe differences between genders particularly at the arch, the lateral side of the foot, the first toe and the ball of the foot. Men have longer and broader feet than women for a given stature (Wunderlich, R.E.; Cavanagh, P.R., 2000), whereby women tend to have a narrower heel in relation to the forefoot and have narrower feet than men in general relative to length (Frey, C., 2000). (1)

The findings of the aforementioned study (1) confirm the existence of significant differences between gender in terms of important anatomical measurements of the foot. Scans of feet and measurements of key anatomical landmarks found that women have narrower feet in the heel and forefoot region than men and their instep height is also lower than mens’. A US size 5 women’s foot (4 men’s – EU 35) was found to be 230 – 237 mm in length. A US size 9 men’s foot (10, women’s – EU 43) was found to be 283-290 mm in length. It is significant that ninety percent of the subjects in the study were athletes. In ski racing, where good skier/ski mechanics depends on the relationship of key mechanical elements of the foot and ski, foot gender differences can have a significant impact on racer performance and especially competitiveness.

The first sketch below shows the width of a men’s foot of a fixed size.

male foot

The sketch below compares the width of a ladies foot of the same size adjusted to depict the gender differences found in the cited study.

female foot

The sketch below compares the proportions of a US Size 9 men’s foot (top) to a US Size 5 ladies foot (US Size 4 men’s). The thick black line running through the both feet represents the inside edge of the outside ski.

Men vs Ladies

As the load W applied to the head of the first metatarsal moves away from the inside edge towards the outside turn aspect of a ski, the mechanics of edge hold become progressively unfavourable as does the competitive disadvantage which will be greatest in GS due to the high transient peaks in GRF. Attempts to rotate ski about its long axis to engage the sidecut of the ski into the snow surface will cause the load W to shift to the default position on the transverse centre of the ski, effectively increasing the moment arm that rotates the ski and foot in inversion.

Although studies on the morphological variances in feet exist such as Gender differences in foot shape, the writer is unaware of the existence of studies on the center-to-center dimension between the heads of the first and second metatarsals and the range of variance in foot size and especially gender differences.  The writer’s is also unaware of any mention of this critical issue in the literature pertaining to skiing. While whole leg rotation of the outside leg can partially negate the initial effects of an inability to position the proximate center of the head of the first metatarsal over the inside edge of the outside ski, this is only effective at minimal sidecut loads.

The sketch below is schematic representation of a ski with a Ski Profile Width of 65 mm. Fe represents the position of the force that would result from a skier with center-to-center dimension between the heads of the first and second metatarsals in the order of 32 to 33 mm with the anatomic center axis of the foot aligned with the transverse centre axis of the ski.

Ski Lever Fe onlyThe sketch below is schematic representation of a ski with the same Ski Profile Width of 65 mm but with a greater sidecut associated with a smaller sidecut radius. Fe represents the force that would result from a skier with center-t0-center dimension between the heads of the first and second metatarsals in the order of 25 mm with the anatomic center axis of the foot aligned with the transverse centre axis of the ski. The resulting misalignment represents a best case scenario.

Ski Lever FemaleFrom a perspective of mechanics and biomechanics, the inability to align the proximate center of the head of the first metatarsal with the inside edge of the outside ski and apply a force aligned in opposition to the snow reaction force acting at the inside edge has significant implications for edge control, ski control, balance and competitiveness. The first nation to address the effects of what should be an obvious inequity, will have a huge competitive advantage.


Gender differences in foot shape http://www.staffs.ac.uk/isbfw/ISBFootwear.Abstracts05/Foot36.Krauss.GenderDifferences.pdf

6 comments

  1. greetings Dave…I ski on WC 165 Sl Vis plate Marker comp 20 sping. 0°delta. My boots ( 27.5 313mm) have 4° of measured ramp. would you comment on upper cuff forward lean angle. you have mentioned that this is a late stance phase condition. Could you also comment on where COP would be located in this stance. Thx!! will share new boot fit with you next! eastern ‘firm’ here at Sugarloaf today 🙂

    1. Hi Steve, 27.5 is an 8.5 US mens. I think you meant 213 mm, not 313 mm. In a future post, I will explain how I arrived at about 3 degrees net ramp angle. Ramp angle doesn’t become a big issue until all the factors of equipment start to approach optimal. Once this happens, I am confident that any elite racer who is given a number of different ramp angles to assess will quickly be able to tell which one works best within a half a degree. Until studies of this issue are done, 3 degrees is a good ballpark number. Four degrees may work well for you. But it would be interesting if you could get some extra boot boards, grind them to different angles and ski with different ramp angles in each boot.

      Shank angle. I could write a lengthy technical description. But this would still not help a racer figure out whether the angle was right when their foot and leg are in a ski boot. After reading your post earlier today, I designed a series of exercises to help racers learn how to find the correct shank angle. When I built boots for racers, a lot of the work was done outside the boots with racers in bare feet and in shorts. The only way I could get their boots right was for them to guide my efforts. Most young racers today have had boots with all the bells and whistles from the first second they were on snow. So it is unlikely they have any idea what the boot should feel like, let alone know what reference shank angle is. At ski flat, COP should be under the proximate center of the head of the first metatarsal.

      At this point in the season, if a racer can improve their equipment set up they have a chance to get some points. So hopefully my recent post will help.

  2. Except…. how does any of this translate to useful information? We all know racers are skiing in downsized boot; downsized in length and width. This means that most often, the foot is wider than the boot. Shells are easily modified to the shape of a specific foot, but it will mostly likely still be narrower than the foot. That means the distance between all the metatarsal heads, are decreased.

    If the position of the 1st metatarsal head is important, then it is also important that the 1st Ray is functional. You do not believe in supporting the foot, thus, all your measurements will mean nothing, as the STJ will unlock, creating a hypermobile 1st Ray.

    1. Judging by your comments the fact that there is a big fat, drop dead, kick in your front teeth obvious unbalanced moment of force on the outside ski and foot of a turn just waiting for a transient spike in ground reaction force to violently rotate the whole leg externally against a well stabilized femur is not a concern of yours. Is this correct? I sincerely hope not.

  3. In your second paragraph detailing equipment rules, specifically profile width under binding, I believe your U18 number is incorrect. In the 2015 USSA Alpine Competition Guide, page 135, it states that for DH, SG and GS, the profile width for U18 and younger, the profile width is unrestricted.

    1. I have a printout of the FIS specification right in front of me. Under USSA (U18), it lists 60 mm as the minimum width of the running surface at binding for all skis. The listings for FIS/ENL and World Cup specifically list the minimum width of the running surface at binding as = 63 mm. If these are the minimum widths, why does the FIS specify <=65 mm? = means equal to or greater. The reference to ‘without tolerance’ means no tolerance in terms of variances; if the specification is 65 <=, 66 mm is not permitted. If it is 60 mm, 59 and 61 are not permitted. This issue aside, there does not appear to be a discernible difference between the minimum width of the running surface at binding of mens and ladies skis. Does any ski maker make a ski less than 63 mm with the minimum width of the running surface at binding? The preceding issues aside, is the position of the proximate centre of the head of the first metatarsal in relation to the inside edge of the outside ski on anyone's radar screen in ski racing? Not as far as I have been able to tell.

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