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.
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.
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.
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.
The 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.
From 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